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J Vet Clin 2023; 40(6): 445-451

https://doi.org/10.17555/jvc.2023.40.6.445

Published online December 31, 2023

Surgery and Metronomic Chemotherapy in a Pet Rabbit (Oryctolagus cuniculus) with Mammary Gland Adenocarcinoma

Jihee Hong , Jeong-Min Lee , Ji-Young Lee , Han-Joon Lee , Dong-Kwan Lee , Joong-Hyun Song , Kun-Ho Song*

Department of Veterinary Internal Medicine, College of Veterinary Medicine, Chungnam National University, Daejeon 34134, South Korea

Correspondence to:*songkh@cnu.ac.kr

Received: July 16, 2023; Revised: October 19, 2023; Accepted: November 7, 2023

Copyright © The Korean Society of Veterinary Clinics.

An 8-year-old female pet rabbit presented at the veterinary clinic for mammary gland palpation due to the presence of a mass. Upon physical examination, a mass was identified in the left fourth mammary gland. Abdominal ultrasonography revealed a 3 × 2 cm mass in the right uterus and general thickening of the endometrium, suggesting uterine sinusitis. Multiple pulmonary nodules suspected to be metastatic lesions were identified on chest radiography. Surgery was performed to mastectomy and ovariohysterectomy (OHE). The histopathological examination of the tumor revealed mammary gland adenocarcinoma (simple-type) with multiple nodules consisting of the proliferation of tumor cells forming tubules containing secretory materials, cellular debris, and solid nests with a central area of necrosis. Metronomic chemotherapy was performed with cyclophosphamide and lomustine (CCNU) based on the histopathological findings. The quality of life has been well maintained, with no specific clinical symptoms observed for 8 months after metronomic chemotherapy. To the best of authors’ knowledge, this study is the first to examine the effects of metronomic chemotherapy with cyclophosphamide and lomustine in a pet rabbit.

Keywords: CCNU, cyclophosphamide, mammary gland adenocarcinoma, rabbit

Recently, as the popularity of rabbits has increased, there has been a growing interest in geriatric diseases and tumors in rabbits (1,16). Mammary gland tumors (MGT), along with uterine carcinoma, are one of the most commonly occurring tumors in female rabbits and can significantly impact the lifespan of rabbits (12). MGT in rabbits occur around the age of 5.5 years on average (1). They are more commonly found in female rabbits and are often associated with uterine hyperplasia or adenocarcinoma, suggesting hormonal factors are involved (1,16).

Clinical signs of MGT in rabbits are diverse and non-specific, including inappetence, weight loss, lethargy, and depression (16). Depending on the local or systemic effects, rabbits may also exhibit symptoms such as hematuria, gastric bloat or gut stasis, and dyspnea (16). When diagnosing the tumor, a comprehensive assessment of the overall condition can be made through blood examination and urinalysis (16). Radiographs and CT scans can be used to determine the size, extent, and invasiveness of the tumor (16).

A definitive diagnosis is confirmed through histopathological examination (1). Histopathological analysis revealed that the majority of tumors (88%) exhibited malignant characteristics, including infiltrative growth, incomplete differentiation, and/or cellular dysplasia (1). The malignancy pattern observed in rabbit mammary tumors significantly differs from that reported in dogs, bearing closer resemblance to the situation observed in cats (1). Similar to cats, rabbits predominantly exhibit malignant tumors, while complex types are relatively rare (1).

In rabbits without metastasis, mastectomy is considered the primary treatment, additionally, chemotherapy is a viable treatment option for non-resectable tumors or rabbits with confirmed metastasis (16). Rabbits can be treated with a range of chemotherapeutic agents, administered either systemically or intralesionally. This flexibility allows for treatment strategies based on individual circumstances (16).

As of now, there are no published specific protocols regarding the clinical use or efficacy of chemotherapeutic agents in rabbits (16). Experimental studies have reported favorable outcomes when using platinum and pirarubicin to treat induced uterine, bladder, or mammary carcinomas in rabbits demonstrating efficacy in managing such tumors in rabbits (16).

This study presents a case of a rabbit diagnosed with mammary gland adenocarcinoma, which was treated with cyclophosphamide and lomustine.

An 8-year-old, 3 kg intact female pet rabbit presented the veterinary clinic with the chief complaint of a mass on the mammary glands. On physical examination, the rabbit was alert and responsive and all vital signs were normal. The mass was firm and had a size of 47 × 32 mm with irregular and asymmetrical shape (Fig. 1). It was palpated below the left fourth mammary gland. The mass was not adherent to adjacent tissues, and no peripheral lymphadenomegaly was detected.

Figure 1.The firm and irregular and asymmetrical mass at the left fourth mammary gland with size of 47 × 32 mm.

The results of the complete blood count (pocH-100iV Diff, Sysmex Korea, Seoul, South Korea) showed a decrease in white blood cell count to 3.7 × 109/L (reference interval; 5-12 × 109/L), an increase in platelet count to 768 × 109/L (reference interval; 290-650 × 109/L), and an elevated lymphocyte percentage of 92.1% (reference interval; 25-60%). The serum biochemistry (Catalyst Dx, IDEXX Korea, Seoul, South Korea) revealed hyperglycemia with a glucose level of 231 mg/dL (reference interval; 75-145 mg/dL) and low alanine transaminase (ALT) activity with a value of 27 U/L (reference interval; 31-53 U/L).

On thoracic radiographs (HF-525 Plus VET, ECORAY, Seoul, South Korea), multiple nodules were observed with increased parenchymal density in the left lower and adjacent lung lobes, suggesting the possibility of pulmonary metastasis (Fig. 2). Abdominal ultrasonography revealed a 3 × 2 cm mass in the right uterine region, along with diffuse endometrial thickening, indicative of uterine adenomyosis (Fig. 3).

Figure 2.(A) Right lateral view and (B) ventral- dorsal view of thoracic radiographs showing the presence of multiple nodules and increased parenchymal density.

Figure 3.3 × 2 cm mass in the right uterine region, along with diffuse endometrial thickening, indicative of uterine adenomyosis.

Fine Needle Aspiration was performed at the mass for diagnosis, but it yielded poor cellularity with the presence of erythrocytes, cellular debris, and fragmented nuclei, making interpretation difficult and unhelpful.

It was decided to proceed with surgery for histopathological examination and treatment, resulting in a total mastectomy and ovariohysterectomy (OHE) being performed. The rabbit was sedated with Ketamine HCl (KESIA inj. South Korea, 10 mg/kg body weight [BW], IM) and midazolam (Bukwang Midazolam Inj. South Korea, 3 mg/kg BW, IM). An intravenous catheter was placed in the left cephalic vein, and hydration was maintained using lactated Ringer’s solution at a rate of 10 mL/kg BW per hour.

The pet rabbit was induced with isoflurane (Ifran Liq; Hana Pharm, South Korea), 3% in oxygen using a V-gel R2 (Docsinnovent; Advanced V-gel for rabbits). Maintenance anesthesia was carried with isoflurane concentrations ranging from 1% to 3%. The rabbit was positioned in dorsal recumbency, and a standard aseptic protocol was followed to prepare the surgical site. A curved 3-cm incision was made around the mass, ensuring its complete removal. The subcutaneous tissues were meticulously sutured using 4-0 Polydioxanone (PDS II; polydioxanone; Ethicon, Somerville, NJ, USA) in a simple continuous pattern, while the skin was sutured using a simple continuous pattern with 4-0 nylon (BLUE NYLON; Polyamide: AILEE. Co., Busan, South Korea). The excised mass was carefully preserved in 10% neutral buffered formalin and sent for histopathological examination. The rabbit experienced a smooth recovery from anesthesia.

After surgery, meloxicam (Metacam, Boehringer Ingelheim Korea, Seoul, South Korea) at a dose of 0.5 mg/kg and marbofloxacin (Marbocyl, Vetoquinol, Gyeonggi-do, South Korea) at a dose of 5 mg/kg were administered subcutaneously. The next day, upon discharge, meloxicam and marbofloxacin were prescribed orally at the same dose.

The histopathological examination of the mammary tissue revealed the presence of multiple nodules composed of proliferating tumor cells exhibiting invasive behavior into the surrounding dermis and subcutaneous tissue (Fig. 4). The tumor cells formed cohesive solid sheets, ductal structures, or tubules, with some luminal spaces containing homogeneous eosinophilic secretions. The tumor cells displayed distinct cuboidal morphology with well-defined cell boundaries and exhibited active proliferation while forming acinar structures. The cytoplasm of the tumor cells showed moderate eosinophilia, and the nuclei appeared euchromatic with finely stippled chromatin. Prominent nucleoli were centrally located within the nuclei. The nuclear-cytoplasmic ratio was mild, and occasional mitotic figures were observed, with an average of 0 to 1 per 400 × field. Significant dilation of normal mammary ducts in the surrounding tissue was evident, while no evidence of tumor emboli or similar findings was identified. Also, the histopathological analysis of the provided tissue revealed the presence of multiple tumor nodules in the dermis.

Figure 4.(A) Neoplastic lobules separated by fibrous connective tissue having central area of necrosis (arrows). Hematoxylin and eosin. (B) Neoplastic cells are cuboidal to columnar epithelial cells forming tubules (arrows). Some tubular lumens contain secretory materials and cellular debris (arrowhead). (C) Neoplastic cells are forming solid nests (arrows) with central are of necrosis (arrowhead). (D) Neoplastic cells are arranged in solid nests (arrows) and tubules (blank arrowhead). Some of the tubules are severely dilated with secretory material and necrotic debris (arrowhead).

Based on the morphological features of the tumor cells and the distinct demarcation of the mass, a diagnosis of malignant neoplasm originating from glandular tissue, including the mammary gland, was established and the patient was diagnosed with simple-type, mammary gland adenocarcinoma.

After 7 days of surgery, the surgical site healed well and the pet rabbit showed good vitality with good appetite, so it was decided to proceed with chemotherapy. Due to the small size of pet rabbit’s blood vessels, oral administration was chosen as the method for administering the chemotherapeutic drugs, instead of intravenous infusion. Cyclophosphamide (Alkyloxan, JW Pharmaceutical Co., Gyeonggi-do, South Korea) was administered orally at a dose of 50 mg/m2 every Monday, Wednesday, and Friday (MWF) and lomustine (CCNU) (GmbH, Wedel, Germany) was administered orally at a dosage of 50 mg/m2 every 3 weeks. In addition, meloxicam was prescribed at a dosage of 0.5 mg/kg once a day.

Side effects of the chemotherapy such as anorexia, fever, and lethargy were monitored through regular physical examinations, complete blood counts (CBC), and blood chemistry tests conducted after three months (Table 1). However, no significant side effects were observed. Thoracic radiography was monitored every three weeks, but no changes were observed in the size or number of metastatic tumors (Fig. 5).

Table 1 Complete blood count and serum chemistry of the patient

IndexResultReference rangeUnit
Day 0Before chemotherapyDay 90
WBC3.74.755.355-12109/L
HGB12.310.513.38-17.6g/dL
RBC5.645.166.614-81012/L
HCT3432.442.630-50%
PLT768540523290-650109/L
WBC-monocyte2.910.39.92-10%
WBC-lymphocyte92.131.838.525-60%
WBC-basophil04.66.42-8%
WBC-eosinophil50.80.60-5%
ALT27-4631-53U/L
ALKP90-70470-145U/L
BUN10-1910-24mg/dL
Creatinine1.1-0.60.8-1.8mg/dL
Glucose231--75-145mg/dL
Protein-total5.8-8.55.5-7.2g/dL
Albumin3.3-5.82.7-4.6g/dL


Figure 5.Thoracic radiographic monitoring of the pulmonary mass after chemotherapy. (A) 3-week, (B) 6-week, (C) 9-week, (D) 12-week.

The treatment was continued for 10 months, and there was no new occurrence or relapse of the tumor.

In this case study, we presented a successful management approach for mammary gland adenocarcinoma in a pet rabbit using metronomic chemotherapy with cyclophosphamide and lomustine (1,6,15,16). To the best of author’s knowledge, this is the first report documenting the effects of metronomic chemotherapy in a rabbit with mammary gland adenocarcinoma (1,12,16).

In this case, the 8-year-old female pet rabbit presented with a palpable mass on the mammary glands, along with a concurrent uterine adenomyosis and metastatic lung tumors. Considering the presence of pulmonary nodules suspected to be metastatic lesions, systemic chemotherapy was initiated using a metronomic approach with cyclophosphamide and lomustine (5,16,17). Conventional cytotoxic drugs are typically intended to be used at the maximum tolerated dose (MTD). Metronomic chemotherapy, characterized by the frequent administration of low-dose chemotherapy agents, aims to inhibit tumor angiogenesis and exert anti-tumor effects while minimizing adverse effects on normal tissues (2,3,8,11). Also, metronomic therapy specifically targets the endothelial cells within the developing blood vessels of the tumor, while conventional therapy focuses on inhibiting the growth of proliferating tumor cells (9). However, these approaches have been successful in various human and veterinary oncology cases, while its application in rabbits has been limited (1,7,11,16). We chose metronomic chemotherapy to minimize side effects due to the limited researches of chemotherapy in rabbits.

The rabbit had an OHE in addition to mastectomy following the standard (14). MGT in rabbits are frequently observed in female individuals and are often associated with uterine hyperplasia or adenocarcinoma, suggesting hormonal influences (1,12). Mammary adenocarcinomas are frequently observed in female rabbits that are between 3 to 4 years old and have had multiparous (14). Without treatment, prognosis with mammary adenocarcinoma is generally poor in female rabbits as metastasis to organs (14). The emphasis is placed on prevention rather than treatment of mammary tumors in dogs and cats (4). This is achieved by performing early OHE, reducing the risk of tumor development in the lifespan of dogs and cats (6). So, in this rabbit, we performed OHE similarly to dogs and cats to minimize tumor recurrence and avoid hormonal influences.

The first treatment for the most of mammary tumors in dogs and cats involves mammary gland excision surgery and spaying/neutering (6). Postoperative chemotherapy is not commonly administered as a routine treatment in dogs and cats (6). However, in cases of advanced mammary cancer such as distant metastases or inoperable tumors, chemotherapy is typically recommended (6). Dogs and cats are more amenable to intravenous access, and particularly in the case of dogs, they experience less environmental stress and mobility stress compared to other animals (6). This makes frequent visits to the veterinary clinic and the use of intravenous chemotherapy more manageable for them (6). Limited information is available regarding the chemotherapy options for rabbits with metastatic mammary adenocarcinomas (12). However, due to nodules suspected to be metastasis of MGT, we applied chemotherapy in the rabbit like in dogs and cats and no additional metastasis or enlargement of pulmonary nodule were observed with any side effects.

In this case, oral administration of cyclophosphamide and lomustine was chosen due to the small body and blood vessel size, making intravenous infusion challenging. Also, to minimize hospital visits and reduce the time spent at the veterinary clinic, oral chemotherapy agents were chosen for rabbits, considering their susceptibility to environmental stress (10). Alkylating agents such as cyclophosphamide, chlorambucil, melphalan, and lomustine have been associated with various side effects in rabbits. These primarily include bone marrow suppression and neutropenia, as well as gastrointestinal toxicity and the risk of hemorrhagic cystitis (16). Furthermore, hepatic and renal toxicity have been specifically reported with the use of lomustine (16). However, no side effects were noticed in this case.

This case highlights several important points. Firstly, in a rabbit with a mammary tumor that potentially metastasized to the lungs, the decision was made to opt for oral chemotherapy due to the lower risk and side effects associated with intravenous chemotherapy in rabbits (6,12,14,16). Additionally, the application of metronomic oral chemotherapy was particularly noteworthy as it minimized side effects and allowed for the management of the rabbit’s condition at home, reducing the stress of hospital visits and intravenous chemotherapy (10,16). This approach significantly improved the pet rabbit’s quality of life over an extended period. According to Shiga et al. (13), the median lifespan of Oryctolagus cuniculus is 7 years. The pet rabbit in this case, after chemotherapy, she survived for 10 months without showing any side effects. When considering the average lifespan and the patient’s age, 10 months is not considered a short period. Therefore, both surgery and chemotherapy are considered to have contributed to extending life. Further studies are needed to develop a chemotherapy protocol in rabbits with mammary gland adenocarcinoma.

To authors’ knowledge, this case describes the first reported case of palliative metronomic chemotherapy in a rabbit with mammary gland adenocarcinoma. Chemotherapy for mammary gland tumors (MGT) is well documented in canines and felines, but its documentation is limited in rabbits. No complications or side effects were observed during the 10 months of follow-up period. Metronomic chemotherapy could be an efficient treatment strategy for rabbits with MGT.

The authors have no conflicting interests.

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Article

Case Report

J Vet Clin 2023; 40(6): 445-451

Published online December 31, 2023 https://doi.org/10.17555/jvc.2023.40.6.445

Copyright © The Korean Society of Veterinary Clinics.

Surgery and Metronomic Chemotherapy in a Pet Rabbit (Oryctolagus cuniculus) with Mammary Gland Adenocarcinoma

Jihee Hong , Jeong-Min Lee , Ji-Young Lee , Han-Joon Lee , Dong-Kwan Lee , Joong-Hyun Song , Kun-Ho Song*

Department of Veterinary Internal Medicine, College of Veterinary Medicine, Chungnam National University, Daejeon 34134, South Korea

Correspondence to:*songkh@cnu.ac.kr

Received: July 16, 2023; Revised: October 19, 2023; Accepted: November 7, 2023

This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

An 8-year-old female pet rabbit presented at the veterinary clinic for mammary gland palpation due to the presence of a mass. Upon physical examination, a mass was identified in the left fourth mammary gland. Abdominal ultrasonography revealed a 3 × 2 cm mass in the right uterus and general thickening of the endometrium, suggesting uterine sinusitis. Multiple pulmonary nodules suspected to be metastatic lesions were identified on chest radiography. Surgery was performed to mastectomy and ovariohysterectomy (OHE). The histopathological examination of the tumor revealed mammary gland adenocarcinoma (simple-type) with multiple nodules consisting of the proliferation of tumor cells forming tubules containing secretory materials, cellular debris, and solid nests with a central area of necrosis. Metronomic chemotherapy was performed with cyclophosphamide and lomustine (CCNU) based on the histopathological findings. The quality of life has been well maintained, with no specific clinical symptoms observed for 8 months after metronomic chemotherapy. To the best of authors’ knowledge, this study is the first to examine the effects of metronomic chemotherapy with cyclophosphamide and lomustine in a pet rabbit.

Keywords: CCNU, cyclophosphamide, mammary gland adenocarcinoma, rabbit

Introduction

Recently, as the popularity of rabbits has increased, there has been a growing interest in geriatric diseases and tumors in rabbits (1,16). Mammary gland tumors (MGT), along with uterine carcinoma, are one of the most commonly occurring tumors in female rabbits and can significantly impact the lifespan of rabbits (12). MGT in rabbits occur around the age of 5.5 years on average (1). They are more commonly found in female rabbits and are often associated with uterine hyperplasia or adenocarcinoma, suggesting hormonal factors are involved (1,16).

Clinical signs of MGT in rabbits are diverse and non-specific, including inappetence, weight loss, lethargy, and depression (16). Depending on the local or systemic effects, rabbits may also exhibit symptoms such as hematuria, gastric bloat or gut stasis, and dyspnea (16). When diagnosing the tumor, a comprehensive assessment of the overall condition can be made through blood examination and urinalysis (16). Radiographs and CT scans can be used to determine the size, extent, and invasiveness of the tumor (16).

A definitive diagnosis is confirmed through histopathological examination (1). Histopathological analysis revealed that the majority of tumors (88%) exhibited malignant characteristics, including infiltrative growth, incomplete differentiation, and/or cellular dysplasia (1). The malignancy pattern observed in rabbit mammary tumors significantly differs from that reported in dogs, bearing closer resemblance to the situation observed in cats (1). Similar to cats, rabbits predominantly exhibit malignant tumors, while complex types are relatively rare (1).

In rabbits without metastasis, mastectomy is considered the primary treatment, additionally, chemotherapy is a viable treatment option for non-resectable tumors or rabbits with confirmed metastasis (16). Rabbits can be treated with a range of chemotherapeutic agents, administered either systemically or intralesionally. This flexibility allows for treatment strategies based on individual circumstances (16).

As of now, there are no published specific protocols regarding the clinical use or efficacy of chemotherapeutic agents in rabbits (16). Experimental studies have reported favorable outcomes when using platinum and pirarubicin to treat induced uterine, bladder, or mammary carcinomas in rabbits demonstrating efficacy in managing such tumors in rabbits (16).

This study presents a case of a rabbit diagnosed with mammary gland adenocarcinoma, which was treated with cyclophosphamide and lomustine.

Case Report

An 8-year-old, 3 kg intact female pet rabbit presented the veterinary clinic with the chief complaint of a mass on the mammary glands. On physical examination, the rabbit was alert and responsive and all vital signs were normal. The mass was firm and had a size of 47 × 32 mm with irregular and asymmetrical shape (Fig. 1). It was palpated below the left fourth mammary gland. The mass was not adherent to adjacent tissues, and no peripheral lymphadenomegaly was detected.

Figure 1. The firm and irregular and asymmetrical mass at the left fourth mammary gland with size of 47 × 32 mm.

The results of the complete blood count (pocH-100iV Diff, Sysmex Korea, Seoul, South Korea) showed a decrease in white blood cell count to 3.7 × 109/L (reference interval; 5-12 × 109/L), an increase in platelet count to 768 × 109/L (reference interval; 290-650 × 109/L), and an elevated lymphocyte percentage of 92.1% (reference interval; 25-60%). The serum biochemistry (Catalyst Dx, IDEXX Korea, Seoul, South Korea) revealed hyperglycemia with a glucose level of 231 mg/dL (reference interval; 75-145 mg/dL) and low alanine transaminase (ALT) activity with a value of 27 U/L (reference interval; 31-53 U/L).

On thoracic radiographs (HF-525 Plus VET, ECORAY, Seoul, South Korea), multiple nodules were observed with increased parenchymal density in the left lower and adjacent lung lobes, suggesting the possibility of pulmonary metastasis (Fig. 2). Abdominal ultrasonography revealed a 3 × 2 cm mass in the right uterine region, along with diffuse endometrial thickening, indicative of uterine adenomyosis (Fig. 3).

Figure 2. (A) Right lateral view and (B) ventral- dorsal view of thoracic radiographs showing the presence of multiple nodules and increased parenchymal density.

Figure 3. 3 × 2 cm mass in the right uterine region, along with diffuse endometrial thickening, indicative of uterine adenomyosis.

Fine Needle Aspiration was performed at the mass for diagnosis, but it yielded poor cellularity with the presence of erythrocytes, cellular debris, and fragmented nuclei, making interpretation difficult and unhelpful.

It was decided to proceed with surgery for histopathological examination and treatment, resulting in a total mastectomy and ovariohysterectomy (OHE) being performed. The rabbit was sedated with Ketamine HCl (KESIA inj. South Korea, 10 mg/kg body weight [BW], IM) and midazolam (Bukwang Midazolam Inj. South Korea, 3 mg/kg BW, IM). An intravenous catheter was placed in the left cephalic vein, and hydration was maintained using lactated Ringer’s solution at a rate of 10 mL/kg BW per hour.

The pet rabbit was induced with isoflurane (Ifran Liq; Hana Pharm, South Korea), 3% in oxygen using a V-gel R2 (Docsinnovent; Advanced V-gel for rabbits). Maintenance anesthesia was carried with isoflurane concentrations ranging from 1% to 3%. The rabbit was positioned in dorsal recumbency, and a standard aseptic protocol was followed to prepare the surgical site. A curved 3-cm incision was made around the mass, ensuring its complete removal. The subcutaneous tissues were meticulously sutured using 4-0 Polydioxanone (PDS II; polydioxanone; Ethicon, Somerville, NJ, USA) in a simple continuous pattern, while the skin was sutured using a simple continuous pattern with 4-0 nylon (BLUE NYLON; Polyamide: AILEE. Co., Busan, South Korea). The excised mass was carefully preserved in 10% neutral buffered formalin and sent for histopathological examination. The rabbit experienced a smooth recovery from anesthesia.

After surgery, meloxicam (Metacam, Boehringer Ingelheim Korea, Seoul, South Korea) at a dose of 0.5 mg/kg and marbofloxacin (Marbocyl, Vetoquinol, Gyeonggi-do, South Korea) at a dose of 5 mg/kg were administered subcutaneously. The next day, upon discharge, meloxicam and marbofloxacin were prescribed orally at the same dose.

The histopathological examination of the mammary tissue revealed the presence of multiple nodules composed of proliferating tumor cells exhibiting invasive behavior into the surrounding dermis and subcutaneous tissue (Fig. 4). The tumor cells formed cohesive solid sheets, ductal structures, or tubules, with some luminal spaces containing homogeneous eosinophilic secretions. The tumor cells displayed distinct cuboidal morphology with well-defined cell boundaries and exhibited active proliferation while forming acinar structures. The cytoplasm of the tumor cells showed moderate eosinophilia, and the nuclei appeared euchromatic with finely stippled chromatin. Prominent nucleoli were centrally located within the nuclei. The nuclear-cytoplasmic ratio was mild, and occasional mitotic figures were observed, with an average of 0 to 1 per 400 × field. Significant dilation of normal mammary ducts in the surrounding tissue was evident, while no evidence of tumor emboli or similar findings was identified. Also, the histopathological analysis of the provided tissue revealed the presence of multiple tumor nodules in the dermis.

Figure 4. (A) Neoplastic lobules separated by fibrous connective tissue having central area of necrosis (arrows). Hematoxylin and eosin. (B) Neoplastic cells are cuboidal to columnar epithelial cells forming tubules (arrows). Some tubular lumens contain secretory materials and cellular debris (arrowhead). (C) Neoplastic cells are forming solid nests (arrows) with central are of necrosis (arrowhead). (D) Neoplastic cells are arranged in solid nests (arrows) and tubules (blank arrowhead). Some of the tubules are severely dilated with secretory material and necrotic debris (arrowhead).

Based on the morphological features of the tumor cells and the distinct demarcation of the mass, a diagnosis of malignant neoplasm originating from glandular tissue, including the mammary gland, was established and the patient was diagnosed with simple-type, mammary gland adenocarcinoma.

After 7 days of surgery, the surgical site healed well and the pet rabbit showed good vitality with good appetite, so it was decided to proceed with chemotherapy. Due to the small size of pet rabbit’s blood vessels, oral administration was chosen as the method for administering the chemotherapeutic drugs, instead of intravenous infusion. Cyclophosphamide (Alkyloxan, JW Pharmaceutical Co., Gyeonggi-do, South Korea) was administered orally at a dose of 50 mg/m2 every Monday, Wednesday, and Friday (MWF) and lomustine (CCNU) (GmbH, Wedel, Germany) was administered orally at a dosage of 50 mg/m2 every 3 weeks. In addition, meloxicam was prescribed at a dosage of 0.5 mg/kg once a day.

Side effects of the chemotherapy such as anorexia, fever, and lethargy were monitored through regular physical examinations, complete blood counts (CBC), and blood chemistry tests conducted after three months (Table 1). However, no significant side effects were observed. Thoracic radiography was monitored every three weeks, but no changes were observed in the size or number of metastatic tumors (Fig. 5).

Table 1 . Complete blood count and serum chemistry of the patient.

IndexResultReference rangeUnit
Day 0Before chemotherapyDay 90
WBC3.74.755.355-12109/L
HGB12.310.513.38-17.6g/dL
RBC5.645.166.614-81012/L
HCT3432.442.630-50%
PLT768540523290-650109/L
WBC-monocyte2.910.39.92-10%
WBC-lymphocyte92.131.838.525-60%
WBC-basophil04.66.42-8%
WBC-eosinophil50.80.60-5%
ALT27-4631-53U/L
ALKP90-70470-145U/L
BUN10-1910-24mg/dL
Creatinine1.1-0.60.8-1.8mg/dL
Glucose231--75-145mg/dL
Protein-total5.8-8.55.5-7.2g/dL
Albumin3.3-5.82.7-4.6g/dL


Figure 5. Thoracic radiographic monitoring of the pulmonary mass after chemotherapy. (A) 3-week, (B) 6-week, (C) 9-week, (D) 12-week.

The treatment was continued for 10 months, and there was no new occurrence or relapse of the tumor.

Discussion

In this case study, we presented a successful management approach for mammary gland adenocarcinoma in a pet rabbit using metronomic chemotherapy with cyclophosphamide and lomustine (1,6,15,16). To the best of author’s knowledge, this is the first report documenting the effects of metronomic chemotherapy in a rabbit with mammary gland adenocarcinoma (1,12,16).

In this case, the 8-year-old female pet rabbit presented with a palpable mass on the mammary glands, along with a concurrent uterine adenomyosis and metastatic lung tumors. Considering the presence of pulmonary nodules suspected to be metastatic lesions, systemic chemotherapy was initiated using a metronomic approach with cyclophosphamide and lomustine (5,16,17). Conventional cytotoxic drugs are typically intended to be used at the maximum tolerated dose (MTD). Metronomic chemotherapy, characterized by the frequent administration of low-dose chemotherapy agents, aims to inhibit tumor angiogenesis and exert anti-tumor effects while minimizing adverse effects on normal tissues (2,3,8,11). Also, metronomic therapy specifically targets the endothelial cells within the developing blood vessels of the tumor, while conventional therapy focuses on inhibiting the growth of proliferating tumor cells (9). However, these approaches have been successful in various human and veterinary oncology cases, while its application in rabbits has been limited (1,7,11,16). We chose metronomic chemotherapy to minimize side effects due to the limited researches of chemotherapy in rabbits.

The rabbit had an OHE in addition to mastectomy following the standard (14). MGT in rabbits are frequently observed in female individuals and are often associated with uterine hyperplasia or adenocarcinoma, suggesting hormonal influences (1,12). Mammary adenocarcinomas are frequently observed in female rabbits that are between 3 to 4 years old and have had multiparous (14). Without treatment, prognosis with mammary adenocarcinoma is generally poor in female rabbits as metastasis to organs (14). The emphasis is placed on prevention rather than treatment of mammary tumors in dogs and cats (4). This is achieved by performing early OHE, reducing the risk of tumor development in the lifespan of dogs and cats (6). So, in this rabbit, we performed OHE similarly to dogs and cats to minimize tumor recurrence and avoid hormonal influences.

The first treatment for the most of mammary tumors in dogs and cats involves mammary gland excision surgery and spaying/neutering (6). Postoperative chemotherapy is not commonly administered as a routine treatment in dogs and cats (6). However, in cases of advanced mammary cancer such as distant metastases or inoperable tumors, chemotherapy is typically recommended (6). Dogs and cats are more amenable to intravenous access, and particularly in the case of dogs, they experience less environmental stress and mobility stress compared to other animals (6). This makes frequent visits to the veterinary clinic and the use of intravenous chemotherapy more manageable for them (6). Limited information is available regarding the chemotherapy options for rabbits with metastatic mammary adenocarcinomas (12). However, due to nodules suspected to be metastasis of MGT, we applied chemotherapy in the rabbit like in dogs and cats and no additional metastasis or enlargement of pulmonary nodule were observed with any side effects.

In this case, oral administration of cyclophosphamide and lomustine was chosen due to the small body and blood vessel size, making intravenous infusion challenging. Also, to minimize hospital visits and reduce the time spent at the veterinary clinic, oral chemotherapy agents were chosen for rabbits, considering their susceptibility to environmental stress (10). Alkylating agents such as cyclophosphamide, chlorambucil, melphalan, and lomustine have been associated with various side effects in rabbits. These primarily include bone marrow suppression and neutropenia, as well as gastrointestinal toxicity and the risk of hemorrhagic cystitis (16). Furthermore, hepatic and renal toxicity have been specifically reported with the use of lomustine (16). However, no side effects were noticed in this case.

This case highlights several important points. Firstly, in a rabbit with a mammary tumor that potentially metastasized to the lungs, the decision was made to opt for oral chemotherapy due to the lower risk and side effects associated with intravenous chemotherapy in rabbits (6,12,14,16). Additionally, the application of metronomic oral chemotherapy was particularly noteworthy as it minimized side effects and allowed for the management of the rabbit’s condition at home, reducing the stress of hospital visits and intravenous chemotherapy (10,16). This approach significantly improved the pet rabbit’s quality of life over an extended period. According to Shiga et al. (13), the median lifespan of Oryctolagus cuniculus is 7 years. The pet rabbit in this case, after chemotherapy, she survived for 10 months without showing any side effects. When considering the average lifespan and the patient’s age, 10 months is not considered a short period. Therefore, both surgery and chemotherapy are considered to have contributed to extending life. Further studies are needed to develop a chemotherapy protocol in rabbits with mammary gland adenocarcinoma.

Conclusions

To authors’ knowledge, this case describes the first reported case of palliative metronomic chemotherapy in a rabbit with mammary gland adenocarcinoma. Chemotherapy for mammary gland tumors (MGT) is well documented in canines and felines, but its documentation is limited in rabbits. No complications or side effects were observed during the 10 months of follow-up period. Metronomic chemotherapy could be an efficient treatment strategy for rabbits with MGT.

Conflicts of Interest

The authors have no conflicting interests.

Fig 1.

Figure 1.The firm and irregular and asymmetrical mass at the left fourth mammary gland with size of 47 × 32 mm.
Journal of Veterinary Clinics 2023; 40: 445-451https://doi.org/10.17555/jvc.2023.40.6.445

Fig 2.

Figure 2.(A) Right lateral view and (B) ventral- dorsal view of thoracic radiographs showing the presence of multiple nodules and increased parenchymal density.
Journal of Veterinary Clinics 2023; 40: 445-451https://doi.org/10.17555/jvc.2023.40.6.445

Fig 3.

Figure 3.3 × 2 cm mass in the right uterine region, along with diffuse endometrial thickening, indicative of uterine adenomyosis.
Journal of Veterinary Clinics 2023; 40: 445-451https://doi.org/10.17555/jvc.2023.40.6.445

Fig 4.

Figure 4.(A) Neoplastic lobules separated by fibrous connective tissue having central area of necrosis (arrows). Hematoxylin and eosin. (B) Neoplastic cells are cuboidal to columnar epithelial cells forming tubules (arrows). Some tubular lumens contain secretory materials and cellular debris (arrowhead). (C) Neoplastic cells are forming solid nests (arrows) with central are of necrosis (arrowhead). (D) Neoplastic cells are arranged in solid nests (arrows) and tubules (blank arrowhead). Some of the tubules are severely dilated with secretory material and necrotic debris (arrowhead).
Journal of Veterinary Clinics 2023; 40: 445-451https://doi.org/10.17555/jvc.2023.40.6.445

Fig 5.

Figure 5.Thoracic radiographic monitoring of the pulmonary mass after chemotherapy. (A) 3-week, (B) 6-week, (C) 9-week, (D) 12-week.
Journal of Veterinary Clinics 2023; 40: 445-451https://doi.org/10.17555/jvc.2023.40.6.445

Table 1 Complete blood count and serum chemistry of the patient

IndexResultReference rangeUnit
Day 0Before chemotherapyDay 90
WBC3.74.755.355-12109/L
HGB12.310.513.38-17.6g/dL
RBC5.645.166.614-81012/L
HCT3432.442.630-50%
PLT768540523290-650109/L
WBC-monocyte2.910.39.92-10%
WBC-lymphocyte92.131.838.525-60%
WBC-basophil04.66.42-8%
WBC-eosinophil50.80.60-5%
ALT27-4631-53U/L
ALKP90-70470-145U/L
BUN10-1910-24mg/dL
Creatinine1.1-0.60.8-1.8mg/dL
Glucose231--75-145mg/dL
Protein-total5.8-8.55.5-7.2g/dL
Albumin3.3-5.82.7-4.6g/dL

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Vol.41 No.1 February 2024

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The Korean Society of Veterinary Clinics

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