Transitional cell carcinoma (TCC) of the urinary tract is the most common neoplasia in dogs, accounting for 1-2% of all canine cancers (4,8,10,14). At the time of diagnosis, 82- 98% of TCC have infiltrated the muscle layer and 14-20% have lymph node or distant metastasis (6,8,9,14). Most TCCs are located in the trigone region of the bladder or urethra, making aggressive surgical resection impossible or difficult (8,16). Many dogs with ureteral or urethral obstruction are euthanized (5). Because the limited feasibility of surgical resection, chemotherapy with or without radiation therapy is prescribed for TCC in dogs (1,4,18).

Piroxicam is a commonly used nonsteroidal anti-inflammatory drug for improving clinical signs of TCC in dogs. It has been reported that it can lead to an objective tumor response rate of 18%, median progression-free interval (PFI) of 4.3 months, and median survival time (MST) of 6 months (11). A combination of piroxicam and mitoxantrone chemotherapy is commonly used for managing TCC, with a tumor response rate of 35%, a PFI of 194 days, and an overall MST of 291 days in dogs (6). In one report, a combination of coarse fraction RT, piroxicam, and mitoxantrone was evaluated in 10 dogs with TCC (18). Nine of 10 dogs had improved clinical signs of urinary tract, five dogs had stable disease (SD), two dogs had partial response (PR), and two dogs had progressive disease (PD) at the time of their last RT with an overall MST of 326 days (18). Compared to results from a study of dogs treated with piroxicam and mitoxantrone without RT (6), this multimodal treatment with coarse fraction RT study did not result in enhanced duration of tumor control or response rate (18). This result might be due to the use of RT with parallel-opposed, two lateral fields and manual planning. This case report describes the tumor response and survival time of a canine TCC treated with piroxicam, mitoxantrone, and once-weekly hypofractionated RT using graphic planning and volumetric modulated arc therapy (VMAT).

A 12-year-old spayed female beagle dog weighing 20 kg was presented for pollakiuria and stranguria. Physical examination was unremarkable. A complete blood count (CBC) showed unremarkable finding. Serum biochemical analyses revealed mildly elevated serum levels of alkaline phosphatase (505 U/L, reference range: 23-212 U/L) and alanine aminotransferase (190 U/L, reference range: 10-100 U/L).

Survey thoracic and abdominal radiography revealed unremarkable findings. Abdominal ultrasonography identified an irregularly marginated and hyperechoic mass in the urethra and trigon area of the bladder (Fig. 1A, B). The apex and body of the urinary bladder wall were normal. A sample was collected by ultrasound-guided traumatic catheterization. Urine sediment dry-mount cytology was examined. Clusters of atypical transitional cells were detected, showing karyomegaly, anisocytosis, anisokaryosis, and binucleation. Neoplastic transitional cells also have presented malignant features including increased nuclear to cytoplasmic ratio and multiple distinct nucleoli (Fig. 2). Cytologic analysis suspected TCC. Bacteria were not observed as there was no growth on urine culture. A total-body computed tomography (CT) (GE lightspeed 16-mulislice helical CT; GE healthcare, little Chalfont, UK) was performed to evaluate distant metastasis and determine the exact extent and invasion of the tumor. The patient was positioned in ventral recumbency with a moldable positioning cushion (Vac-Lok, Civco Medical, Orange City, IA, USA) having scanning parameters of 120 kV, 200 mA, and 1.25 mm slice thickness. The patient received 600 mgI/kg iohexol (Omnipaque 300^®, GE Healthcare, Ireland) at a dose 2 mL/Kg for contrast enhancement. Following placement of a foley catheter, the urinary bladder was emptied and 4 mL/Kg of 0.9% sterile saline was infused into the bladder. There was a heterogeneous mass in the trigone area leading to a urethra with evidence of invasion to the bladder wall (Fig. 3A, B). Gross tumor volume was 16 cm³. Other abdominal organs and regional lymph nodes were normal. Based on cytology and imaging, the tumor was diagnosed tentatively as TCC of the urinary bladder and urethra at clinical stage T₂N₀M₀ (17).

Figure 1. Ultrasonographic images of the trigon area of bladder. (A, B) Abdominal ultrasonography revealing an irregular, marginated, and hyperechoic mass (arrow) in the urethra and trigon area of the bladder. (C) Ultrasound examination after fraction number 3 of RT and before receiving mitoxantrone revealing that the margin of the tumor (arrow) became smooth and the diameter was reduced. (D) Ultrasonography at two months after the end of RT and 3 cycles of mitoxantrone confirming that the size of the tumor (arrow) located in the trigon of the bladder was reduced.

Figure 2. Microscopic appearance of the cytology collected from traumatic catheterization. Clusters of atypical transitional cells were detected, showing karyomegaly (arrow), anisocytosis, anisokaryosis, and binucleation (arrowhead). Neoplastic transitional cells also have presented malignanct features including increased nuclear to cytoplasmic ratio and multiple distinct nucleoli.

Figure 3. Contrast-enhanced computed tomography (CT) images of the coronary and sagittal plane before multimodal treatment (A, B) and at 6 months after the end of RT (C, D). (A, B) Contrast-enhanced CT images before multimodal treatment revealing a heterogeneous mass (arrow) in the trigone area leading to the urethra. (C, D) Contrast-enhanced CT images at 6 months after the end of RT showing a reduction in tumor size without tumor in the bladder or the urethra or evidence of regional or distant metastasis.

Treatment was planned to proceed with piroxicam, mitoxantrone, and adjuvant RT (once weekly fractionated RT). Pre- and post-contrast images were imported into an external beam planning system (Eclipse^TM; Varian Oncology Systems, Palo Alto, CA, USA). VMAT treatment was planned with this planning system. Gross tumor volume (GTV), clinical tumor volume (CTV), and planning target volume (PTV) were delineated. GTV included the visible tumor in contrast-enhanced CT images. CTV included a 5 mm isotropic expansion from GTV. PTV included a 5 mm isotropic expansion from CTV. The dose was planned to the 95% isodose line with the GTV receiving at 100% of the dose and the PTV receiving 95% of the dose (Fig. 4). Organs at risk included skin, small intestine, spinal cord, descending colon, and rectum. Radiation was performed with megavoltage radiation delivered with a linear accelerator (Vital Beam; Varian Medical Systems) using a 6 MV photon. The patient received 6 fractions of 6 Gy for a total dose of 36 Gy over a 6-week period (7,20). The urinary bladder was kept constant through catheterization, urine removal, and 4 mL/Kg saline infusion for each RT. Piroxicam was administered orally at a dose of 0.3 mg/kg, once daily until 6 months after the end of RT. Three weeks after the start of RT, the patient received mitoxantrone by intravenous administration at a dosage of 5.5 mg/m² every 3 weeks for five treatments. CBC was performed at one week after mitoxantrone administration and repeated before the next mitoxantrone treatment to monitor hematologic toxicity.

Figure 4. Transversal plane (A) and sagittal plane (B) of dose distribution and dose-volume histogram (C) of the patient.

After fraction number 3 of RT and before receiving mitoxantrone, ultrasound examination revealed that the margin of the tumor became smooth and the diameter was reduced to 10 mm (Fig. 1C). Two months after the end of RT and 3 cycles of mitoxantrone, clinical signs (stranguria and pollakiuria) improved remarkably. Ultrasonography confirmed that the size of the tumor located in the trigon of the bladder was reduced to 4 mm (Fig. 1D). No adverse effects of RT were observed (12). The chemotherapy was well-tolerated without occurrence of any side effect.

A follow-up CT was repeated at six months after the end of RT, showing a reduction of the tumor size. No tumor was identified (Fig. 3C, D). There was no evidence of regional or distant metastasis. At 13 months after the end of multimodal treatment and 17 months after the start of RT, the patient became severely anorectic and lethargic. A CBC revealed anemia (16.2%, reference range: 37-55%) and mild elevation of white blood cells (19.6 × 10⁹/L, reference range: 6-17 × 10⁹/L). Serum biochemical analyses revealed elevated levels of serum alkaline phosphatase (480 U/L, reference range: 23-212 U/L), alanine aminotransferase (317 U/L, reference range: 10-100 U/L), blood urea nitrogen (50 mg/dL, reference range: 7-27 mg/dL), and creatinine (2.2 mg/dL, reference range: 0.5-1.8 mg/dL). Ultrasound examination revealed a hyperechoic mass in the apex area of the bladder. The trigone area of the bladder and urethra appeared normal. Multiple hypoechoic nodules of various sizes were found in the liver and spleen. Echogenic peritoneal effusion was revealed (Fig. 5). Liver and splenic metastasis were tentatively diagnosed based on ultrasound examination. A few days later, the patient was humanely euthanized at the request of the owner.

Figure 5. Ultrasonographic images of bladder, spleen, and liver at 17 months after the start of RT. (A) Ultrasound examination revealing a hyperechoic mass (arrow) in the apex area of the bladder. (B) Normal appearance of the trigone area of the bladder and urethra. (C, D) Multiple hypoechoic nodules of various sizes found in the spleen (arrow) and liver (arrowhead). (E) Echogenic peritoneal effusion was revealed.

RT is an important therapy in the management of cancer patients. It uses radiation to kill cancer cells while causing minimal damage to normal tissues and cells (22). Depending on the goal of treatment, RT can be divided into RT with palliative intent and RT with curative intent (22). Typical curative (definitive) RT dose for the bladder is 54-58 Gy delivered in 20 daily fractions on a Monday to Friday basis (15), whereas palliative RT can effectively alleviate pain and clinical signs with a smaller dose (typical 34.5 Gy delivered in six weekly fractions) (18). The use of definitive RT with intensity-modulated and image-guided RT (IMRT/IGRT) without or with adjunctive chemotherapy in one study has reported an MST of 614 days (15). Despite benefits of definitive RT to increase survival time, this RT protocol is limited when severe side effects are expected, patients have metastatic disease receiving short courses RT, or when frequent anesthesia is not possible for patients in veterinary medicine (3,15,22). Palliative RT protocols such as ten fractions of 2.7 Gy given once daily (Monday to Friday) for 12 days (1) and 6 fractions of 5.75-6.5 Gy given once weekly (18,19) have been reported. Our case was treated with a combined protocol of piroxicam, mitoxantrone, and once weekly palliative RT. Our patient was treated with a combined protocol of piroxicam, mitoxantrone, and once weekly palliative RT, taking into account the time and cost of owner.

A previous study has reported that MST and response rate of piroxicam and mitoxantrone with hypofractionated RT are not significantly different from those of piroxicam and mitoxantrone without RT for dogs with TCC (6,18). Treatment protocol with hypofractionated RT might be due to the radiation dose did not irradiate the tumor using parallel-opposed, two lateral fields, and manual planning. However, this treatment protocol was well-tolerated, even when a large dose per fraction was administered. The aim of our study was to descript tumor response and survival time of a dog treated with once weekly palliative RT through computer-assisted planning and VMAT. Manual planning is carried out through the manual manipulation of standard isodose charts on to patient body contour that was generated by lead wire representation or direct tracing (2). Manual forward based treatment planning, which is based on an error and trial approach by experienced dosimetrist, is giving way to inverse computer-assisted planning, which makes use of dose optimization techniques to satisfy the user specified criteria for the dose to the target and critical structures (2). While Parallel-opposed pair is poor conformality, VMAT (RapidArc) improves conformality and reduces the time and monitor units used to deliver the radiation (20). VMAT is a novel radiation technique, which can achieve highly conformal dose distributions with improved sparing of normal tissues and target volume coverage (20). Although our patient was irradiated with a large dose per fraction, no side effects were observed. Survival time was defined as the time from the start of treatment to death. The survival time of our patient was 17 months. This result is expected because the VMAT method has a more accurate dose than the previous method (21). However, since this study was conducted with only one patient. Thus, it is unlikely to be comparable. Further studies with a large number of patients are needed.

The most difficult part of RT in patients with bladder tumor is to make the bladder position and size the same. The reason why it is difficult to have the same bladder position and size is because the position of the bladder can dramatically change depending on the degree of stool in the descending colon and the amount of urine in the bladder (13). Accordingly, we tried to keep the bladder constant through catheterization and urine removal with 4 mL/Kg saline infusion for each RT. Before each RT fraction, IGRT with 3D cone beam CT was used to ensure positioning accuracy of bladder.

A follow-up CT was repeated at 6 months after the end of RT, showing a reduction in tumor size in the bladder or the urethra. Accordingly, the patient was classified as having a complete response. However, the patient was euthanatized at 17 months after the start of RT due to metastasis.

As RT progressed, the tumor margin became smoother and the size decreased on ultrasound examination. On ultrasonography at 17 months after treatment, the tumor was not identified in the urethra or the trigon of the bladder. However, it was confirmed that the tumor was newly grown in the apex of the bladder. For urinary bladder tumor, it remains questionable what volume should be defined as CTV (partial urethra/bladder or whole urethra/bladder RT) (13). In general, side effects after RT in patients with bladder tumor include bladder fibrosis, cystitis, urinary incontinence, colitis, fecal incontinence, colonic stricture, and perforation (1). In our case, since the dose per fraction was large, the possibility of side effects was higher as partial bladder and urethra were treated with RT. In our case, the possibility of side effects was high because the dose per fraction was large. Therefore, PTV included a 10 mm isotropic extension of the GTV and PTV was defined as the partial bladder and urethra. For this reason, the recurrence on the apex of bladder might be because it was not irradiated.

This case described tumor response and survival time of a dog treated with once weekly palliative RT using computer-assisted planning and VMAT. This treatment protocol was well tolerated and tumor response was identified. Thus, weekly palliative RT can be considered when a definitive RT protocol cannot be applied for a dog with TCC.

This work was supported by the Animal Medical Institute of Gyeongsang National University and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2020R1G1A1007886).

The authors have no conflicting interests.