A dermoid cyst, also known as a mature cystic teratoma, is an uncommon tumor-like developmental anomaly that can occur in various species including dogs (1,7,15,16,18). This cyst arises from entrapment of ectodermal elements along lines of embryonic closure. It is comprised of both ectodermal and mesodermal elements, including epithelium and adnexal structures such as hair follicles and sebaceous glands. Although they are benign, dermoid cysts can lead to clinical issues depending on their size and location, often causing discomfort or complications due to compression of adjacent tissues or secondary infection (2,19). In dogs, a dermoid cyst is relatively rare. It is often discovered incidentally during imaging or physical examinations conducted for other reasons (3,10,13).

Approximately 7% of dermoid cysts affect the head and neck in humans, with 23% of these located on the floor of the mouth (8,22). Similarly, in dogs, dermoid cysts are rare in the neck. Most reports in this area involve dermoid sinus rather than dermoid cysts (6,12,17). Dermoid cysts have not been reported in the ventrolateral deep region of the lower neck in dogs. They have only been reported in the dorsal midline subcutaneous region of the upper neck (21).

In humans, dermoid cysts appear mixed echogenicity with both hyperechoic and hypoechoic areas due to the presence of various tissue types, including hair and sebaceous material on ultrasound (9,11,14). On CT scans, they show well-circumscribed with smooth margins and non-contrast-enhanced material (9,14). However, in veterinary medicine, there are few reports on CT and ultrasonographic features of dermoid cysts (5).

The purpose of this report was to describe ultrasonographic and CT features of a dermoid cyst occurring in the ventrolateral deep region of the neck, a location not typical for these cysts. This report emphasizes the importance of diagnostic imaging in identifying and managing such incidental findings.

A 13-year-old castrated male mixed-breed dog was present for health screening with a small nodule on the left hindlimb. The dog was diagnosed with a heartworm infection six years ago. It has been successfully treated except for subclinical pulmonary thromboembolism. Radiographs reveal enlargement of the right side of the heart and pulmonary arteries caused by the previous heartworm infection. A small (0.6 × 0.5 cm) freely movable soft nodule was identified in the subcutaneous of left hindlimb on physical examination. The small soft nodule was confirmed as a mast cell tumor on fine needle aspiration. Complete blood count and serum biochemistry showed no specific findings.

To evaluate surgical margins and metastasis of the hindlimb mast cell tumor, pre- and post-contrast CT (Aquilion Lightning 160, Canon Medical Systems, Japan) examinations were performed under general anesthesia. The dog was premedicated with butorphanol (0.2 mg/kg, administered intravenously [IV], Butophan Injection^®; Myungmoon Pharm, Korea), midazolam (0.2 mg/kg, IV, Midazolam Inj^®; Bukwang, Korea), and glycopyrrolate (0.01 mg/kg, SC, Mobinul^®; Myungmoon Pharm, Korea). Anesthesia was induced with propofol (6 mg/kg, IV) and maintained with 2% isoflurane. Iodinated contrast medium (Omnipaque 300 mg/mL, GE Healthcare, Princeton, NJ, USA) was injected intravenously (3.0 mL/kg, IV). The CT scan incidentally revealed a well-marginated, oval-shaped, non-contrast-enhancing, homogeneous, fat-fluid attenuating mass (–5-23 HU) in the left lower neck region, cranio-dorsal to the manubrium (Fig. 1). The mass was located between the left sternohyoid muscle and the esophagus, adjacent to the bifurcation of the left brachiocephalic vein. The mass measured 1.9 × 1.9 × 2.8 cm (width × height × length). An extended lesion protruding into the sternohyoid muscles cranioventrally was identified. No metastatic lesions were identified.

Figure 1. Postcontrast CT images of the lower neck mass. A well-marginated, non-contrast-enhancing, homogeneous, and fluid-attenuating mass was identified at the lower neck region incidentally. (A, B) Two-dimensional (2D) transverse view. (B) The mass (asterisk) is located between the esophagus (open arrowhead) and sternohyoid muscle (close arrowhead) and medial to the left brachiocephalic vein (dashed arrow). The mass is compressing the sternohyoid muscle, but no significant mass effect was identified on other structures such as the esophagus and trachea. (C) Maximum intensity projection (MIP) image of the dorsal view. The mass does not involve other organs, such as the thyroid (close arrow). (D) 2D sagittal view. A focal protruding lesion extending into the sternohyoid muscles is identified as cranioventral of the mass (open arrow). (E) Three-dimensional (3D) volume-rendered image, sagittal view, showing an ovoid mass (yellow) craniodorsal to the manubrium.

Ultrasonography revealed that the mass was well-capsulated except for a cranio-ventrally protruding lesion extending into the sternohyoid muscles, as observed in the CT examination. The protruding lesion was composed of two parts measuring 4.4 × 3.8 mm and 2.3 × 1.7 mm (Fig. 2B, C). While the mass appeared well-capsulated, muscular involvement could not be ruled out due to this protruding lesion. The outer layer of the mass had a hypoechoic thick wall and the internal parenchyma contained hyperechoic foci and multiple hyperechoic parallel lines (Fig. 2D). The initial differential diagnosis for the lower neck mass included granuloma, dermoid cyst, and ectopic thyroid tumor.

Figure 2. Ultrasonography images of the lower neck mass. (A) The mass is well capsulated with internal hyperechoic substances and a thick, hypoechoic wall. (B) Linear hyperechoic lines (open arrow) are identified internally. (C, D) Cranioventrally protruding lesion (close arrow, dashed arrow) is extended into the sternohyoid muscle (close arrowhead).

After midline excision on the ventral cervical region at the level of vertebra C5, surgical removal of the unusual mass was performed. The excised mass (1.8 × 2.8 cm in diameter) was well-defined and encapsulated with a clear membrane containing blood vessels. Upon incision, cystic structures filled with flaky white material were observed (Fig. 3). Three months after surgery, additional abnormalities in the neck region were not found.

Figure 3. Surgical removal of the unusual mass. (A, B) Intraoperative images showing surgical exposure and dissection of the unusual mass on the ventral neck region at the level of vertebra C5. The mass is being carefully separated from surrounding tissues using surgical instruments. (C) Gross appearance of the excised mass measures approximately 2.8 cm in length. The cyst is encapsulated with a clear membrane, containing visible blood vessels. (D) Bisected view of the excised mass revealing its internal structure. The cyst is filled with flaky white material.

Histologically, within the subcutis and abutting portions of adjacent skeletal muscle, there was a well-demarcated, expansile, unilocular cyst lined by a stratified layer of cornifying squamous epithelial cells containing many deeply basophilic intracytoplasmic granules (keratohyalin granules). The lumen of the cyst contained abundant lamellae of keratinaceous debris and a few admixed hair shaft fragments. Contiguous with and radiating from the cyst wall, there were several small folliculosebaceous units. In the microscopic image of the cystic material, numerous hair shaft fragments were confirmed (Fig. 4). When the material was cultured under cell growth conditions, proliferated and expanded cell colonies consisting of neoplasm-like or fibroblast-like cells were presented. Based on these findings, the mass was identified as a dermoid cyst.

Figure 4. Histopathological and microscopic images of the unusual mass. (A) Histopathological image of the excised mass showing a well-demarcated, unilocular cyst lined by a stratified layer of cornifying squamous epithelial cells. The lumen of the cyst contains abundant lamellae of keratinaceous debris and a few admixed hair shaft fragments (H&E staining). (B) Histopathological image displaying the cyst wall, which includes sebaceous glands and hair follicles consistent with adnexal structures (H&E staining). (C, D) Microscopic images of the cystic content showing numerous hair shaft fragments and keratinous material, consistent with typical findings of dermoid cysts. Images illustrate characteristic features of the internal debris within the cyst (scale bar = 100 µm).

In this case, CT examination confirmed the presence of a well-marginated, non-contrast-enhancing abnormal mass in the lower neck region, leading to a differential diagnosis of a benign tumor such as a dermoid cyst, granuloma, or an ectopic tissue origin tumor. On ultrasonography, the mass was found to contain hyperechoic components (foci and lines), similar to that caused by hair fragments in a dermoid cyst (4). Subsequent histopathological examination confirmed the existence of cystic materials and hair fragments, leading to a final diagnosis of a dermoid cyst.

Dermoid cysts are teratomatous lesions that contain both ectodermal and mesodermal elements, often including hair follicles, sebaceous glands, and keratinous debris. The etiology of dermoid cysts is thought to be related to embryological development errors, where ectodermal tissue becomes trapped in subcutaneous or deeper tissues (20). They are most commonly found in the skin. They can also occur in other tissues, including the central nervous system, thorax, and abdomen. In subcutaneous tissues, they typically present as slow-growing, non-painful masses. However, when located in deeper tissues or critical areas such as the spinal cord or thoracic cavity, they may cause more severe clinical signs due to compression of vital structures.

The dermoid cyst is one of the categories in the dermoid sinus. Most cases of cervical dermoid sinus in dogs have been reported in Rhodesian Ridgeback. They are located in the dorsal midline of the upper neck subcutaneous region (vertebrae C1-C3) (5). However, in the present case, it occurred in a mixed-breed dog. It was presented in the ventrolateral deep region of the lower neck. In humans, similar to this case, cervical dermoid cysts have been reported in the ventrolateral region. In one case, a dermoid cyst has been found at a location similar to this case on CT examinations (deep to the sternocleidomastoid and strap muscles) (11).

The location, shape, and internal material of the mass are helpful clues for prioritizing the differential diagnosis. Diagnostic imaging examinations such as CT and ultrasound are crucial for diagnosis. Histopathology is necessary for confirmation. Dermoid cysts have heterogeneous density on CT scans because they contain a mix of fat, fluid, and sometimes calcified elements. These cysts are usually well-circumscribed with smooth, defined borders. Typically, dermoid cysts do not enhance significantly with contrast material (5,9,14). The presence of these features on CT helps differentiate dermoid cysts from other cystic masses. On ultrasound, these cysts typically show mixed echogenicity with both hyperechoic and hypoechoic areas due to the presence of various tissue types, including hair and sebaceous material. They often have well-defined margins. They may show posterior acoustic enhancement due to their cystic nature (9,11,14). In the present case, the mass was located in the lower neck region with no involvement of other structures. The margin of the mass was well-encapsulated except for focal protruding lesions. The internal material was suspected to be fluid or poorly vascularized tissue. Additionally, CT and ultrasonographic findings were consistent. Particularly on ultrasonography, hyperechoic structures, which could be caused by hair shafts, were identified.

This case report describes a dermoid cyst in the ventrolateral neck region of a dog and highlights the necessity for thorough diagnostic imaging in identifying and managing incidental findings that might have important clinical implications. Even if a cyst-like mass is located in a region other than the dorsal subcutaneous area, a dermoid cyst should be included in the differential diagnosis if the mass is well-encapsulated, non-contrast-enhancing, and containing internal hyperechoic substances suspected to be hair shafts.

This work was supported by the Institute of Animal Medicine of Gyeongsang National University.

The authors have no conflicting interests.