Anal sac mast cell tumor in a dog

Abstract

An 11-year-old castrated mixed-breed dog was diagnosed with a unilateral anal sac mass. Anal sacculectomy was performed without complication. Histopathology of the mass was consistent with a well-differentiated mast cell tumor. Analyses for the Ki-67 protein, KIT expression pattern, and the presence of c-kit mutations were done. Recovery from surgery was unremarkable and repeat staging revealed no evidence of mast cell disease at 4, 8, and 12 mo after surgery. To the authors’ knowledge, primary mast cell neoplasia of the anal sac has not previously been reported in the veterinary literature.

Key clinical message:

Although mast cell tumors of the perineal region are commonly encountered in veterinary practice, the anal sac represents a novel location for this disease. Cytology of anal sac masses can be a useful diagnostic tool to confirm the diagnosis, guide staging, and assist in treatment decisions.

Apocrine gland anal sac adenocarcinoma (AGASACA) is the most common neoplasm of the canine anal sac. These tumors can show aggressive biologic activity and readily metastasize to locoregional lymph nodes. Additionally, distant metastasis and paraneoplastic hypercalcemia often occur with AGASACA, which can affect both the prognosis and the approach to surgical and medical management (1). Therefore, complete staging before surgical intervention is recommended to assess for the presence of metastasis, hypercalcemia, and comorbidities. Cutaneous mast cell tumors (cMCTs) are the most frequently diagnosed cutaneous neoplasms in the dog, with most of these tumors located on the trunk, perineum, limbs, head, and neck (2). Surgery remains the mainstay of therapy for solitary cMCTs when feasible and clinically appropriate. Tumor location may affect prognosis and clinical case management (2–6). To date, mast cell tumors are not considered a differential diagnosis for a palpable anal sac mass. We describe the presentation, management, and prognosis of a dog with mast cell neoplasia of the anal sac.

Case description

An 11-year-old castrated male mixed-breed dog weighing 34 kg was seen for routine annual examination. The dog had a history of 2 cutaneous mast cell tumors (cMCTs) surgically excised ~12 mo before. These were from the left lateral thorax and left inguinal regions. Results of histopathologic evaluations of both masses were consistent with low-grade Kiupel/grade 2 Patnaik cMCTs with complete excision. The mass from the left lateral thorax was reported to have a mitotic count (MC) of 0, and the inguinal mass was reported to have an MC of 1 in representative fields of 2.37 mm². Physical examination was unremarkable aside from a well-demarcated, 2- to 3-millimeter left-sided anal sac mass. Routine complete blood (cell) count (CBC), serum chemistry profile, total T4, and urinalysis results were unremarkable. The dog was sedated with butorphanol, 0.15 mg/kg body weight (BW), IV and dexmedetomidine hydrochloride 7.3 μg/kg BW, IV. Fine-needle aspiration of the left anal sac mass was conducted and cytopathologic evaluation reported small numbers of well-differentiated, fully granulated mast cells, small numbers of eosinophils, and rare fibroblasts. Three-view thoracic radiographs showed no evidence of pulmonary or cardiac abnormalities. A complete abdominal ultrasound revealed no abnormalities, including a detailed evaluation of the liver, spleen, and iliosacral lymph nodes. Ultrasound-guided fine-needle aspiration of the spleen was completed. Cytopathologic examination revealed normal splenic tissue with no evidence of metastatic mast cell neoplasia.

The dog was anesthetized with acepromazine maleate, 0.015 mg/kg BW, IM and hydromorphone, 0.1 mg/kg BW, IM, as premedication; and midazolam, 0.25 mg/kg BW, IV with ketamine hydrochloride, 5 mg/kg BW, IV, as induction. After induction and endotracheal intubation, anesthesia was maintained with sevoflurane (2 to 4%) and oxygen. A purse-string suture was placed to close the anal orifice and the perineum was surgically prepared in routine manner. Perioperative antimicrobial therapy with cefazolin, 22 mg/kg BW, IV was administered. A standard closed anal sacculectomy was done without complication. The dog recovered uneventfully. Carprofen, 2.14 mg/kg BW, PO, q12h and codeine, 0.9 mg/kg BW, PO, q8h were prescribed for postoperative pain management.

Histopathologic examination confirmed the cytologic diagnosis of a mast cell tumor. The excised tissue revealed small aggregates of neoplastic mast cells spanning nearly 2 to 3 mm in diameter in the region of the submucosa (Figure 1 A). Neoplastic cells were round, with discrete cell borders. The cells displayed moderate amounts of well-granulated cytoplasm. Nuclei were ovoid, with stippled chromatin and punctate nucleoli. Anisocytosis and anisokaryosis were mild. No mitotic figures were identified in ~1.5 high-power fields, 400×. The narrowest surgical margin was composed of 290 μm of collagen. Immunohistochemistry was done for CD117 (c-kit) and Ki-67. The neoplastic cells demonstrated a strong membranous immunohistochemical pattern of expression of CD117 (KIT pattern 1) (Figure 1 B) and were diffusely negative for immunoreactivity to Ki-67 (Figure 1 C). The results of polymerase chain reaction (PCR) amplification to detect internal tandem duplication (ITD) mutations of exon 8 and exon 11 in the c-kit gene were negative.

Figure 1.

Histopathology. A — Mast cell tumor in an 11-year-old male dog. Hematoxylin and eosin staining. Anisocytosis and anisokaryosis were mild. The asterisk indicates the lumen of the anal sac. “M” indicates the mucosa of the anal sac. The tumor is below, located in the submucosa, immediately subjacent to the epithelium. B — Immunohistochemistry for CD117 (KIT pattern I), with membranous staining. C — Immunohistochemistry for Ki-67; Ki-67 immunolabeling was diffusely negative. Scale bar = 50 μm in A–C.

Physical examinations and repeat abdominal ultrasounds were done 4 mo and 8 mo after surgery. The dog was doing well with no owner concerns at each of these visits. Results of complete physical examinations, including digital rectal examinations, were unremarkable. No palpable abnormalities affecting the previous surgery site or contralateral anal sac were detected. At the 8-month postoperative recheck, routine CBC revealed a mild monocytosis (1442 cells/μL; reference range: 0 to 840 cells/μL). The serum chemistry profile revealed a mildly elevated alanine transaminase (ALT, 174 IU/L; reference range: 12 to 118 IU/L). The total T4 and urinalysis results were unremarkable.

Approximately 12 mo following the initial consultation for the left anal sac mass, repeat physical examination including digital rectal examination was unremarkable, aside from 2 new dermal masses. Fine needle aspirates confirmed additional cMCTs in the left axilla and left lateral thorax. The mass on the left lateral thorax was ~20 cm distant from the previous mast cell tumor removed from this general location 2 years before. Thoracic radiographs obtained before anesthesia revealed no abnormalities. Repeat abdominal ultrasound was once again unremarkable, with no evidence of iliosacral lymphadenopathy. Ultrasound-guided fine-needle aspiration of the liver and spleen was completed, revealing no evidence of visceral metastatic disease. Routine CBC before surgery was unremarkable. The serum chemistry profile revealed a mildly elevated ALT (135 IU/L; reference range: 12 to 118 IU/L). Both masses were surgically removed in a routine fashion without complication. Histopathology for both masses was consistent with completely excised, low-grade Kiupel/grade 2 Patnaik cMCTs. No mitotic figures were observed in either mass in representative fields of 2.37 mm².

Discussion

Neoplasms of the perineum, including cMCTs, are common in dogs. However, this report describes a mast cell tumor arising from the anal sac, which, to the authors’ knowledge, has not been previously described in the veterinary literature; its prognosis is thus unknown. The anal sacs are paired invaginations of the skin located between the internal and external sphincters of the anus, lined by keratinizing stratified squamous epithelium (7). Sebaceous glands and apocrine glands are located subjacent to the connective tissue supporting the epithelium of the anal sac (8). The sebaceous glands are limited to the duct of the anal sac, whereas apocrine glands are concentrated in the fundus (8). Tumors arising from the anal sacs demonstrate both a distinct biological behavior and differ histologically when compared to the more common perianal gland tumors (9). Primary neoplasia of the canine anal sac is relatively uncommon, accounting for 17% of perianal tumors and 2% of all cutaneous tumors (2). Apocrine gland anal sac adenocarcinoma (AGASACA) is the most common neoplasm of the canine anal sac. Other canine primary anal sac tumors, which are infrequently encountered, include squamous cell carcinoma, malignant melanoma, and a single reported case each of a benign papillary cystadenoma and a collision tumor consisting of an AGASACA and a soft tissue sarcoma (10–13). Diagnosis of anal sac neoplasia involves a thorough rectal examination in conjunction with cytologic assessment or histopathologic confirmation. Apocrine gland anal sac adenocarcinoma is considered an aggressive malignancy that commonly metastasizes to regional lymph nodes, liver, spleen, and bone (1,9,14). Removal of the anal sac tumor and metastatic lymph nodes, if present, is the standard of therapy and should be pursued if feasible (14). Additional therapy, if indicated, may include radiation, electrochemotherapy, or chemotherapy, although the role of the latter in extending survival times is currently unclear (1,9,14).

Cutaneous mast cell tumors have been well-described and extensively documented in the veterinary literature. Mast cell tumors are the most reported cutaneous neoplasm in the dog, representing 16 to 21% of all cutaneous tumors (2). Most cMCTs are located on the trunk and perineal region (50%), limbs (40%), and head and neck regions (10%) (2). Less commonly, mast cell neoplasia can occur at subcutaneous, mucosal, extracutaneous/extramucosal sites, and rarely as systemic mast cell leukemia (15). The biologic activity of cMCTs is strongly correlated with histopathological grading (2,16–19). Surgery remains the cornerstone of therapy for amenable tumors. Additional therapies may include radiation therapy, systemic chemotherapy or tyrosine kinase inhibitor therapy, electrochemotherapy, palliative therapy, and recently, tigilanol tiglate injection (Stelfonta; QBiotics Group, Taringa, Queensland, Australia) (1,2,19–24). Differences in the biologic behavior of canine cMCTs based on anatomic location have been documented. Of particular interest are dogs with cMCTs of the inguinal or perineal regions, including scrotal and preputial locations. Debate exists as to whether these animals have a worse prognosis. Multiple studies have failed to document a survival difference based on location (16,25). Two papers specifically looking at prognosis in dogs with inguinal and perineal cMCTs showed that, with aggressive therapy, many of these animals benefited with prolonged survival times (3,4). However, dogs with cMCTs of the scrotum and preputial regions had a shorter disease-free interval when compared to those with cMCTs at locations other than the perineum and inguinal region, and were more likely to receive systemic therapy (4). In addition, cMCTs located on the muzzle, head/neck, and paw may also carry a worse prognosis (5,6).

Histologic grading of canine cMCTs is most predictive of outcome and prognosis, and is instrumental in case management (6,16,18,26,27). Histologic grading applies only to cMCTs, and not to subcutaneous, mucosal, or extracutaneous/ extramucosal mast cell tumors. Currently, the Kiupel and Patnaik grading systems are most widely used for the classification of cMCTs. The Patnaik grading system assigns a grade of 1, 2, or 3 based on well-, intermediate-, or poorly-differentiated tumors, respectively (28,29). Tumor grade is significantly related to survival time, with 93% of grade 1, 44% of grade 2, and 6% of grade 3 patients surviving to 1500 d (28). The Kiupel grading system is a 2-tiered system that divides cMCTs into either high-grade or low-grade tumors based on several criteria, including the number of mitotic figures, multinucleation of cells, presence of bizarre nuclei, and karyomegaly (30). Using this grading system, the median survival of dogs with high-grade tumors was 3.65 mo, compared to 23 mo for those assigned as low-grade (30). Currently, the Oncology-Pathology Working Group (OPWG), which is a joint initiative of the Veterinary Cancer Society and the American College of Veterinary Pathologists, recommends that both the Patnaik and Kiupel grading systems be included with canine cMCT histopathology reports (29).

Mitotic index (MI) is a quantitative evaluation, of number of cells undergoing mitosis divided by the number of cells not in mitosis, that provides an indirect assessment of cellular proliferation. The MI has been shown to significantly correlate with metastatic rate and median survival time (MST), but not with tumor recurrence (31). Romansik et al (31) demonstrated that the MST for dogs with non-metastatic cMCT with an MI ≤ 5 was 80 mo, compared to 3 mo for those patients with non-metastatic cMCT with an MI > 5. A validation study investigating these cut-points showed an MST of 8 mo for dogs with an MI > 5, whereas the MST was not reached for dogs with an MI ≤ 5 (32). The mitotic count (MC), the number of mitotic figures within a given area, is the term preferred over MI by OPWG, to better standardize the number of mitotic figures reported (29).

A subset of low-grade cMCTs with an MC ≤ 5 may still behave in an aggressive clinical fashion (29). Given this variability in the biologic activity, numerous prognostic markers have been investigated to further assess the potential for local regrowth, the development of distant metastasis, and mast cell tumor-associated morbidity and mortality. It is important to remember that no single marker should be used as a prognostic factor; rather, markers should be used in concert with the histopathologic grade, MC, clinical presentation, and clinician experience. Immunostaining for Ki-67, a marker of cellular proliferation, has been evaluated in numerous studies to better determine the biologic behavior of mast cell tumors. Increased Ki-67 counts have been associated with tumor regrowth, the development of metastasis, and overall increased tumor-related mortality (16,26). The KIT protein (CD117) pattern localization evaluated by immunohistochemistry has shown differing patterns correlating with tumor aggressiveness. The CD117 protein is a Type-III tyrosine kinase protein involved in mast cell growth and differentiation (18). Membrane-associated (pattern 1) KIT pattern staining of neoplastic cells is not associated with recurrence or a decrease in survival time. Conversely, staining patterns 2 (focal to stippled cytoplasmic staining with decreased membrane-associated staining) and 3 (diffuse cytoplasmic staining) are both significantly associated with a shorter overall survival time and a higher rate of local recurrence (33). Evaluation for the presence of internal tandem duplications (ITD) in exons 11 and 8 of the c-kit oncogene by PCR has been evaluated in multiple studies. Internal tandem duplication mutations of exon 11 in c-kit in cMCTs have been associated with a higher risk of mast cell metastasis, a shorter disease-free interval, increased mortality due to mast cell tumor-related death, and increased tumor recurrence when compared to tumors lacking ITD mutations in c-kit (16,17,27). The presence of ITD mutations of exon 8 in c-kit may indicate a less aggressive biologic activity in cMCTs (31). Like tumors without ITD mutations, cMCTs with mutations of exon 8 do not have a significant increase in proliferation markers (34). Conversely, tumors with ITD mutations of exon 11 are associated with an increased MC, aberrant protein localization of KIT, increased Ki-67 labeling index, and higher grade in both the Kiupel and Patnaik grading systems (16,27).

The reported prevalence of multiple cMCTs at the time of initial evaluation varies widely, ranging from 9 to 44% of patients (21,35). The presence of multiple cMCTs and their effect on prognosis has been evaluated in various studies with conflicting results. It has been stated that dogs with multiple synchronous cMCTs at the time of diagnosis that are treated with surgery alone have a worse overall prognosis (25). However, in several other studies, the MST of dogs with multiple cMCTs was not reached (19,21,35). Additionally, 18 to 44% of dogs with multiple cMCTs that received surgery alone or surgery with adjuvant therapy (chemotherapy and/or radiation therapy) developed additional cMCTs at sites distant from the original surgery site, with no effect on overall survival (19,35).

This report describes a mast cell tumor arising from the anal sac, which has not been previously described in the veterinary literature; its prognosis is thus unknown. Twelve months following the surgical removal of the mass via anal sacculectomy, no local recurrence or evidence of metastatic disease was evident. The anal sac mast cell tumor in this report had a membranous CD117 staining pattern, stained diffusely negative for Ki-67, had an MC of 0, and was negative for c-kit mutations, suggesting a more favorable biologic behavior. Had this mass been in a more conventional cutaneous location, it would have been graded as low-grade Kiupel/grade 2 Patnaik cMCT. The anal sac mast cell tumor in this case appeared to behave biologically like low-grade cMCTs arising from more traditional cutaneous locations. In dogs with a history of a cMCT and a palpable anal sac mass, cytological evaluation is important to confirm the diagnosis and guide staging and therapeutic options.