Initial diagnostic imaging of dogs with suspected anal sac adenocarcinoma should include abdominal radiography, abdominal
ultrasonography, and three-view thoracic radiography. Thoracic radiography is still considered the first screening test for
pulmonary metastatic disease, and fluoroscopic evaluation may help verify that nodules are within lung parenchyma. Computed
tomography has higher sensitivity in the detection of pulmonary nodules, however, and should be considered in patients with
questionable lesions on survey radiographs.
Moderate to severe sublumbar lymph node enlargement will be detectable as a sublumbar mass on abdominal radiographs. Mild
lymph node enlargement and metastases to the liver or spleen may go undetected on survey radiographs, however, so abdominal
ultrasonography should be performed for complete staging. Because ultrasonography does not permit differentiation between
subcategories of malignancy and between reactive lymphadenopathy and metastatic neoplasia, ultrasound-guided tissue sampling
should be performed to obtain a cytologic or histopathologic diagnosis.6
2. Cytologic examination of a fine-needle aspirate of an anal sac apocrine gland adenocarcinoma reveals dense papillary cell
clusters with abundant, scattered free nuclei. Intact cells are round to polygonal and are occasionally observed in an acinar-like
arrangement (inset) (Wright’s stain; background, 200X; inset, 1,000X).
Cytologic samples of anal sac adenocarcinoma are typically highly cellular and consist of dense, often papillary, cell clusters.7 Cells are fragile, and samples often contain many free nuclei (i.e. "naked nuclei") from disrupted cells. Intact cells are round to polygonal with a small amount of lightly basophilic cytoplasm;
they are often observed in short rows and, occasionally, acinar-like arrangements (Figure 2). Cells often appear quite uniform, although some tumors may have more striking features of malignancy. Metastatic lesions
(most commonly iliac lymph nodes) appear similar cytologically and are often identified before the primary lesion.7 The typical cytomorphology of this tumor is different from that of perianal gland (hepatoid) adenoma; however, distinguishing
between the two tumor types reliably with cytology requires some experience.
Anatomic pathology perspective Amanda J. Crews, DVM
Amanda J. Crews, DVM
Grossly, anal sac adenocarcinoma is usually unilateral, ventrolateral to the anus, in close association with the anal sac,
and not attached to the overlying epidermis. Ulceration is rare. Growth is frequently inward toward the pelvic canal. Mass
removal should include the entire anal sac. All cut surfaces of the mass and associated tissue should be painted with surgical
marking ink before fixation if margin evaluation is desired. On cut section, the mass is typically tan and multilobulated.
Neoplastic cells are often surrounded by a fibroblastic response (desmoplasia), which makes the tumor firm on palpation.
3A & 3B. The histologic appearance of anal sac adenocarcinoma. Figure 3A shows a rosette pattern. Figure 3B shows solid, rosette,
and tubular patterns in the same tumor (hematoxylin-eosin stain; 40X).
Histologically, anal sac adenocarcinoma can be readily differentiated from other perianal tumors, such as anal sac adenoma,
perianal (hepatoid) gland adenoma, perianal gland epithelioma, perianal gland adenocarcinoma, and sebaceous tumors. Anal sac
adenocarcinoma cells form in three distinct patterns: solid, rosette, and tubular. Frequently, a combination of all three
patterns is present in an individual tumor (Figures 3A & 3B). Similar tumor cell morphology and patterns have been reported in tumors from other parts of the body, so location must
be provided to the pathologist to ensure accurate interpretation and diagnosis. Metastatic tumor cells resemble those of the
primary tumor but cannot be reliably differentiated from other types of carcinoma cells, particularly endocrine and neuroendocrine,
without information about primary tumor location.
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