When CAA Isn't CAA: A Second Opinion Case of Oral Papillary Squamous Cell Carcinoma
Case Spotlight · Canine · Oral Pathology, Oncology
Acanthomatous ameloblastoma and oral papillary squamous cell carcinoma share enough histologic features that the distinction between them can be genuinely challenging — particularly on a punch biopsy where architectural context is limited. This case illustrates exactly that diagnostic challenge: a second opinion submission where the original diagnosis was acanthomatous ameloblastoma, and the correct diagnosis was canine oral papillary squamous cell carcinoma (COPSCC). The distinction matters enormously. Not because the surgical approach changes — wide excision is appropriate for both — but because the prognosis, the staging workup, and the conversation with the owner are fundamentally different.
Signalment and presentation
The patient was a middle-aged spayed female Golden Retriever presenting to a veterinary dental specialist following referral from the primary care veterinarian. The original examination had identified an abnormality at the maxillary left second premolar (tooth 204), initially noted as a suspected draining tract from a fistula. After further evaluation, radiographs were taken and a punch biopsy was submitted to a reference laboratory, which returned a diagnosis of acanthomatous ameloblastoma.
The dental specialist, planning a maxillectomy, requested a second opinion before proceeding with surgery. The submitted specimen was the same punch biopsy tissue, reviewed independently. The clinical differential submitted with the case was SCC versus acanthomatous ameloblastoma, reflecting the dental team's own uncertainty about the original diagnosis.
The initial presentation detail worth noting
The referring veterinarian's initial impression of a draining tract or fistula is a clinically interesting detail. COPSCC is a locally invasive tumor with common bone involvement, and the exophytic papillary surface can ulcerate, become secondarily infected, and produce discharge that mimics a periapical abscess or oronasal fistula in gross appearance. The presenting sign that led to the diagnostic workup — a suspected draining tract at 204 — is consistent with the locally invasive and ulcerative character of this tumor. It is a reminder that oral masses presenting with draining tracts or apparent fistulae warrant biopsy rather than empirical treatment for dental disease.
Histopathologic findings
Sections of the oral mass over tooth 204 demonstrated an exophytic, nodular, densely cellular, poorly demarcated neoplasm. The neoplastic cells were arranged in papillary projections and trabeculae within a fine fibrovascular stroma. The cells were polygonal with a moderate amount of eosinophilic cytoplasm and distinct cell borders, with round nuclei and finely stippled chromatin. A moderate inflammatory infiltrate of neutrophils with fewer mixed inflammatory cells was admixed with the neoplastic cells. The mitotic count was 11 per 2.37 mm².
The diagnosis was canine oral papillary squamous cell carcinoma. While some features in the sections were shared with ameloblastoma — polygonal cells, distinct cell borders, a trabecular component — the defining architectural feature was the exophytic papillary growth pattern projecting from the mucosal surface. This is the characteristic and discriminating feature of COPSCC and is not a feature of acanthomatous ameloblastoma.
Why these two tumors can be confused
Acanthomatous ameloblastoma and COPSCC share a number of histologic features that make them genuinely difficult to distinguish, particularly on small punch biopsies. Both are epithelial tumors with polygonal cells, distinct intercellular junctions, and a tendency toward peripheral palisading. Both invade adjacent bone. Both arise in the gingival and alveolar mucosa of dogs. In a small biopsy where the full architectural context of the lesion is not represented, the two can look strikingly similar.
The distinction lies in growth pattern and cell arrangement. Acanthomatous ameloblastoma grows in trabeculae and islands with a consistent peripheral palisading pattern — the basal cells at the periphery of each island are columnar, polarized away from the basement membrane, and separated from the central cells by a looser stellate reticulum-like zone. COPSCC grows in exophytic papillary projections — finger-like fronds of neoplastic epithelium supported by fibrovascular cores, extending outward from the mucosal surface. The papillary architecture is the key diagnostic feature and is not a feature of CAA.
A punch biopsy that samples the base of a COPSCC rather than the papillary tips may show predominantly trabecular architecture without well-developed papillary fronds, which is where the confusion arises. This case is a reminder that when the architectural pattern is ambiguous on a small biopsy, the gross description of the lesion — exophytic versus endophytic, papillary versus nodular — and clinical photographs are valuable supplementary data.
The second opinion process: why it mattered here
This case went through a formal second opinion process, and the value of that process is worth acknowledging explicitly. The original diagnosis — acanthomatous ameloblastoma from a reference laboratory — is not an unreasonable interpretation of a challenging oral biopsy. The dental specialist who reviewed the case had sufficient uncertainty about the original diagnosis that they requested independent review before committing to a surgical plan. That decision to seek a second opinion before surgery is exactly the right clinical instinct when the diagnosis and the surgical approach are inseparable.
Because the second opinion disagreed with the original diagnosis, a further consultation with a second pathologist was obtained, who agreed with COPSCC. The agreement of two independent pathologists on a diagnosis that disagrees with the original laboratory result is the appropriate standard of evidence for a diagnosis change of this clinical significance. It is also a concrete example of why second opinions on oral pathology cases — particularly those involving a surgical plan — are worth requesting.
COPSCC: what the diagnosis means for prognosis and management
Canine oral papillary squamous cell carcinoma is a rare but histologically and biologically distinct subtype of canine oral SCC. It is not conventional oral SCC with a papillary growth pattern — it is a recognized entity with a substantially more favorable prognosis than oral SCC of other types. Published case series document long-term disease control following wide surgical excision, with survival times that are meaningfully better than those reported for conventional oral SCC at equivalent stages.
The defining gross and histologic feature is the exophytic papillary growth pattern. Despite this relatively favorable behavior, COPSCC is locally invasive and commonly involves adjacent bone at presentation. Achieving complete excision can be challenging given the locally infiltrative character of the tumor, and the margin requirement is the same as for other locally aggressive oral tumors — wide excision with the goal of clear histologic margins.
Regional lymph node metastasis is uncommon but recognized in COPSCC. Published case reports include dogs with confirmed lymph node involvement, and the most recent literature suggests that metastatic behavior may occur more frequently than early case series indicated. Lymph node evaluation — at minimum fine needle aspiration of the regional nodes — and thoracic staging with radiographs or CT are recommended at the time of diagnosis.
For this patient, the diagnosis of COPSCC rather than acanthomatous ameloblastoma changes the prognosis conversation significantly. CAA does not metastasize and carries an excellent prognosis with adequate surgery. COPSCC has a more favorable prognosis than conventional oral SCC but is a carcinoma with metastatic potential, and the owner's understanding of long-term disease monitoring, lymph node evaluation, and the possibility of recurrence is meaningfully different from what a CAA diagnosis would require.
Teaching points for oral mass submissions
This case reinforces several principles for oral mass biopsy submissions. Clinical photographs and gross description of the lesion — particularly whether it is exophytic or endophytic, papillary or nodular — are diagnostically relevant and should accompany every oral biopsy submission. A gross description of exophytic papillary projections from the mucosal surface would have raised COPSCC on the differential list from the outset and may have influenced the initial interpretation.
Radiographic and CT findings should be included. Both CAA and COPSCC involve bone, but the pattern and extent of bone involvement may differ, and imaging context helps frame the histologic interpretation.
When the histologic findings on a punch biopsy are ambiguous between two diagnoses with significantly different prognoses, a second opinion before surgery is appropriate standard of care. The cost of a second opinion is substantially lower than the cost of a surgical revision if the initial diagnosis was incorrect. In this case, the second opinion changed not only the diagnosis but the entire prognostic and staging discussion that would precede surgery.
Case diagnosed by Eric Snook, DVM, PhD, DACVP — Vetopathy. All patient and clinician details anonymized. Second opinion consultations are available for oral pathology cases and other diagnostically challenging submissions.