Dog’s Aggressive Tumor Exposed by PET Scan After Biopsy Missed the Mark

A striking case report from veterinary researchers in South Korea has documented, for the first time, how a powerful nuclear imaging technique can reveal the true aggressiveness of a rare canine cancer when conventional biopsy results tell only part of the story. The study, published in Frontiers in Veterinary Science, describes a nine-year-old Alaskan Malamute whose seemingly moderate-grade skin tumor turned out to be a highly malignant leiomyosarcoma, a finding foreshadowed by the intense metabolic signal captured on fluorodeoxyglucose positron emission tomography combined with computed tomography, known as 18F-FDG PET/CT.

The dog arrived at the veterinary teaching hospital with a hemorrhagic, ulcerative lesion on the back of its right elbow that the owner had first noticed three weeks earlier. Initial cytological examinations, including an impression smear and fine needle aspiration, revealed bacterial infection and atypical cells with concerning features such as marked variation in cell and nuclear size, but no definitive mesenchymal tumor cells could be identified. The atypical epithelioid-appearing cells were interpreted as likely reactive changes associated with the ulcerated wound rather than the underlying malignancy. Empirical antimicrobial and antifungal treatment was started, but the lesion failed to respond and instead rapidly enlarged into a distinct mass.

A biopsy was then performed, and histopathological examination revealed spindle-shaped cells with pleomorphic nuclei within the subcutaneous tissue, consistent with a soft tissue sarcoma. Using the standard three-tiered grading system for canine soft tissue sarcomas, which combines differentiation, mitotic activity, and necrosis into a summed score, the tumor was assigned a grade 2. In general terms, grade 2 soft tissue sarcomas carry a median survival of one to two years and a metastasis rate of up to twenty percent, so the clinical team expected a moderately aggressive but manageable disease. Yet the lesion was progressing faster than such a grade would typically suggest, prompting the clinicians to look deeper.

That deeper look came in the form of 18F-FDG PET/CT, a hybrid imaging modality that merges the anatomical detail of computed tomography with the functional insight of positron emission tomography. The technique relies on a radiolabeled glucose analog that accumulates in tissues with high metabolic activity, a hallmark of many malignant tumors. The dog received an intravenous injection of 239.02 megabecquerels of the tracer, equivalent to 6.29 megabecquerels per kilogram of body weight, followed by a sixty-minute uptake period before scanning. The resulting images showed intense tracer accumulation within the elbow mass, yielding a maximum standardized uptake value of 10.1 and a mean standardized uptake value of 4.6. Critically, no abnormal tracer-avid lesions were detected elsewhere in the body, including the regional lymph nodes, ruling out distant metastasis and confirming the elbow mass as the primary tumor.

Surgical resection was performed approximately two weeks after the PET/CT scan, and the excised tissue underwent definitive histopathological analysis. The results were sobering: the tumor was a poorly differentiated mesenchymal neoplasm with a mitotic count exceeding forty figures per ten high-power fields, producing a summed grade of 3, the most aggressive category. Moreover, the excision was incomplete, with neoplastic cells extending to the deep surgical margin. The preoperative biopsy had therefore underestimated the final tumor grade, while the high fluorodeoxyglucose avidity observed on PET/CT aligned far more closely with the tumor’s true biological behavior. Immunohistochemical staining for desmin and alpha smooth muscle actin, both markers of smooth muscle differentiation, confirmed the diagnosis of subcutaneous leiomyosarcoma, a rare malignancy arising from smooth muscle cells.

Leiomyosarcoma in dogs most commonly originates in visceral organs such as the gastrointestinal tract, liver, or uterus, where it carries a poor prognosis with a reported median survival of roughly ten months. Non-visceral forms, including cutaneous and subcutaneous variants, are far less common and remain poorly characterized. Some researchers argue that subcutaneous leiomyosarcoma should be classified independently from conventional soft tissue sarcomas because of its distinct biological behavior and anatomical location. In human medicine, subcutaneous leiomyosarcoma is considered more aggressive than its purely cutaneous counterpart, with higher local recurrence rates and a worse clinical course, though whether the same distinction holds in dogs remains uncertain due to the scarcity of published cases.

The PET/CT findings in this case are notable when compared against benchmarks from human oncology. In human soft tissue sarcomas, maximum standardized uptake values have been reported at averages of 1.3, 2.7, and 4.5 for grades 1, 2, and 3 respectively, with thresholds of 3.9 for the maximum value and 2.5 for the mean proposed to separate grade 2 and 3 tumors from grade 1. The dog’s maximum value of 10.1 exceeded even the reported human range for grade 3 tumors. The only prior veterinary PET/CT study of canine soft tissue sarcomas documented maximum values between 2.0 and 10.6, but tumor grades were not recorded, limiting direct comparison. A single earlier report of canine leiomyosarcoma, a grade 1 gastric tumor, recorded a maximum value of 6.234, substantially lower than the 10.1 observed in this grade 3 subcutaneous case, suggesting a possible relationship between tracer uptake and histologic grade in this tumor type as well.

The clinical outcome underscored the accuracy of the metabolic signal. Although limb amputation and adjuvant chemotherapy were recommended following the grade 3 diagnosis with incomplete margins, the owner declined both options and opted for palliative care with prednisolone and analgesics. One month after surgery, thoracic radiographs showed no metastatic lesions. But three months postoperatively, the dog developed a cough, and imaging revealed a pulmonary nodule alongside local tumor recurrence at the surgical site. Follow-up imaging three weeks later showed both the size and number of lung nodules increasing. The dog expired six months after surgery, a course consistent with the poor prognosis associated with high-grade tumors and incomplete resection, and one that vindicates the early warning provided by the PET/CT scan.

The case carries important implications for veterinary oncology practice. Because soft tissue sarcomas are biologically heterogeneous, preoperative biopsies can misgrade tumors, with studies showing that pretreatment biopsy does not always reflect the true grade due to intratumoral heterogeneity. Such underestimation can lead to suboptimal surgical planning, inadequate margins, and missed opportunities for aggressive intervention. In this case, the markedly elevated fluorodeoxyglucose uptake served as a functional red flag that the tumor was more dangerous than its biopsy grade implied, supporting the argument for more proactive management such as amputation, adjuvant chemotherapy, and intensified postoperative surveillance when PET/CT reveals high metabolic activity.

The researchers acknowledge limitations inherent to a single case report. Fluorodeoxyglucose is not tumor-specific, and the active purulent inflammation within the ulcerated lesion may have contributed to the elevated uptake, although the authors argue that inflammation alone is unlikely to explain the magnitude of the signal. Additional immunohistochemical markers for smooth muscle differentiation, such as h-caldesmon and calponin, were not evaluated, and Ki-67 proliferative staining was not performed. The suspected pulmonary metastases were never histologically confirmed, though their progressive growth on serial radiographs was highly suggestive. Even so, as the first documented application of 18F-FDG PET/CT to canine subcutaneous leiomyosarcoma, the report opens a window onto a diagnostic approach long established in human sarcoma care and now demonstrating its value in veterinary medicine, where metabolic imaging may soon complement the scalpel and the microscope in deciding how hard to fight a tumor that biopsy alone cannot fully characterize.

Subject of Research: Use of 18F-FDG PET/CT imaging to assess tumor aggressiveness in canine subcutaneous leiomyosarcoma

Article Title: Case Report: 18F-FDG PET/CT for precision assessment of tumor aggressiveness in canine subcutaneous leiomyosarcoma

Article References: Chae, Y., Lee, D., Koo, Y., Yun, T., Kim, H., & Kang, B.-T. (2026). Case Report: 18F-FDG PET/CT for precision assessment of tumor aggressiveness in canine subcutaneous leiomyosarcoma. Frontiers in Veterinary Science, 13, Article 1948200. https://doi.org/10.3389/fvets.2026.1948200

Image Credits: AI Generated

DOI: 10.3389/fvets.2026.1948200

Keywords: 18F-FDG PET/CT, leiomyosarcoma, soft tissue sarcoma, veterinary oncology, tumor grading, diagnostic imaging, canine cancer, standardized uptake value, immunohistochemistry, case report, metabolic imaging, surgical margins