CASE REPORT: Suspected Radiation-Induced Osteosarcoma in a Cat

Tumours induced by radiation have occasionally been reported in humans and dogs, but no such cases have previously been described in cats so this may be the first known case.

It started with the referral of a three-year-old castrated male domestic shorthair cat to the Ontario Veterinary College Health Sciences Center for the evaluation of progressive tail paresis over a period of one week. Despite an unremarkable physical examination, the cat’s neurological assessment showed some impairments and a lesion was located between the lumbosacral junction and third sacral special cord segment.

A mass within the spinal canal was then identified on MRI. Fine needle aspirates of the spinal cord lesion were taken with cytology confirming the presence of a spindle cell tumour. Palliative radiation therapy was initiated and a tapering dose of prednisolone was administered orally.  By the third week, tail was fully functional.

Three years after this therapy, the cat return to the hospital and a firm intrapelvic mass was identified during the physical examination. Further exams were performed and CT images were consistent with the mass not being a recurrence or metastasis of the previous tumour. A left total hemipelvectomy was elected aiming to cytoreduce the growth and relieve secondary obstipation. No relevant events were reported post-surgically and the patient was defecating and urinating normally after two days. Histopathology findings were consistent with osteosarcoma (OSA). The tumour is suspected to be a consequence of irradiation as it complies with four of the five established criteria of radiation-induced OSA in humans:

• Development from an area within the irradiated field;
• Growth in bone not known to have a lesion at the time of radiotherapy;
• Histological diagnosis of OSA which appeared distinct from the initial mass;
• Low frequency of tumour occurrence at this site.

Confirmation of this hypothesis would require histological assessment but neither biopsy (high risk of iatrogenic neurological damage) nor necropsy (euthanasia was performed 127 days after the surgery) were done.

Nevertheless, this study outlines the importance of considering the possible onset of these types of tumours in cats undergoing radiation therapy and demonstrates hemipelvectomy as an effective and well-tolerated method for palliative management of tumours when complete resection is not possible. Additional data is required to better understand feline OSA cases.

Swieton, N. et al. Suspected Radiation-Induced Osteosarcoma in a Domestic Shorthair Cat. Journal of Veterinary Diagnostic Investigation 2019, Vol. 31(1) 103–106. https://doi.org/10.1177/104063871881212