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Using cytology to diagnose canine osteosarcoma

How do the sensitivity and specificity of cytology compare to histopathology as a diagnostic test for canine osteosarcoma?

Canine osteosarcoma is the most common malignant bone tumour in dogs, accounting for up to 85 percent of primary skeletal malignant neoplasms (Sabattini et al., 2017). Clinicians often rely on signalment, history, clinical findings and radiographic abnormalities to make a presumptive diagnosis of a malignant bone tumour. However, more benign disorders such as osteomyelitis may share similar clinical and radiographic features with malignancies which complicates diagnosis.

Incisional biopsy and histopathological examination is often recommended prior to surgery and is considered the gold standard for the diagnosis of primary bone lesions; however, several technique-related complications such as pathological fractures, haematomas or even local seeding of neoplastic cells have been reported with bone biopsies. The procedure also requires the patient to be under general anaesthesia and is reportedly associated with enhanced patient pain (Mankin et al., 1996).

Fine needle aspiration (FNA) cytology is a cost-effective and less invasive alternative to preoperative histopathology, typically associated with minimal complications and a short sample turnaround time

Fine needle aspiration (FNA) cytology is a cost-effective and less invasive alternative to preoperative histopathology, typically associated with minimal complications and a short sample turnaround time. Therefore, you want to explore the sensitivity and specificity of cytology as a test for canine osteosarcoma when compared to histopathology.

The evidence

Four studies were critically appraised; one was a retrospective study in which the cytological and histopathological samples were retrieved and re-examined, another was a retrospective study in which the initial cytological and histopathological diagnoses were utilised, and the other two were prospective studies. In all of the studies, the sensitivity of cytology for the diagnosis of canine osteosarcoma was measured using histopathology as the definitive benchmark.

In all of the studies, the sensitivity of cytology for the diagnosis of canine osteosarcoma was measured using histopathology as the definitive benchmark

In the first study, 27 dogs with histologically confirmed osteosarcomas underwent preoperative cytological and histopathological examinations (Loukopoulos et al., 2005). After excluding non-diagnostic specimens from the analysis, the sensitivity of cytology for diagnosing the malignancy was 79.2 percent, and the exact histotype was 25.0 percent.

In a study by Britt et al. (2007), FNA cytological examination was performed, as well as histological examinations after preoperative incisional or surgical excisional biopsy, in 36 dogs with aggressive appendicular bone lesions. Osteosarcomas were diagnosed histologically in 30 specimens, and of those, cytology classified 29 as sarcomas.

The sensitivity of cytology for identifying osteosarcomas as sarcomas was 96.7 percent after excluding the non-diagnostic specimens from the analysis. The specificity was 100 percent, as the prevalence of osteosarcoma was very high in the study which resulted in a positive bias in the calculation of specificity.

In one prospective study, FNA cytological examination, core aspirate (CA) cytological examination and histopathological examination (after an incisional biopsy) were performed in 27 dogs with lytic and/or proliferative bone lesions (Neihaus et al., 2011). Of the 27 cases, 20 were diagnosed as osteosarcomas histologically. Of these 20, FNA cytology identified 17 as sarcomas, while CA cytology identified 19 as sarcomas. The sensitivity of FNA and CA cytology for identifying osteosarcomas as sarcomas was 85.0 percent and 95.0 percent, respectively. Specificity for both FNA and CA cytology was 100 percent, although this result was likely biased by the high prevalence of osteosarcoma in the study.

Specificity for both FNA and CA cytology was 100 percent, although this result was likely biased by the high prevalence of osteosarcoma in the study

In a study by Sabattini et al. (2017), FNA cytological examination and histopathological examinations after preoperative incisional biopsy and after surgical excisional biopsy or post-mortem examination were performed in 68 dogs with bone lesions. Of those, 40 dogs had osteosarcomas; in five histologically confirmed osteosarcomas, cytological examination was not performed. The sensitivity of FNA cytology for providing a diagnosis of sarcoma was 80.0 percent (28 out of 35), and the sensitivity for identifying the histotype was 42.9 percent (15 out of 35). The specificity of cytology was 88.9 percent (16 out of 18) for the histotype and 100 percent (18 out of 18) for providing a generic diagnosis of sarcoma. The sensitivity of preoperative histopathology was 45.5 percent (5 out of 11) for the histotype and 72.8 percent (8 out of 11) for malignancy.

Conclusion

Overall, there is evidence of a moderate strength that cytology appears to be reasonably sensitive and highly specific for identifying malignancy when diagnosing histologically confirmed osteosarcomas as sarcomas, although the high prevalence of osteosarcoma in the studies has likely induced a positive bias for specificity. The sensitivity of cytology identifying the exact histotype (a specific diagnosis of osteosarcoma) was lower: only 25 percent in the study by Loukopolous et al. (2005) and 42.9 percent in the study by Sabattini et al. (2017). Specificity was found to be quite high (88.9 percent); however, this was only reported by Sabattini et al. (2017), so the evidence for this is weak.

Cytology appears to be reasonably sensitive and highly specific for identifying malignancy when diagnosing histologically confirmed osteosarcomas as sarcomas

Surprisingly, the sensitivity of preoperative histopathology after incisional biopsy was lower than that of FNA cytology (Sabattini et al., 2017). This finding could be of clinical relevance as cytology is minimally invasive when compared to preoperative biopsy; however, further studies are needed. Future studies that focus on the diagnostic performance of cytology compared to preoperative histopathology would be useful to provide evidence on the use of these techniques for the diagnosis of canine osteosarcomas prior to surgery.

The full Knowledge Summary can be read in RCVS Knowledge’s open access journal Veterinary Evidence.