Hidden in plain sight: searching for occult micrometastases and isolated tumour cells in feline mammary carcinomas

Regional lymph node status is a key prognostic factor in neoplastic lesions in cats and humans alike. The presence and clinical relevance of micrometastases and isolated tumour cells (ITCs) have been studied extensively in human breast cancer. However, comparable data in feline mammary carcinomas (FMCs) remain scarce; a single recent study addressed the presence of occult metastases. In this study, a multilevel sectioning protocol was applied to lymph nodes from 11 node-negative FMCs at first evaluation to search for occult metastases. The tissue in each paraffin-wax block was completely serially sectioned at 50 μm intervals, with one slide prepared at each tissue section and additional sections collected at every 150 μm interval for immunohistochemistry (IHC). ITCs were detected in two of the 11 cases on haematoxylin and eosin-stained tissue sections and confirmed by IHC for cytokeratin. No additional metastases were identified following IHC. These findings suggest that the assessment of multiple lymph node tissue levels may improve detection of metastases. Larger studies are necessary to determine the prevalence and clinical relevance of occult metastases in FMCs and their potential implications on treatment decisions.