Tumor seeding, the local spread of viable tumor cells during a medical procedure, has been reported following surgery, biopsy, and image-guided fine-needle aspirations (FNA) or radiofrequency ablation of tumors. In animals, needle-tract seeding is most common for transitional cell carcinomas. Six months after placement of a cardiac pacemaker in a 7-year-old cairn terrier, thoracic radiographs revealed a mass in the left caudal lung lobe. Ultrasound-guided FNA revealed pulmonary carcinoma. The lung lobe was resected and no sign of metastases was found; pulmonary adenocarcinoma was confirmed via histopathology. Twelve months later, the dog was of decent health but the owner reported intermittent episodes of holding up the left forelimb and reluctance to use stairs. A mass was found on the left dorsal aspect of the thorax. Examination revealed mild left-sided superficial cervical and axillary lymphadenopathy. Computed tomography–guided FNA of the mass was performed and cytology was consistent with pulmonary adenocarcinoma. The dog received palliative analgesic therapy, as surgical resection was not an option. The site of the mass corresponded exactly with the path of the needle during the original FNA. Twelve months later, the mass had enlarged and the dog has continued to receive analgesics.

Commentary
A definitive diagnosis is essential for designing a cancer treatment protocol and is, with rare exception, required before chemotherapy or radiation therapy. However, for surgery patients, the timing can sometimes be flexible. A preoperative biopsy is indicated when the diagnosis will help the surgeon plan definitive surgery. Ideally, preoperative biopsy should be performed so the biopsy tract can be surgically removed during the procedure. If this is not possible, the risk–benefit associated with preoperative biopsy should be carefully considered. If the result might not directly impact definitive surgery, it is reasonable to forgo the preoperative biopsy. All tissue removed during definitive surgery should be submitted for histopathology.—Dennis Bailey, DVM, DACVIM (Oncology)

Source
Pulmonary adenocarcinoma seeding along a fine needle aspiration tract in a dog. Warren-Smith CMR, Roe K, de la Puerta B, et al. VET REC 169:181, 2011.