The behavior and treatment of cancer can vary between tumor types, between tumor stages and between patients with the same tumor type and stage. In order to properly fight a tumor, specific information is necessary. The more is known about the tumor, the better it can be treated.
For example, your dog may have a bone tumor with an unknown origin. The mass can originate from bone (osteosarcoma), cartilage (chondrosarcoma) or even be a metastasis towards the bone (as can be the case for eg a prostate carcinoma).
The behavior of an osteosarcoma is far more aggressive than that of a chondrosarcoma and requests a different approach. For these reasons it is important to detect this difference. Tumors can be differentiated via the study of their tissue (histopathology) for which a biopsy can be taken, or cells (cytology) for which a needle can be inserted into the mass and the retrieved cells examined under a microscope.
Via medical imaging (radiography, CT/MRI scan, ultrasound, scopy, etc.) information can be gathered about where precisely the tumor is situated. This facilitates the treatment planning for surgery and/or other treatments, which reduces the risk of relapse.