jack,
Often with orthopedic tumors, the type can be determined from patient and tumor characteristics (age, gender, location, what bone, etc), the x-ray appearance, and with ancillary studies (CT scan, MRI, lab tests, etc). Chest x-ray, chest CT, and bone scans are usually done to determine if the tumor has metastasized. For the determination of the exact cell type, a biopsy has to be done.
So, ask the oncologist what tumor does he/she thinks it is. Is it malignant? Has it metastasized? What are the treatment options? What are the risks and benefits of each treatment option? What would happen if treatment was not selected? How soon does treatment have to start?
Of course, ask any questions you might have. Be sure you understand what he/she is telling you. You might want to have someone with you. As, at times like this we often get overwhelmed.
Good luck.