Most experts in oncology are knowledgeable in such fields as cancer of the breast, lung, colon, tongue, melanoma, larynx, head and neck, esophagus, thyroid, stomach, colon, rectum, anus, skin , liver, kidneys, bladder, pancreas, ovary, testicle, hepatoma, and sarcoma, as well as soft tissue malignancies such as non-Hodgkin's and Hodgkin's lymphoma, the acute and chronic leukemia's, multiple myeloma, anemias, leukopenia, thrombocytopenia, and pancytopenia.

Typical issues that a General Surgeon, Surgical Oncologist and Bariatric Surgeon in private practice will deal with are:
Surgical treatment of obesity (open, laparoscopic, and endoscopic) Surgical Oncology
Laparoscopic Adjustable Gastric Banding LAP-BAND and REALIZE BAND certified
Diseases of the Breast and Gastrointestinal Tract
Intraoperative Lymphatic Mapping and Sentinel Lymph Node Dissection Liver Resection, 
Cryoablation and Radiofrequency Ablation (open and laparoscopic) for primary and metastatic 
liver tumors.

Radiation Oncology is an important field which expert witnesses deal with:

As with chemotherapy and surgery radiation treatment can be beneficial. However, it has the potential to be dangerous or at least hazardous when misapplied. When an expert witness receives a case where there is an allegation of radiation injury it is his or her job to quantify the level of radiation exposure and make a judgment whether that dose could be responsible for the injury. Modern computer-driven technology allows experts to quantify radiation dosage at any level or CAT scan slice in the treated area. A medical physicist is often requested to aid an expert witness in quantifying the radiation doses.

http://www.witnessdirectory.com/oncology