Most radiosensitive tumor is

Ref Robbins 8/e p989 In general, radiation therapy is mainly used for patients with seminoma, which is very sensitive to radiation. Sometimes it's used after orchiectomy (the operation to remove the testicle) and is directed at the lymph nodes at the back of the abdomen (the retroperitoneal lymph nodes). This is to kill any tiny bits of cancer in those lymph nodes that can't be seen. It can also be used to treat small amounts of seminoma that have spread to the nodes (based on changes seen on CT and PET scans).