201Tl accumulates well in the malignant tumors. Since high uptake ratio of tumor to blood can be obtained soon after injection, a small lesion can be detected by SPECT imaging. It has been observed that 201Tl imaging is useful to detect recurrent lesions or metastatic lesions from thyroid cancer, lung cancer, brain tumor, breast cancer, mediastinal tumor, bone and soft tissue tumor, pancreas cancer and colon cancer. The clinical application of 201Tl is expected to expand. Although 201Tl accumulates in benign lesions, the accumulation in malignant lesions retained more longer than in benign lesions. This unique characteristic is useful to separate benign from malignant lesions. 201Tl is expected useful for evaluating response of tumor to radiation therapy and/or chemotherapy as 201Tl accumulates in viable tumor cells.