Abscopal Effect of Radiotherapy in Imatinib-resistant Dermatofibrosarcoma Protuberans.

Local tumor control and symptom relief have been the major advantage of radiotherapy in clinical practice. In the past years, the systemic anti-tumor effect of radiotherapy, also known as the abscopal effect, has been reported with limited studies. With the advent of immunotherapy, the frequency of the abscopal effect has increased in patients who receive sequential treatment with radiotherapy and immunotherapy or patients who receive radiotherapy after acquiring resistance to immunotherapy. A novel cancer treatment modality, such as molecular targeted therapy, has been associated with the immune response within the tumor but its systemic anti-tumor effect, when combined with radiotherapy, is yet to be documented. There have been few studies to date assessing the immunological effects of imatinib on tumors; however, the mechanism of tumor regression or resistance acquisition is poorly understood. We present a 56-year-old male diagnosed with dermatofibrosarcoma protuberans (DFSP) who developed resistance to imatinib after five months of treatment. Following subsequent local radiotherapy to the primary tumor, he had complete clinical remission of the primary and metastatic lesions.