Repeated Abscopal Effect After Two Courses of Stereotactic Body Radiotherapy for Multiple Postoperative Metastases of Malignant Melanoma of the Buccal Mucosa.

We report a case of repeated abscopal effect (AE) following two courses of stereotactic body radiotherapy (SBRT) for multiple postoperative metastases of malignant melanoma in the buccal mucosa during treatment with nivolumab. A 90-year-old female presented with malignant melanoma of the right buccal mucosa. She underwent partial resection of the right buccal mucosa and right cervical dissection at our hospital in January 2022. In August 2022, positron emission tomography (PET)-CT revealed recurrence in the left cervical lymph node and metastatic lung tumors at S5 of the left lung and S9 of the right lung. SBRT (40 Gy in five fractions) was performed for the recurrent left cervical lymph node. In December 2022, PET-CT showed regression of the irradiated lymph nodes as well as an unirradiated left lung S5 lesion, which was considered the first AE. In addition to the existing S9 lesion in the right lung, a new metastatic lung tumor was identified in S4 of the right lung, and SBRT of 42 Gy in four fractions was performed on the right S9 lesion in April 2023. In July 2023, PET-CT showed regression of both the irradiated right S9 lesion and the unirradiated right S4 lesion, representing the second AE. Both the irradiated and unirradiated lesions associated with AE remained reduced on PET-CT in October 2023. Repeated AEs after two SBRT courses during nivolumab therapy are extremely rare. We report this case together with a literature review.