Unusual behavior of foreign body granuloma that grew rapidly in the radiation field during radiation therapy.

We report our experience with two cases of a rapidly growing benign tumor in the radiation field despite radiotherapy. Case 1 was a 75-year-old man who was diagnosed as having postoperative recurrence of esophageal carcinoma in the right chest wall and underwent radiotherapy. A small nodule in the recurrent lesion grew to 20 mm in diameter at 70 Gy. The biopsy specimen was diagnosed as foreign body granuloma (FBG) with no malignancy. Case 2 was a 78-year-old woman who was diagnosed with axillary lymph node metastases of operated lung cancer and underwent lymphadenectomy and postoperative radiotherapy. No subcutaneous tumor was seen at the beginning of radiotherapy. However, a small nodule appeared in the radiation field at 40 Gy, and it had grown to 30 mm in diameter at 70 Gy. The biopsy specimen was diagnosed as FBG, showing granulation because of a remnant of surgical suture. Both FBGs disappeared within 3-6 months after radiotherapy. In cases in which a tumor has arisen from a site where surgical treatment had been performed and the tumor shows unnatural growth despite radiotherapy, FBG should be considered in the differential diagnosis.