Secondary malignancies following low-dose-rate brachytherapy for prostate cancer in a single institution.

Abstract

142 Background: Low-dose-rate brachytherapy (BT) is the internal radiation therapy for non-metastatic prostate cancer by implanting radioactive isotope Iodine 125 into the prostate. The carcinogenetic effect of radiation for the treatment of cancer has been proven during the last decades. However, few studies have been reported about brachytherapy-induced secondary malignancies. In this study, we assessed the BT-induced secondary malignancies in our institution. Methods: We reviewed 487 patients treated with low-dose-rate BT from 2006 to 2016 in our hospital. We checked the occurrence of metachronous malignancies (MM) in all the patients and examined the features of MM including the number, organ sites, histology, period from BT to the occurrence, and analyzed whether the features meet the criteria of radiation-induced malignancies. We also examined the relationship between the BT-related radiation doses and the occurrence of MM. We adopted the criteria of radiation-induced cancer proposed by Sakai et al in 1981; different pathological feature from the organ of origin, the follow-up duration after radiation therapy (over 5 years), and whether the lesion is located in the irradiated field. Results: After a median follow-up period of 57 months (range 10-125 months), 23 MM occurred, which were bladder cancer in 4 cases (0.8%), esophageal in 2 (0.4%), gastric in 5 (1.0%), colon in 4 (0.8%), rectal in 3 (0.6%), pancreatic in 1 (0.2%), lung in 3 (0.6%), and brain in 1 (0.2%). Time from initial treatment to MM was 36 months (range 1-85 months). None of the cases had experienced biochemical recurrence. Average radiation dose to the urethra and the rectum including UD90, UD5, U200, R100, R150 showed no significant difference compared to the patients without occurrence of MM. According to the Sakai’s criteria, one bladder and one rectal cancer were judged as radiation-induced secondary malignancies (0.2%). Conclusions: The risk of secondary malignancies in patients undergoing BT was very low. Further studies with longer follow-up are required because it has been only about 10 years since low-dose-rate brachytherapy started in Japan.