Reevaluating BED in Cervical Cancer HDR Brachytherapy: Source Decay and Tissue-Specific Repair Significantly Impact Radiobiological Dose

Suyan Bi¹Jiaomei Zhou²Meiling Xu²Zhitao Dai^3*

• ¹Hospital Universiti Sains Malaysia, Health Campus, Universiti Sains Malaysia, Kota Bharu, Kelantan Darul Naim, Malaysia
• ²National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital & Shenzhen Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Shenzhen, China
• ³Department of Radiation Therapy, Cancer Hospital Shenzhen Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Shenzhen, China

Objective This study aims to explore the influence of intrafraction DNA damage repair on biologically effective dose (BED) in Ir-192 high-dose-rate (HDR) brachytherapy (BT) for cervical cancer. Specifically, we aim to comprehensively investigate the BED variations triggered by source decay at diverse treatment time points across different tumor cell lines and normal tissues.Methods Instead of the simplified BED (BED) formula, which fails to account for intrafraction and interfraction repair as well as repopulation, we utilized the full-form BED (BEDg) formula.Subsequently, BED values for various subtypes of cervical cancer tissues and Organs at Risk (OARs) were calculated by applying both BED formulas throughout a source exchange cycle.The findings reveal that the BEDg values are notably smaller and exhibit a significant decrease as the source activity decays and the treatment time extends, in contrast to the BED values. Regarding the tumor, when α/β = 10, the maximum changes in BED (∆BED = BED -BEDg) amounted to 3.05±0.47% in D 90% of High-Risk Clinical Tumor Volume (HRCTV) in BT. Upon considering the subtypes of cervical cancer tissues, the three largest ∆BED (%) are determined to be 14.06±1.67, 9.92±1.19, and 7.57±1.05 in D 90% of HRCTV for α/β = 10 (stage I and II cervix carcinoma), α/β = 16.46 (HX156c), α/β = 11.40 (HX155c). Analogous results are also detected in OARs.As the source decays, the maximum ∆BED(%) ( α/β = 3) at D 0.1cc of the bladder, rectum, sigmoid, and small intestine reached 13.37±2.27, 11.92±2.10, 12.45±2.27, and 11.91±2.62, respectively. Conclusions This study validates that source decay has a substantial impact on the BED of cervical cancer tissues with distinct cell lines and normal tissues. Through the comparison between the full-form BED formula and the simplified one, it is evident that the latter has a tendency to overestimate the BED value, particularly when the source activity is 2 Ci.