Rectal Dose and Source Strength of the High-Dose-Rate Iridium-192 Both Affect Late Rectal Bleeding After Intracavitary Radiation Therapy for Uterine Cervical Carcinoma

Purpose: The purpose of this study was to reconfirm our previous findings that the rectal dose and source strength both affect late rectal bleeding after high-dose-rate intracavitary brachytherapy (HDR-ICBT), by using a rectal dose calculated in accordance with the definitions of the International Commission on Radiation Units and Measurements Report 38 (ICRU_RP) or of dose-volume histogram (DVH) parameters by the Groupe Européen de Curietherapie of the European Society for Therapeutic Radiology and Oncology. Methods and Materials: Sixty-two patients who underwent HDR-ICBT and were followed up for 1 year or more were studied. The rectal dose for ICBT was calculated by using the ICRP_RP based on orthogonal radiographs or the DVH parameters based on computed tomography (CT). The total dose was calculated as the biologically equivalent dose expressed in 2-Gy fractions (EQD₂). The relationship between averaged source strength or the EQD₂ and late rectal bleeding was then analyzed. Results: When patients were divided into four groups according to rectal EQD₂ (≥ or <median dose) and source strength (≥ or <2.4 cGy.m².h^-1), the group with both a high EQD₂ and a high source strength showed a significantly greater probability of rectal bleeding for ICRU_RP, D_2cc, and D_1cc. The patients with a median rectal dose above the threshold level did not show a greater frequency of rectal bleeding unless the source strength exceeded 2.4 cGy.m².h^-1. Conclusions: Our results obtained with data based on ICRU_RP and CT-based DVH parameters indicate that rectal dose and source strength both affect rectal bleeding after HDR-ICBT.