Point vs. volumetric bladder and rectal doses in combined intracavitary-interstitial high-dose-rate brachytherapy: Correlation and comparison with published Vienna applicator data

Purpose: We correlated rectal and bladder point and volumetric dose data in patients treated for advanced cervix cancers with combined intracavitary-interstitial high-dose-rate (HDR) brachytherapy (BT). The results are compared with published Vienna applicator data. Methods and Materials: We retrospectively analyzed 30 individual combined intracavitary plus interstitial implants from 10 patients treated with external beam radiation therapy (EBRT) followed by HDR BT for locally advanced cervix carcinoma. EBRT consisted of 45 Gy to the pelvis followed by 9-14.4 Gy boost to involved parametria. BT consisted of a total dose of 21 Gy delivered in 7 Gy fraction. For each implant, CT-image-based simulation and image-guided BT treatment planning was performed. Bladder and rectal doses were evaluated and analyzed using both International commission on Radiation Units and Measurements (ICRU) reference points and dose-volume histograms. The cumulative doses to the rectum and bladder were calculated by combining contributions from external beam therapy and BT. To facilitate comparison with published literature, the total doses were normalized to equivalent dose in 2-Gy fractions (EQD₂) using the equation EQD_2total = EQD_2EBRT + EQD_2BT. Results: For the patient population considered, the mean ICRU bladder dose was 75 (±4) Gy₃ compared to bladder D_{0.1 cc} and D_{2 cc} doses of 84 (±4) and 78 (±3) Gy₃, respectively. The mean ICRU rectal dose was 73 (±4) Gy₃ compared to rectal D_{0.1 cc} and D_{2 cc} doses of 79 (±5) and 74 (±4) Gy₃, respectively. For rectum, the mean dose ratios (D_{0.1 cc}/D_ICRU) and (D_{2 cc}/D_ICRU) were 1.08 and 1.01, respectively, compared to Vienna applicator study mean dose ratios of 1.08 and 0.93, respectively. ICRU rectal dose correlated with volumetric rectal doses and best with volumetric D_{2 cc} dose (r_S = 0.91, p = 0.0003); however, ICRU bladder dose did not correlate with volumetric bladder dose. Conclusions: Our study findings reveal a strong correlation between ICRU rectal reference dose and volumetric rectal D_{2 cc} dose in combined intracavitary-interstitial HDR brachytherapy. This surrogate rectal-dose relationship is valuable in establishing rectal tolerance dose levels in transitioning from traditional two-dimensional to image-based three-dimensional dose planning.