2179 - Comparison of Cumulative Dose Assessment Strategies for External Beam Radiotherapy and Brachytherapy in Gynecological Cancers

Purpose/Objective(s): Gynecological radiotherapy routinely combines external beam radiotherapy (EBRT) for pelvic disease control with brachytherapy for localized tumor boost. This requires an accurate cumulative dose assessment to ensure optimal planning. However, substantial anatomical changes due to organ filling and applicator displacement can affect the accuracy of the dosimetric assessment. We sought to compare two techniques of cumulative dose assessment.

Materials/Methods: We retrospectively evaluated 20 patients treated at our institution with combined CT-based EBRT (45 Gy in 25 fractions with SIB to 56.25 Gy for 13 cases) and MRI-based brachytherapy (10 interstitial and 10 intracavitary tandem and ring implants). For each case, we compared the cumulative dose in EQD₂ to the rectum, bladder, and high-risk CTV calculated using two different approaches. First, we used a dose summation of D_2cc for organs at risk (OARs) and D_90% for targets in the EBRT and brachytherapy plans. D_2cc on the EBRT planning CT was evaluated only within the geometric overlap with a variable isotropic expansion (ranging from 0.5 cm to 3 cm) of the uterus, cervix, and vagina. For target D_90%, we assumed 100% of the EBRT prescription dose. Second, dose accumulation on the brachytherapy planning MRI was performed using a state-of-the-art large deformation diffeomorphic metric mapping registration algorithm. The accuracy of the deformable image registration (DIR) was validated using the dice similarity coefficient (DSC; 0 = no similarity, 1 = perfect similarity) and Jacobian determinant. We assessed the dosimetric differences between both approaches.

Results: The DIR achieved an accurate alignment with a DSC of 0.88(±0.07) and 0.81(±0.09) for the bladder and rectum, respectively, and did not exhibit non-physiological tissue folding. With the dose summation approach, the OAR D_2cc increased with the expansion but remained constant after approximately 2 cm. Using this 2 cm expansion, the D_2cc for the bladder and rectum were 1.1(±0.5) GyEQD₂³ and 1.0(±0.7) GyEQD₂³ higher than the dose accumulation approach (t-test; p<0.001). Larger differences of more than 2 GyEQD₂³ (up to 6 GyEQD₂³) between both approaches were observed in four cases with SIB regions close to OARs. Deformable dose accumulation resulted in a significantly larger D_90% for the high-risk CTV (1.4 GyEQD₂¹⁰ on average). No statistically significant differences were observed between interstitial and intracavitary treatments (p=0.16).

Conclusion: The dose differences between the deformable dose accumulation and the dose summation approaches were statistically significant, although the overall magnitude was small. Employing a 2 cm isotropic expansion of the uterus, cervix, and vagina to assess the relevant OAR doses from EBRT proved to be a robust and conservative approach, potentially easier to implement in the clinic. However, a visual inspection of dose maps is recommended if an SIB volume is in close proximity to an OAR.