A. M. Pitre1, A. Yen2, S. Zhang3, Z. Iqbal4, B. A. Hrycushko4, X. Zhong2, M. H. Lin2, and K. V. Albuquerque2; 1Texas College of Osteopathic Medicine, Houston, TX, United States, 2Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, 3UT Southwestern Medical Center, Dallas, TX, 4Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, TX
Purpose/Objective(s): Adaptive radiation therapy (ART) is a new treatment modality for cervical cancer. Daily ART with reduced margins has shown to significantly improve target coverage while sparing organs at risk. However, daily adaptive therapy is time and resource intensive. Adapt-on-demand (AOD), or adaptations per physician’s discretion can spare OARs, but the degree of improvements is significantly reduced when compared to daily ART. Thus, improved utilization and patient selection is required to optimally employ ART. In this study, the dosimetry from patients treated with ART were compared to traditional IGRT plans to determine the optimal timepoint to adapt. Additionally, patient and dosimetric predictors were analyzed to determine which patients benefit most from ART. Materials/
Methods: A retrospective cohort of 41 patients with cervical cancer (stage IIA-IIIC2) were analyzed. 28 patients received AOD and 13 patients received daily ART with a total of 424 fractions between September 2021 and July 2023. Dosimetric values of V40/V45 bowel bag, and V40/V30 bladder and rectum were obtained for both adapted and pre-planned IGRT plans. Patients had variable uterocervix CTV margins ranging from 0.5 - 1.5 cm per physicians’ discretion and adaptation regimens. Patient and tumor characteristics such as stage, BMI, initial uterocervix CTV size, and change in CTV size were collected. First, a linear mixed effect model was utilized to determine the optimal week for adaptation. Second, a mixed logistic regression model was used to determine which patients would benefit most from ART. Weeks and various demographic/clinical variables were evaluated as covariates. Both models include patient random effects to account for correlation among measurements from the same patients. Statistical significance was defined as p < 0.05. Results: When compared to IGRT pre-plans, adapted plans only had improvements in bladder V40 and V30. Starting week 3, bladder V40 was significantly improved when treated with ART, while bladder V30 was significantly improved with ART starting week 1. Reduction in tumor size, from week 1 to week 3 and initial bowel bag V40 were predictors for benefit from ART. Initial uterocervix CTV size trended towards predicting benefit from ART, but was not statistically significant (p = 0.053) Conclusion: Results from our study suggests the most beneficial timepoint to start adapting is week 3 and onwards. Our study also showed that patients with significant change in uterocervix CTV size and large initial bowel bag V40 may benefit more for ART. These results can help clinicians optimize timing and patient selection for ART with potential to reduce toxicity and improve target coverage.
