3764 - 8 Mm and 3 Mm: The Optimal PTV Margins for Uterine and Cervical Ctvs In Online Adaptive Radiotherapy
Presenter(s)
X. Gong1, S. Sun1, Y. Liang1, L. He1, X. Sun2, L. Jia3, W. Zhang2, F. Zhang1, and K. Hu1; 1Department of Radiation Oncology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, China, 2Shanghai United Imaging Healthcare Co., Ltd., Shanghai, China, 3Radiotherapy Business Unit, Shanghai United Imaging Healthcare Co., Ltd, Shanghai, China
Purpose/Objective(s): Online adaptive radiotherapy (oART) accounts for interfractional target motion through daily plan adaptation, thereby limiting the role of the planning target volume (PTV) to encompassing only intrafraction motion and enabling margin reduction. This study aims to determine the optimal PTV margins for the uterine and cervical clinical target volume (CTV) in cervical cancer patients undergoing definitive oART.
Materials/Methods: Fifteen patients with FIGO 2018 stage IB3–IIIC1 cervical cancer who received definitive daily oART under fan-beam CT (FBCT) guidance were enrolled. For each treatment fraction, FBCT scans were acquired before and immediately after the adaptive workflow, with the CTV of the uterus (CTV-U) and cervix (CTV-C) independently delineated on both scans. In a training cohort comprising 278 fractions from 10 patients, PTVs were generated by applying varying isotropic margins to the pre-treatment CTV (CTVpre) and rigidly registered to the post-treatment FBCT to calculate CTVpost coverage. The minimal margin achieving CTV coverage =98% in =90% of fractions was identified. In a validation cohort of 140 fractions from 5 patients, the identified optimal margins were assessed for CTV coverage compliance. Dosimetric parameters were compared between plans using these optimal oART margins and conventional image-guided radiotherapy (IGRT) margins (15mm isotropic margins for both CTV-U and CTV-C).
Results: In the training cohort, an isotropic margin of 8mm for CTV-U resulted in CTV coverage =98% in 90.6% (252/278) of fractions, while a 3mm margin for CTV-C achieved this in 92.8% (258/278) of fractions. In the validation cohort, CTV coverage =98% was maintained in 92.9% (130/140) and 93.6% (131/140) of fractions for CTV-U and CTV-C, respectively. Dosimetric analysis confirmed that plans using both optimal oART and conventional IGRT margins met the prescription goal (CTV D98% > 50.4 Gy). However, the optimal oART margins significantly reduced organs-at-risk (OARs) doses, with lower bladder D50% (34.81 Gy vs. 46.48 Gy, P<0.001), rectum D50% (35.41 Gy vs. 50.47 Gy, P<0.001), and small bowel V47Gy (92.69 cc vs. 119.28 cc, P<0.001).
Conclusion: For definitive cervical cancer oART, optimal PTV margins of 8mm and 3mm for CTV-U and CTV-C, respectively, ensure robust target coverage while significantly reducing OAR exposure.