Class solution development for volumetric modulated arc therapy planning of cervical cancer and the investigation of source-to-skin distance changes during radiotherapy treatment

Abstract

Background: Cervical cancer continues to be a leading cause of morbidity and mortality worldwide. Advancements in radiotherapy, particularly volumetric modulated arc therapy, reduces treatment-related toxicity by providing highly conformal dose distributions. The implementation of class solutions facilitates consistent and efficient treatment planning, essential for optimising patient outcomes. Patient’s anatomy may also change during radiotherapy, which can alter source-to-skin distances, affecting dose distributions and necessitating adjustments in treatment plans. Current practices in Malta reassess treatment plans if monitor unit variations exceed ± 2 % due to source-to-skin distance changes, although this threshold lacks scientific evidence. Objectives The study aimed to develop a generalisable volumetric modulated arc therapy treatment planning class solution for cervical cancer which complies with the national clinical protocol based on the EMBRACE II studies, and to facilitate future treatment planning. In addition, it aimed to investigate the dosimetric effects of source-to-skin distance changes in cervical cancer patients by applying different margin adjustments to establish an evidence-based threshold for when a monitor unit adjustment is required. Research Methodology: The research methodology was quantitative, focusing on data analysis from the optimisation and validation of the volumetric modulated arc therapy class solution and examining absorbed dose variations after applying source-to-skin distance margins of different sizes and directions. An experimental and comparative approach was used to develop a generalisable class solution and investigate the effects of source-to-skin distance changes mimicking patient anatomical fluctuations. CT scans from 20 cervical cancer patients prescribed 45 Gy in 25 fractions were used, together with the Monaco® Treatment Planning System v5.11.03 and the Monte Carlo algorithm as the data collection tools. Results: The class solution achieved a 97.48 % pass rate for hard dose constraints and 46.30 % for soft dose constraints, suggesting it provides a good foundation but requires further patient-specific optimisation. Significant changes in dose distributions were observed with the smallest margin changes (± 0.5 cm), indicating that even minor modifications in treatment plans may require monitor unit adjustment. Conclusions and Recommendations: The study successfully developed a generalisable volumetric modulated arc therapy class solution for cervical cancer that adhered to the national clinical protocol. A revised monitor unit percentage difference threshold of 1.4 ± 0.4 % was also established, serving as an evidence-based clinical recommendation.