Factors influencing dose delivery in rectal cancer radiotherapy using CBCT-guided intensity modulated techniques

This study investigates factors that affect dose delivery in rectal cancer radiotherapy and evaluates the resulting differences between planned and delivered treatment doses using intensity-modulated techniques. Twenty patients were included with 140 cone beam computed tomography (CBCT) images. A correlation analysis was conducted to ascertain the impact of detected misalignment on the Penta-Guide phantom on the planning target volumes (PTVs) and volumes of organs at risk (OARs), and whether shifts from CBCT were affected by patient mass changes. To facilitate a comparative analysis and evaluate dose delivery, Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) plans were simulated in the Monaco treatment planning system and further adapted to each CBCT using the Adapt-to-Shape (ATS) function. Moderate to strong correlations between Penta-Guide detected misalignments and the PTVs, bladder and bowel bag were observed, as well as a strong correlation between translational and rotational shifts and patient mass modifications during treatment. Thus, a reduction in target volume coverage was observed in comparison to the initial plans on CT, with VMAT providing a PTV coverage of nearly 95%, while IMRT resulted in an under-irradiated tumour volume with 93.4% coverage for calculated plans on CBCT. Regarding OARs, a slight decrease was observed in parameter values analyzed on CBCT for bladder (up to 5% for V40 with VMAT), while for bowel bag the evaluated parameters were up to 11% higher for IMRT and 10% for VMAT on CBCT. These results suggest that monitoring organ filling, patient mass, and quality assurance (QA)-related misalignments is essential in rectal cancer radiotherapy to minimize dose discrepancies.