The influence of average electron density assignment on dose accuracy in MR-guided radiotherapy of rectum cancer

To study the impact of assigning average electron density on the dose accuracy of patients in adapt to shape (ATS) workflow of MR-guided adaptive radiotherapy (MRgRT). Ten rectum cancer patients, who underwent treatment with MRgRT, were included for this study. For each patient, 2 plans were created. The first plan, referred to as the average electron density (AVED) plan, was generated using bulk anatomical average density method based on anatomical contours derived from the CT image. The second plan, equivalent to the actual electron density (ACED) plan, was recalculated using the actual electron density while maintaining the same segment shapes and monitor units as the AVED plan. Consequently, the ACED plan represents the actual adaptive dose. Various parameters, including the dose conformity index (CI), homogeneity index (HI), as well as the dose to the planning target volume (PTV) and organs at risk (OARs), were analyzed. Additionally, the dose verification of the ACED plans was conducted using an MR-compatible ArcCHECK phantom. The gamma passing rate of the ACED plan in comparison to the AVED plan was calculated using 3-dimensional gamma analysis software. In the ACED plan, compared with the AVED plan, the coverage of prescribed dose in PTV was significantly low (p < 0.05), with the difference of 2.11% and PTV exhibited poorer values for both the HI and CI (p < 0.05). Additionally, the V₅₀ of colon, mean dose of bladder, left femur and right femur were significantly low (p < 0.05) in the ACED plans. The gamma pass rates for ACED plans exceeded 90% (ArcCHECK vs TPS) under 2 mm/3% and 2 mm/2% criteria with a 10% dose threshold. Compared to AVED plans, ACED plans achieved average pass rates of 99.39% ± 0.34% and 97.92% ± 0.65%, respectively. When using average electron density for dose calculations in MRgRT of rectum cancer, it is essential to note that there can be discrepancies between the calculated and actual doses, particularly in homogeneity of PTV.