WE-G-17A-07: Investigation of the Influence of the Electron Return Effect (ERE) On the Dose Distribution in Rectal Cancer Patients On a 1.5T MR-Linac.
PURPOSE
The purpose of this planning study is to investigate the influence of the ERE on the day-to-day dose distribution in rectal cancer patients, where changes in gas-pockets frequently occur.


METHODS
Daily CT scans of 5 patients treated neo-adjuvant with 5×5Gy for rectal cancer were used. We optimized two plans on the planning CT (Monaco, 1 mm3 dosegrid), a conventional 7-field 6MV IMRT plan (Dconv) and a plan in the presence of a 1.5T field (Dmrl). We recalculated the plans on all repeat-CT scans and evaluated under/over-dosage of the daily CTVs. Changes of more than 1% were considered significant. In the bowel area, we investigated the relative dose changes due to the ERE, where the contribution of the ERE was separated from other effects such as attenuation.


RESULTS
Both plans were comparable and compliant with ICRU 62 for all patients. For 2 fractions in one patient under-dosage in the CTV was significant, due to a disappearing gas-pocket. Here the V95 was 96.82 and 97.36% in in Dmrl compared to 98.85 and 98.66% in Dconv, respectively. For 3 fractions in another patient appearing gas-pockets resulted in significant over-dosage of the CTV. In these fractions the V107 was 1.88-2.68% in Dmrl compared to 0.33-1.27% in Dconv. In the bowel area the dose changes attributable to the ERE were approximately ± 5% in 1cc, at low dose levels.


CONCLUSION
We were able to calculate acceptable treatment plans with and without a magnetic field. The ERE was present in the Dmrl, but the volumetric effect within the CTV was limited. Outside the CTV relative dose differences were similar, but on small volumes at lower, less relevant dose levels. This suggests that there is no clinical relevant ERE on dose distributions in rectal cancer patients on a 1.5T MR-Linac.