ESTRO 2021 Abstract Book

S1270

ESTRO 2021

create a smooth dose gradient in the junction a structure-based optimisation method was used (Figure 1b). A treatment workflow was simulated using an MR from the second week of treatment, with the plan adapted to the nodal region first using an adapt-to-position (ATP) technique. The plan for the primary disease was fully re-optimised via the adapt-to-shape (ATS) technique onto the nodal region plan using the bias dose method. Additionally, the plans were recalculated without adaptation to simulate a standard workflow. The dosimetry of the adapted and non-adapted plans were compared against the reference plan. Finally an investigation into the effect of intra-fraction motion was performed. Relative shifts between the two isocentres on the adapted plans were shifted superiorly (sup) and inferiorly (inf) by 3mm and 6mm, and recalculated.

Results The mean PTV lengths of the NN and NP patients were 19.5 cm and 18.8 cm respectively. To allow room for daily adaptation to anatomical changes none of these patients were eligible for the MRL. Using a dual isocentre technique resulted in field lengths averaging 12.8 cm for isocentre 1 and 7.4 cm for isocentre 2, allowing them to be treated on the MRL. Fig 2 shows dual isocentre adapted plans give better coverage than non-adapted plans. This is true for both NN and NP patients but a larger benefit is seen for nodal boost volumes in the NP cases, with the difference to the reference plan for the D98% being 0.4 Gy and -3.5 Gy for the adapted and recalculated plans respectively. Shifting the adapted plans sup didn’t affect the percentage coverage by more than 3% but shifting them inf caused the PTV coverage on the plan to reduce by ~2.2% per mm for the NP and ~3.2% for the NN cases.