Robustness of VMAT TBI Plans Against Interfractional Variations Under Different Placements of Upper Limbs
Abstract
Purpose
To quantify the impact of interfractional variations on the dosimetric quality of VMAT TBI plans, primarily caused by potentially inconsistent upper limb placement at treatment; and to determine a clinically optimal setup for plan robustness against such variations.
Methods
This study was retrospectively performed on three patients, each treated with myeloablative VMAT TBI at 12Gy prescription dose in eight fractions. Placements of the upper limbs during simulations are: 1. elbows flexed with palms on abdomen, 2. posterolateral to torso, 3. anterolateral to torso with palms resting on legs. The planning CT and the associated structure set were deformed to the extended CBCT from each fraction in Varian Velocity. Doses from each fraction were recalculated on the deformed structure sets with preset plan parameters. Fractional doses were accumulated on the original planning CT by applying the inverse deformations. DVH endpoints were calculated with accumulated fractional doses and compared with the original plan.
Results
Non-negligible changes in both PTV (cropped from C2 to femoral heads) coverage and dose to the OARs were found. For Patient 1, D98% of cropped_PTV dropped from 85.2% to 80.83%, while mean dose to kidneys increased from 67.6% to 70.2%. For Patient 2, D98% and V100% of cropped_PTV dropped from 87.6% to 81.8% and 84.8% to 80.9%, respectively; while mean dose to lungs-1cm increased from 45.0% to 46.9% and mean dose to the kidneys from 74.7% to 79.3%. For Patient 3, no significant change in PTV coverage was found while mean dose to the lungs marginally increased from 57.9% to 59.2%.
Conclusion
Interfractional variations, in particular variations in upper limb positions, have a significant impact on the dosimetric quality of VMAT TBI plans. Placing arms anterolateral to the torso with palms resting on the legs helps reducing interfractional changes.