P205] The impact on pencil beam scanning (PBS) proton therapy for mediastinal lymphoma from deep inspiration breath-hold (DIBH) variability

Abstract

Purpose Pencil beam scanning (PBS) proton therapy for mediastinal lymphoma has the potential to reduce lung and heart doses compared to photon based radiotherapy. In combination with deep inspiration breath-hold (DIBH) technique, the dose distribution can be improved even further. DIBH technique offers an almost static treatment situation preventing interplay effects. However, it requires the fractional dose to be delivered in several breath-holds. Small anatomical inter breath-hold variations may therefore disturb the planned dose distribution. The aim of this study was to investigate the impact of such DIBH variability on PBS proton therapy in DIBH for mediastinal lymphoma. Methods DIBH positional variations were simulated in a two field robust optimised PBS plan. In order to reflect the number of breath-holds needed for the actual treatment delivery sequence, the PBS fields were divided into 8 separate sub-fields. 10 different breath-hold images, all within a 3 mm range in vertical chest position, were acquired using MR-scanning. The anatomy was then transformed by deformable image registration onto the CT-study used for treatment planning, creating 10 different DIBH CT-studies. The dose of one single fraction was generated from randomly selected 8 sub-field doses, recalculated in the 10 DIBH CT-studies. Finally, a set of 10 complete treatment series were created by summing up 17 unique fraction doses (in total 29.75 Gy-RBE) generated from the sets of randomly assigned breath-holds in each fraction. Results The results show that there generally is a noticeable discrepancy between the planned dose distribution and a single simulated fraction dose. The CTV D 98 decreases and D 2 increases by 3% on average and in rare cases even up to 8% for a single fraction. However, averaged over a complete series of 17 fractions the deviations from the planned dose distribution are small, for CTV within 1%, indicating that the DIBH technique is a suitable delivery mode for PBS proton therapy for mediastinal lymphoma. Conclusions The results support PBS proton therapy in DIBH for mediastinal lymphoma. When averaging over a complete serie of fractions, the impact of inter-BH variability is minimal.