Towards near real-time adaptive proton therapy: the partial adaptation strategy

Purpose/Objective The preparation for an online-adaptive replanning in proton therapy (OAPT), including e.g. daily anatomy assessment and contouring of relevant structures, can considerably prolong the treatment session. We therefore propose for the first time a concept of partial adaptation as a step towards near real-time OAPT: the first non-adapted field is delivered during the replanning process while the remaining adapted fields shall also compensate for the suboptimal dose from the first field. The dosimetric consequences are demonstrated for head and neck cancer (HNC) patients. Materials/Methods A partial adaptation workflow was implemented in RayStation (v.11.0.100) via the dose tracking module and the consideration of field-wise background dose. For six HNC patients, simultaneous integrated boost plans with three fields delivering 54Gy/66Gy to the low-risk/high-risk clinical target volume (CTVLow/CTVHigh) in 33 fractions were robustly optimized. Partially adapted fraction doses considering a non-adapted posterior field were generated on 3 control CTs (cCT) per patient, acquired in the first week, at mid of treatment and in the last week. Results were compared with doses from the non-adapted plan and from fully adapted plans on the cCT anatomy by analysing CTV coverage (D98%>95%), overdose (D1%) and organ at risk (OAR) sparing. Paired t-tests were used to indicate significant differences (p<0.05). Results In general, dose distributions from the partial and full adaptations/partial and full adaptation strategy were quite similar, while non-adapted plans resulted in different individual deficits (Fig.1). In all 18 fractions (Fig.2), partial as well as full adaptation led to sufficient coverage of both CTVs without significant differences (median D98% for both strategies: 97.2%/98.5% for CTVLow/CTVHigh), while the D98% of non-adapted plans was significantly lower, even below 95% in 6 and 3 cases and had median values of 95.3% and 97.5% for CTVLow and CTVHigh, r