Can bolus range shifting improve plan quality in the IMPT of head-and-neck cancer?

Michiels, Steven [KUL] Barragan Montero, Ana Maria [UCL] Souris, Kevin [UCL] Poels, Kenneth [KUL] Crijns, Wouter [UCL] Lee, John Aldo [UCL] Sterpin, Edmond [UCL] Nuyts, Sandra [KUL] Haustermans, Karin [UCL] Depuydt, Tom [UCL] et al.[show all]

Purpose/Objective In intensity-modulated proton therapy (IMPT) of head-and-neck cancer (HNC), range shifter (RS) air gap is known to widen the pencil beam spots reaching the patient. The actual effect on IMPT dose distributions has remained overall unassessed. Moreover, emerging technologies such as 3D printing enable to implement RS as bolus, hereby removing the air gap and reducing the risk of collision compared to nozzle-mounted RS. In this study, we assess the impact of clinically applied air gaps on IMPT plan quality, the potential of Monte Carlo (MC) dose calculation treatment plan optimization to mitigate air gap effects, and the potential advantage of bolus RS compared to currently used RS solutions. Materials and Methods Oropharynx patients were selected for IMPT based on potential reduction of normal tissue complication probability (NTCP) for xerostomia and dysphagia. For the plan generation, a 5mm CTV-to-PTV-margin was used. Prescription doses were 54Gy to PTVElective and 66Gy to PTVBoost. Table 1 shows the considered organs-at-risk (OAR). Gantry angles were 50°, 180° and 310°, with 4cm PMMA RS mounted on the nozzle, on a snout accessory or applied as bolus. Pencil beams from clinical beam data were placed with spot spacing and target margin equal to 5mm. Beamlets were calculated with a 2x2x2mm³ grid size using a fast-MC dose engine (Souris et al. Med Phys 2016). Beamlet weight optimization was performed using the non-linear solver IPOPT. For each patient, 3 cases were compared: RS on a 25cm Ø[? Diameter?] snout and RS on the 40x30cm² nozzle, where the snout/nozzle was moved as close as possible to the skin but with 3cm safety gap, and RS applied as bolus. The air gap effect was assessed by recalculating the bolus-optimized treatment plan with MC for the snout and nozzle case without re-optimization. Loss in PTV coverage (D98%) was mitigated by re-optimizing the snout and the nozzle case. Differences in OAR dose and NTCP between all case-specific optimal plans were calculated. Results Results for the first 3 patients were compiled in Table 1. Suboptimal planning by disregarding the air gap yielded reductions in D98% ranging from 3.4Gy to 7.7Gy for PTVBoost and from 3.8Gy to 10.1Gy for PTVElective. This effect increases with distance, as for the same safety distance the air gap for the nozzle is typically 2cm larger than for the snout. Geometry-specific optimizing restored PTV coverage, but increased the mean dose to most OARs due to individual pencil beam widening. Compared to the bolus RS plan, these OAR dose increments translated into xerostomia NTCP increases between 3.9% and 7.1% and dysphagia NTCP increases between 1.5% and 5%. Conclusions The actual impact of RS air gaps in clinically relevant circumstances was quantified for IMPT. Bolus RS can considerably improve plan quality in HNC. NTCP reductions for xerostomia and dysphagia achieved with bolus RS were not negligible compared to the 10% threshold proposed by the model-based IMPT patient selection (Langendijk et al. Radiother Oncol 2013).