Spot scanning proton arc therapy reduces toxicity in oropharyngeal cancer patients

Abstract

Background: Proton arc technology has recently shown dosimetric gains for various treatment indications. The increased number of beams and energy layers (ELs) in proton arc plans, increases the degrees of freedom in plan optimization and thereby flexibility to spare dose in organs at risk (OARs). A relationship exists between dosimetric plan quality, delivery efficiency, the number of ELs -and beams in a proton arc plan. Purpose: This work aims to investigate the effect of the number of beams and ELs in a proton arc plan, on toxicity and delivery time for oropharyngeal cancer patients (OPC) selected for intensity modulated proton therapy (IMPT) based on the Dutch model-based approach. Methods: The EL reduction algorithm iteratively selects ELs from beams equidistantly spaced over a 360 • arc. The beams in the final plan may contain multiple ELs, making them suited for static delivery on the studied treatment machine. The produced plans can therefore be called "step and shoot" proton arc plans. The number of beams and ELs were varied to determine the relationship with the planning cost function value, normal tissue complication probability (NTCP) and delivery time. Proton arc plans with robust target coverage and optimal energy layer reduction (ELR) settings to reduce NTCP, were generated for 10 OPC patients. Proton arc plans were compared to clinical volumetric modulated arc therapy (VMAT) and IMPT plans in terms of integral dose, OAR dose, NTCP for xerostomia and dysphagia and delivery time. Furthermore, dose-weighted average linear energy transfer (LET d ) distributions were compared between the IMPT and proton arc plans. A dry run delivery of a plan containing 20 beams and 360 ELs was performed to evaluate delivery time and accuracy. Results: We found 360 ELs distributed over 30 beams generated proton arc plans with near minimal expected plan toxicity. Relative to corresponding IMPT and VMAT plans, an average reduction of 21 ± 3% and 58 ± 10% in integral dose was observed. D mean was reduced most in the pharyngeal constrictor muscle (PCM) medius structure, with on average 9.0 ± 4.2 Gy(RBE) (p = 0.0002) compared to the clinical IMPT plans. The average NTCP for grade≥2 and grade≥3 xerostomia at 6 months after treatment significantly decreased with 4.7 ± 1.8% (p = 0.002) and 1.7 ± 0.8% (p = 0.002), respectively, while the This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

FLASH instead of proton arc therapy is a more promising advancement for the next generation proton radiotherapy

“…Based on this concept, publications from different research institutions have shown the potential clinical benefit of proton arc therapy for a wide range of clinical indications, 55–62 drawing significant interest among radiation oncology communities, including the ESTRO, 63 AAPM, 64 and PTCOG 65 . Multiple international consortiums have been established, aiming to join‐force in pushing the technique forward 66,67 .…”

“…8 As a result, the concept of particle arc therapy was not at the center of the research development until the introduction of the first robust spot-scanning particle arc therapy optimization algorithm 8 and the demonstration of its feasibility using a clinical system. 54 Based on this concept, publications from different research institutions have shown the potential clinical benefit of proton arc therapy for a wide range of clinical indications, [55][56][57][58][59][60][61][62] drawing significant interest among radiation oncology communities,including the ESTRO, 63 AAPM, 64 and PTCOG. 65 Multiple international consortiums have been established, aiming to join-force in pushing the technique forward.…”

Partitioning of discrete proton arcs into interlaced subplans can bring proton arc advances to existing proton facilities

“…This approach is similar, but not identical to the energy layer reduction (ELR) technique. 8,9 In ELF the arc is set up by a start and stop gantry angle, a number of irradiation directions, an initial and final number of energy layers. Throughout the optimization, the lowest weighted energy layers are filtered out to reach the final number of energy layers.…”

“…When this method was employed to 10 oropharyngeal cancer patients, it was found that 360 layers distributed over 30 directions produced near optimal plans with reduced toxicity to normal tissue. 9,10 The disadvantage of this method is that a large number of beams will lead to very long and possibly even unacceptable treatment times, if introduced using conventional IMPT delivery. Technological advances, such as automatic sequencing of consecutive IMPT beams or delivery as a single stepand-shoot arc beam with no additional time between discrete directions other than the rotational time, could remove part of this problem.…”

“…In treatment planning studies proton arcs have shown potential benefits with respect to reduced organ-at-risk (OAR) doses, 9,[11][12][13][14][15][16][17] reduced normal tissue complication (NTCP), 9,10 improved LET distributions, [18][19][20] and consequently improved variable radiobiological effective (RBE) dose distributions compared to IMPT. 21 More recently, there have been indications that proton arc treatments on the other hand could suffer from worse interfractional target robustness compared to conventional IMPT, as has been seen in studies for head and neck cancer patients.…”