Shortening delivery times for intensity-modulated proton therapy by reducing the number of proton spots: an experimental verification

Abstract

Delivery times of intensity-modulated proton therapy (IMPT) can be shortened by reducing the number of spots in the treatment plan, but this may affect clinical plan delivery. Here, we assess the experimental deliverability, accuracy and time reduction of spot-reduced treatment planning for a clinical case, as well as its robustness. For a single head-and-neck cancer patient, a spot-reduced plan was generated and compared with the conventional clinical plan. The number of proton spots was reduced using the iterative ‘pencil beam resampling’ technique. This involves repeated inverse optimization, while adding in each iteration a small sample of randomly selected spots and subsequently excluding low-weighted spots until plan quality deteriorates. Field setup was identical for both plans and comparable dosimetric quality was a prerequisite. Both IMPT plans were delivered on PSI Gantry 2 and measured in water, while delivery log-files were used to extract delivery times and reconstruct the delivered dose via Monte-Carlo dose calculations. In addition, robustness simulations were performed to assess sensitivity to machine inaccuracies and errors in patient setup and proton range. The number of spots was reduced by 96% (from 33 855 to 1510 in total) without compromising plan quality. The spot-reduced plan was deliverable on our gantry in standard clinical mode and resulted in average delivery times per field being shortened by 46% (from 51.2 to 27.6 s). For both plans, differences between measured and calculated dose were within clinical tolerance for patient-specific verifications and Monte-Carlo dose reconstructions were in accordance with clinical experience. The spot-reduced plan was slightly more sensitive to machine inaccuracies, but more robust against setup and range errors. In conclusion, for an example head-and-neck case, spot-reduced IMPT planning provided a deliverable treatment plan and enabled considerable shortening of the delivery time (∼50%) without compromising plan quality or delivery accuracy, and without substantially affecting robustness.

Dosimetric influence of deformable image registration uncertainties on propagated structures for online daily adaptive proton therapy of lung cancer patients

“…However, with current developments in controlled breath-hold [54], it is possible to hold the breath up to several minutes. Combined with recent investigations about decreasing delivery times [55], the delivery of the entire field in future will be possible within one breath-hold. Additionally, previous studies have also shown a high reproducibility of DIBH for NSCLC patients [32,56].…”

A quantitative FLASH effectiveness model to reveal potentials and pitfalls of high dose rate proton therapy

“…In order to remove low‐weighted spots and potentially increase the dose rate, the number of spots in all treatment plans was greatly reduced by applying the spot‐reduction algorithm presented by Van de Water et al. 20 …”

“…Please note that no air gap was modeled, representing a best‐case scenario for the range shifter plans in terms of spot sizes and thus of plan quality and integral dose. In order to remove low‐weighted spots and potentially increase the dose rate, the number of spots in all treatment plans was greatly reduced by applying the spot‐reduction algorithm presented by Van de Water et al 20 …”

“…Please note that no air gap was modeled, representing a best-case scenario for the range shifter plans in terms of spot sizes and thus of plan quality and integral dose. In order to remove low-weighted spots and potentially increase the dose rate, the number of spots in all treatment plans was greatly reduced by applying the spot-reduction algorithm presented by Van de Water et al 20 Treatment plans were generated for five different treatment sites: brain (BR), lung (LU), nasal cavity (NC), pancreas (PA), and prostate (PR). Each of these five treatment sites is represented in this study by two separate patient geometries, that is, 10 geometries were investigated in total.…”

Treatment planning for Flash radiotherapy: General aspects and applications to proton beams

“…The energy of the shoot-through beams was set to 230 MeV (see Table 1), with a corresponding spot width of 3.5 mm (one sigma, in-air at isocenter). Spot-reduced treatment planning was performed using the "pencil beam resampling" technique, 36 which was implemented as an inhouse developed extension of the open-source treatment planning toolkit "matRad." 37 It is an iterative planning approach, with each iteration consisting of addition of a relatively small sample of randomly selected candidate spots, followed by prioritized multicriteria dose optimization, and finally iterative exclusion of low-weighted spots while maintaining dosimetric plan quality.…”