Quantitative Assessment of 3D Dose Rate for Proton Pencil Beam Scanning FLASH Radiotherapy and Its Application for Lung Hypofractionation Treatment Planning

Abstract

To quantitatively assess target and organs-at-risk (OAR) dose rate based on three proposed proton PBS dose rate metrics and study FLASH intensity-modulated proton therapy (IMPT) treatment planning using transmission beams. An in-house FLASH planning platform was developed to optimize transmission (shoot-through) plans for nine consecutive lung cancer patients previously planned with proton SBRT. Dose and dose rate calculation codes were developed to quantify three types of dose rate calculation methods (dose-averaged dose rate (DADR), average dose rate (ADR), and dose-threshold dose rate (DTDR)) based on both phantom and patient treatment plans. Two different minimum MU/spot settings were used to optimize two different dose regimes, 34-Gy in one fraction and 45-Gy in three fractions. The OAR sparing and target coverage can be optimized with good uniformity (hotspot < 110% of prescription dose). ADR, accounting for the spot dwelling and scanning time, gives the lowest dose rate; DTDR, not considering this time but a dose-threshold, gives an intermediate dose rate, whereas DADR gives the highest dose rate without considering any time or dose-threshold. All three dose rates attenuate along the beam direction, and the highest dose rate regions often occur on the field edge for ADR and DTDR, whereas DADR has a better dose rate uniformity. The differences in dose rate metrics have led a large variation for OARs dose rate assessment, posing challenges to FLASH clinical implementation. This is the first attempt to study the impact of the dose rate models, and more investigations and evidence for the details of proton PBS FLASH parameters are needed to explore the correlation between FLASH efficacy and the dose rate metrics.

A Novel Proton Pencil Beam Scanning FLASH RT Delivery Method Enables Optimal OAR Sparing and Ultra-High Dose Rate Delivery: A Comprehensive Dosimetry Study for Lung Tumors

“…van Marlen et al [ 23 ] first defined spot peak dose rate (SPDR) as the max dose rate of the central axis of a spot to quantify single spot dose rate. Based on the beam current and delivery mechanism, we can quantify the dose rates using different proposed metrics, including DADR [ 18 ], ADR [ 19 ], and DTDR [ 20 ]. A 2 ms delivery time and ~140 nA beam current in the treatment room corresponds to a minimum MU/spot of 400 and ~640 Gy/s SPDR [ 20 ].…”

“…Based on the beam current and delivery mechanism, we can quantify the dose rates using different proposed metrics, including DADR [ 18 ], ADR [ 19 ], and DTDR [ 20 ]. A 2 ms delivery time and ~140 nA beam current in the treatment room corresponds to a minimum MU/spot of 400 and ~640 Gy/s SPDR [ 20 ]. Recently, transmission efficiency of 86% from the cyclotron to the treatment room [ 31 ] was achieved for the PSI gantry 1, and if similar efficiency is assumed for the ProBeam system, then the maximum nozzle beam current is ~690 nA, equivalent to a minimum MU/spot of ~1970 assuming a 2 ms delivery time and ~2800 Gy/s SPDR in the treatment room.…”

“…Recently, transmission efficiency of 86% from the cyclotron to the treatment room [ 31 ] was achieved for the PSI gantry 1, and if similar efficiency is assumed for the ProBeam system, then the maximum nozzle beam current is ~690 nA, equivalent to a minimum MU/spot of ~1970 assuming a 2 ms delivery time and ~2800 Gy/s SPDR in the treatment room. The SPDR in the near-Bragg peak region is typically ~40% lower compared to the plateau region for a single spot [ 20 ], especially in the presence of an air gap between the range shifter and the patient surface [ 20 ]. Therefore, using Bragg peak planning is necessary to use a high minimum MU/spot, i.e., higher nozzle current, to achieve a sufficient dose rate.…”

“…Currently, there is no consensus in PBS dose rate definition, and a variety of dose rate calculation methods have been proposed for 3D dose rate assessment for FLASH RT planning studies [ 18 , 19 , 20 ]. Unlike double scattering (DS) proton or electron treatment, the dose delivery in a PBS field runs via scanning magnets, which scan hundreds of spots to cover the entire target layer-by-layer.…”

“…Verhaegen et al [ 26 ] investigated transmission FLASH proton treatment plans by assuming a normal tissue protection factor of two, and their results suggested significant dose reduction in normal tissues while conforming to the dose constraints without beam optimization. Kang et al [ 20 ] studied the intensity-modulated proton therapy (IMPT) transmission plans for hypofractionated lung cancer based on beam parameters, including the beam current and minimum spot delivery time, to fully assess the dosimetry and dose rate performances using multiple single-energy proton beams.…”