Computation of Voxel-by-Voxel Dose Rates in Patients for Proton Pencil Beam Dose Delivery

Abstract

<h3>Purpose/Objective(s)</h3> Proton beam radiotherapy is most commonly delivered using the pencil beam scanning (PBS) technique. However, the vast majority of clinical outcome data is based on patients treated with the passive scattering technique. The overall dose distributions delivered with these techniques are fairly similar, with an advantage in conformity noted for PBS. Major differences between these two modalities can be observed in the temporal dose delivery pattern. The passive scattering technique uniformly delivers dose across the cross section of the entire target simultaneously, while the dose in PBS is delivered point-by-point, resulting in a much higher local instantaneous dose rate. Recent studies have shown that ultra-high dose rate (FLASH) irradiations can change the biological response to radiation. This work presents an analytical tool to compute voxel-wise maximum dose rates for clinical PBS treatment plans. <h3>Materials/Methods</h3> The scripting environment of our treatment planning system was used to develop software to compute dose rate distributions for clinical pencil beam scanning treatment plans. The basis of the computation is an accurate model of the delivery timing properties of the accelerator and scanning system. The minimum temporal resolution supported is 1 ms. This captures the timing of single spots but excludes the beam pulse structure of our cyclotron (ns). With the timing model and a given spot map we compute the average dose rate over a time interval of interest for each voxel. The latter is an input parameter determined by the user. To validate the tool, test beams in a simple solid water geometry were generated in the treatment planning system and corresponding dose rate distributions produced. Single point measurements with custom-made diode and electronics were used to verify computations against experimentation for a range of proton energies and depths. <h3>Results</h3> For dose rate calculations recording the maximum value observed in any 1 ms time interval during the field's delivery, we observe peak values of 186 Gy/min and 273 Gy/min for 110 MeV and 150 MeV protons, respectively. On average, the agreement of calculations and measurements was -0.9% with a standard deviation of 4.1%. <h3>Conclusion</h3> The software enables accurate local instantaneous dose rate calculations for clinically deliverable treatment plans. This may be important to use as a basis to better understand clinical results. In addition to standard PBS treatments, the observed dose rates may be relevant when considering the out of field areas in FLASH treatments.