Commissioning of a clinical pencil beam scanning proton therapy unit for ultra‐high dose rates (FLASH)

Abstract

The purpose of this work was to provide a flexible platform for FLASH research with protons by adapting a former clinical pencil beam scanning gantry to irradiations with ultra-high dose rates. Methods: PSI Gantry 1 treated patients until December 2018. We optimized the beamline parameters to transport the 250 MeV beam extracted from the PSI COMET accelerator to the treatment room, maximizing the transmission of beam intensity to the sample. We characterized a dose monitor on the gantry to ensure good control of the dose, delivered in spot-scanning mode. We characterized the beam for different dose rates and field sizes for transmission irradiations. We explored scanning possibilities in order to enable conformal irradiations or transmission irradiations of large targets (with transverse scanning). Results: We achieved a transmission of 86% from the cyclotron to the treatment room. We reached a peak dose rate of 9000 Gy/s at 3 mm water equivalent depth, along the central axis of a single pencil beam. Field sizes of up to 5 × 5 mm 2 were achieved for single-spot FLASH irradiations. Fast transverse scanning allowed to cover a field of 16 × 1.2 cm 2 . With the use of a nozzle-mounted range shifter, we are able to span depths in water ranging from 19.6 to 37.9 cm. Various dose levels were delivered with precision within less than 1%. Conclusions: We have realized a proton FLASH irradiation setup able to investigate continuously a wide dose rate spectrum, from 1 to 9000 Gy/s in single-spot irradiation as well as in the pencil beam scanning mode. As such, we have developed a versatile test bench for FLASH research.

Future Developments in Charged Particle Therapy: Improving Beam Delivery for Efficiency and Efficacy

“…For ion beams there are difficulties with reaching the required dose rates, which demand an increase in beam current by several orders of magnitude for rapid irradiation of a clinically relevant volume. A number of CPT facilities have been able to modify their accelerators (mostly isochronous cyclotrons and synchrocyclotrons) for FLASH with proton beams (171), and photon beams have been studied at large scale synchrotron research facilities (172). FLASH with different ions such as carbon and helium is also being examined (173)(174)(175).…”

FLASH Radiotherapy Using Single-Energy Proton PBS Transmission Beams for Hypofractionation Liver Cancer: Dose and Dose Rate Quantification

“…FLASH radiotherapy (RT), characterized by an ultra-high-dose rate of >40 Gy/s as reported previously, has shown superior normal tissue protection and effective tumor control in many pioneering in vivo studies based on mice (1-5), minipig, cats (6), zebrafish (7), and first human treatment for a cutaneous lymphoma (8). While most of the abovementioned FLASH studies were based on electron beams (9), there have also been several investigations using photon beams (10) and increasingly using proton beams (11)(12)(13)(14)(15)(16)(17). The state-of-the-art proton beam delivery technique, pencil beam scanning (PBS), can precisely position each proton beamlet using scanning magnets, thereby providing outstanding target conformity and organ-at-risk (OAR) sparing.…”

“…In proton FLASH-RT, transmission proton beams have been the most favorable choice of delivery owing to the sufficiently high beam current achievable with existing clinical systems. Recent efforts have reported combining transmission proton PBS with FLASH-RT, to translate the technology from bench to preclinic experiments, in aspects including proton systems (11)(12)(13)(14)(15), treatment planning (18)(19)(20)(21)(22)(23), and biological investigations (24,25). Proton PBS FLASH treatment planning plays a crucial role just as conventional treatment planning, but it faces new and unique challenges as the dose rate considerations must be included when evaluating the plan quality.…”

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

“…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. 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. This work explores the dosimetric potential by applying a different minimum MU/spot under currently realistic and achievable machine settings [ 19 , 31 ] for Bragg peak plans compared to transmission plans.…”