Maximum proton kinetic energy and patient‐generated neutron fluence considerations in proton beam arc delivery radiation therapy

Abstract

Several compact proton accelerator systems for use in proton therapy have recently been proposed. Of paramount importance to the development of such an accelerator system is the maximum kinetic energy of protons, immediately prior to entry into the patient, that must be reached by the treatment system. The commonly used value for the maximum kinetic energy required for a medical proton accelerator is 250 MeV, but it has not been demonstrated that this energy is indeed necessary to treat all or most patients eligible for proton therapy. This article quantifies the maximum kinetic energy of protons, immediately prior to entry into the patient, necessary to treat a given percentage of patients with rotational proton therapy, and examines the impact of this energy threshold on the cost and feasibility of a compact, gantry-mounted proton accelerator treatment system. One hundred randomized treatment plans from patients treated with IMRT were analyzed. The maximum radiological pathlength from the surface of the patient to the distal edge of the treatment volume was obtained for 180°continuous arc proton therapy and for 180°split arc proton therapy ͑two 90°arcs͒ using CT# profiles from the Pinnacle™ ͑Philips Medical Systems, Madison, WI͒ treatment planning system. In each case, the maximum kinetic energy of protons, immediately prior to entry into the patient, that would be necessary to treat the patient was calculated using proton range tables for various media. In addition, Monte Carlo simulations were performed to quantify neutron production in a water phantom representing a patient as a function of the maximum proton kinetic energy achievable by a proton treatment system. Protons with a kinetic energy of 240 MeV, immediately prior to entry into the patient, were needed to treat 100% of patients in this study. However, it was shown that 90% of patients could be treated at 198 MeV, and 95% of patients could be treated at 207 MeV. Decreasing the proton kinetic energy from 250 to 200 MeV decreases the total neutron energy fluence produced by stopping a monoenergetic pencil beam in a water phantom by a factor of 2.3. It is possible to significantly lower the requirements on the maximum kinetic energy of a compact proton accelerator if the ability to treat a small percentage of patients with rotational therapy is sacrificed. This decrease in maximum kinetic energy, along with the corresponding decrease in neutron production, could lower the cost and ease the engineering constraints on a compact proton accelerator treatment facility.

Risk of radiogenic second cancers following volumetric modulated arc therapy and proton arc therapy for prostate cancer

“…Even though the distribution of neutrons throughout the patient anatomy might have differed slightly because of the different beam arrangement, this approximation provided a conservative estimate of the total neutron production for proton arc therapy, due to the larger number of neutrons created by the higher beam energy and the larger amount of tissue traversed for parallel-opposed beam proton therapy (Zheng et al 2008, Sengbusch et al 2009). …”

The physics of proton therapy

“…Some of these are enumerated here, with an emphasis on those questions that will require physics research and development to reduce cost, improve treatment quality and efficiency, and create previously new treatment capabilities of clinical importance. Can novel techniques, such as proton arc therapy (Sandison et al , 1997; Sengbusch et al , 2009; Rechner et al , 2012), be developed to improve the quality of treatment, reduce treatment time, and increase cost-competitiveness and -effectiveness?Can cost-competitiveness or treatment capability be increased significantly through incremental improvements to existing accelerator technologies, e.g. , fixed-field alternating gradient synchrotron (Johnstone et al , 1999) and superconducting cyclotron accelerators (Blosser et al , 1997), or novel linear accelerators, e.g.…”

“…Can novel techniques, such as proton arc therapy (Sandison et al , 1997; Sengbusch et al , 2009; Rechner et al , 2012), be developed to improve the quality of treatment, reduce treatment time, and increase cost-competitiveness and -effectiveness?…”

Proton Therapy Facility Planning From a Clinical and Operational Model

“…Further, higher energy leads to increased secondary neutrons and radioactivity both in the gantry and in the patient. Using Monte Carlo simulation it was shown that the total neutron fluence increases with E 3.76 where E is proton beam energy (16), and a reduction in beam energy from 240 MeV to 200 MeV decreased the secondary neutron fluence by 2.3 folds from 3.5 3 10 23 to 1.5 3 10 23 neutrons/p. There is no actual need for higher energies except potentially for proton computed tomography.…”