Monte Carlo modeling of proton therapy installations: a global experimental method to validate secondary neutron dose calculations

Abstract

Monte Carlo calculations are increasingly used to assess stray radiation dose to healthy organs of proton therapy patients and estimate the risk of secondary cancer. Among the secondary particles, neutrons are of primary concern due to their high relative biological effectiveness. The validation of Monte Carlo simulations for out-of-field neutron doses remains however a major challenge to the community. Therefore this work focused on developing a global experimental approach to test the reliability of the MCNPX models of two proton therapy installations operating at 75 and 178 MeV for ocular and intracranial tumor treatments, respectively. The method consists of comparing Monte Carlo calculations against experimental measurements of: (a) neutron spectrometry inside the treatment room, (b) neutron ambient dose equivalent at several points within the treatment room, (c) secondary organ-specific neutron doses inside the Rando-Alderson anthropomorphic phantom. Results have proven that Monte Carlo models correctly reproduce secondary neutrons within the two proton therapy treatment rooms. Sensitive differences between experimental measurements and simulations were nonetheless observed especially with the highest beam energy. The study demonstrated the need for improved measurement tools, especially at the high neutron energy range, and more accurate physical models and cross sections within the Monte Carlo code to correctly assess secondary neutron doses in proton therapy applications.

Measurement of stray radiation within a scanning proton therapy facility: EURADOS WG9 intercomparison exercise of active dosimetry systems

“…[1][2][3] Neutrons are of particular concern owing to their high relative biological effectiveness 4 and have thus been extensively studied in the past. [5][6][7][8] Neutron ambient dose-equivalent, H * (10) has been measured by several authors using a wide variety of proportional counters. 5,6,8,9 Similarly, measurements of neutron spectral fluence within a proton therapy facility were previously done using dedicated Bonner sphere spectrometry (BSS) systems.…”

“…[5][6][7][8] Neutron ambient dose-equivalent, H * (10) has been measured by several authors using a wide variety of proportional counters. 5,6,8,9 Similarly, measurements of neutron spectral fluence within a proton therapy facility were previously done using dedicated Bonner sphere spectrometry (BSS) systems. 5,8,10,11 Neutron organ dose measurements have also been conducted using passive detectors such as superheated emulsions (bubble detectors), thermoluminescent detector (TLD), and solid state nuclear track detector (SSNTD) mainly.…”

“…5,6,8,9 Similarly, measurements of neutron spectral fluence within a proton therapy facility were previously done using dedicated Bonner sphere spectrometry (BSS) systems. 5,8,10,11 Neutron organ dose measurements have also been conducted using passive detectors such as superheated emulsions (bubble detectors), thermoluminescent detector (TLD), and solid state nuclear track detector (SSNTD) mainly. 3,8,[12][13][14] On the one hand, all these studies have shown that stray neutron radiation involved in proton therapy is not negligible (up to 10 mSv/proton Gy outside the clinical target volume), is facility dependent (with major effects arising from beam line elements and shielding design), and implicates neutrons of high energy (equal to the proton beam energy).…”

Conception of a New Recoil Proton Telescope for Real-Time Neutron Spectrometry in Proton-Therapy

“…For a proton-therapy spectrum corresponding to an irradiation of the head with a 178 MeV proton beam [13], a reconstruction of the neutron spectrum is obtained between 5 to 30 MeV with a 16.3 ± 0.3% resolution. As can be seen in figure 7, the main reason for this resolution is the high level of background that can be seen in the tail corresponding to a ∆E/E smaller than -0.4.…”

“…However, it requires a complex procedure of deconvolution and both the measurement and the deconvolution are time-consuming, which excludes a real-time measurement. More problematic for this application, it is cumbersome and it has been proven in [13] to be limited to a fluence of 4 × 10 4 n/cm 2 /s, which is two orders of magnitudes below the 6 × 10 6 n/cm 2 /s that can be found in treatment rooms [1]. Because of that, the Bonner Sphere System cannot be used in treatment rooms, and is far from being optimal for multiparameter studies.…”