Monte Carlo simulation estimates of neutron doses to critical organs of a patient undergoing x‐ray LINAC‐based radiotherapy

Abstract

Absorbed photoneutron dose to patients undergoing 18 MV x-ray therapy was studied using Monte Carlo simulations based on the MCNPX code. Two separate transport simulations were conducted, one for the photoneutron contribution and another for neutron capture gamma rays. The phantom model used was of a female patient receiving a four-field pelvic box treatment. Photoneutron doses were determinate to be higher for organs and tissues located inside the treatment field, especially those closest to the patient's skin. The maximum organ equivalent dose per x-ray treatment dose achieved within each treatment port was 719 microSv/Gy to the rectum (180 degrees field), 190 microSv/Gy to the intestine wall (0 degrees field), 51 microSv/Gy to the colon wall (90 degrees field), and 45 microSv/Gy to the skin (270 degrees field). The maximum neutron equivalent dose per x-ray treatment dose received by organs outside the treatment field was 65 microSv/Gy to the skin in the antero-posterior field. A mean value of 5 +/- 2 microSv/Gy was obtained for organs distant from the treatment field. Distant organ neutron equivalent doses are all of the same order of magnitude and constitute a good estimate of deep organ neutron equivalent doses. Using the risk assessment method of the ICRP-60 report, the greatest likelihood of fatal secondary cancer for a 70 Gy dose is estimated to be 0.02% for the pelvic postero-anterior field, the rectum being the organ representing the maximum contribution of 0.011%.

AAPM TG158: Measurement and calculation of doses outside the treated volume from external‐beam radiation therapy

“…Such models have shown agreement with fluence measurements within 20% on average and with measured dose‐equivalent metrics within 10% on average . As a simplification, it is also possible to represent the linac as a beam‐line structure (or neutron source) encased in heavy metal shells or an otherwise simplified shielding structure . Such an approach makes the modeling much easier, but as it is a simplification, it results in poorer agreement with measurement.…”

“…93 As a simplification, it is also possible to represent the linac as a beam-line structure (or neutron source) encased in heavy metal shells or an otherwise simplified shielding structure. 272,278,279 Such an approach makes the modeling much easier, but as it is a simplification, it results in poorer agreement with measurement. Fluence calculations and dose equivalents have been found to agree within~40% as compared to complete models and measurements.…”

Neutron spectra and dose equivalents calculated in tissue for high‐energy radiation therapy

“…In evaluating the total dose equivalent to a particular point in the patient, secondary gammas must be accounted for as they may deposit a comparable dose equivalent to the neutrons, particularly at deeper depths. 9 Generally, in an evaluation of the total dose equivalent, these secondary gammas are accounted for through photon measurements carried out using a separate, photon-specific dosimeter. 5,7,8 Because a photon-specific dosimeter does not distinguish between the secondary gammas arising from neutron capture and photons due to head leakage or patient scatter, all photon doses are generally bundled together.…”

“…These neutrons pose a health risk for any patient undergoing high-energy radiation therapy because they increase the out-of-field radiation dose and, thus, the corresponding risk of secondary malignancies. [5][6][7][8][9] Studies have measured or used Monte Carlo to calculate neutron fluence and energy spectrum, but the majority of these studies have evaluated such neutron properties in air or on the patient's surface rather than within a phantom or patient where organs at risk are actually found. [5][6][7][10][11][12][13] This observation is not surprising because measurements within a patient or even a phantom are very difficult due to substantial challenges in neutron detection.…”

Calibration of a neutron detector based on single event upset of SRAM memories

“…Production of secondary particles, like neutrons, protons and nuclear fragments during radiotherapy (RT) with high energy photon and ion beams can contribute significantly to the tumour and healthy tissue dose and, consequently, increase the risk of the induction of secondary tumours (Barquero et al, 2005;Hall and Wuu, 2003;Kry et al, 2005;Vanhavere et al, 2004;Ruben et al, 2008;Hall, 2009). In particular, delivery of modern Intensity Modulated Radiotherapy (IMRT) treatments with linac primary electron beams above 8e10 MeV, with a large total number of Monitor Units (MU), may lead to photo-neutron fluences in excess of 10 8 cm À2 at the isocenter (Pena et al, 2005).…”