Semi-experimental assessment of neutron equivalent dose and secondary cancer risk for off-field organs in glioma patients undergoing 18-MV radiotherapy

Abstract

Neutron contamination as a source of out-of-field dose in radiotherapy is still of concern. High-energy treatment photons have the potential to overcome the binding energy of neutrons inside the nuclei. Fast neutrons emitting from the accelerator head can directly reach the patient’s bed. Considering that modern radiotherapy techniques can increase patient survival, concerns about unwanted doses and the lifetime risk of fatal cancer remain strong or even more prominent, especially in young adult patients. The current study addressed these concerns by quantifying the dose and risk of fatal cancer due to photo-neutrons for glioma patients undergoing 18-MV radiotherapy. In this study, an NRD model rem-meter detector was used to measure neutron ambient dose equivalent, H*(10), at the patient table. Then, the neutron equivalent dose received by each organ was estimated concerning the depth of each organ and by applying depth dose corrections to the measured H*(10). Finally, the effective dose and risk of secondary cancer were determined using NCRP 116 coefficients. Evidence revealed that among all organs, the breast (0.62 mSv/Gy) and gonads (0.58 mSv/Gy) are at risk of photoneutrons more than the other organs in such treatments. The neutron effective dose in the 18-MV conventional radiotherapy of the brain was 13.36 mSv. Among all organs, gonads (6.96 mSv), thyroid (1.86 mSv), and breasts (1.86 mSv) had more contribution to the effective dose, respectively. The total secondary cancer risk was estimated as 281.4 cases (per 1 million persons). The highest risk was related to the breast and gonads with 74.4 and, 34.8 cases per 1 million persons, respectively. Therefore, it is recommended that to prevent late complications (secondary cancer and genetic effects), these organs should be shielded from photoneutrons. This procedure not only improves the quality of the patient’s personal life but also the healthy childbearing in the community.

Association between radiotherapy and risk of second primary malignancies in patients with resectable lung cancer: a population-based study

“…Emerging studies have demonstrated that radiotherapy may substantially increase the risk of developing SPM in multiple cancers, such as breast cancer [ 13 ], nasopharyngeal carcinoma [ 14 ] and prostate cancer [ 15 ]. And some studies have investigated the optimal doses and theoretical strategies to reduce the risk of SPM for patients receiving radiotherapy [ 16 – 18 ]. The annual incidence of developing second primary lung cancer (SPLC) was 1.10% per patient, with an exceptionally high risk among young women with lung cancer [ 19 ].…”

How much should you worry about contaminant neutrons in spatially fractionated grid radiation therapy?

“…Since the threshold energy for (γ, n) reaction is at least 8 MeV [6], neutron contamination in radiation therapy is of concerns in treatment with high-energy photons (> 10 MV). Recently, many researchers in the field of radiation protection have addressed the role and consequences of unwanted doses to the patient caused by photoneutrons [7][8][9]. Some efforts focused on reducing neutron production in the linac head through optimization of the Bremsstrahlung target [10].…”

“…For example, the unwanted dose to the breasts of a patient undergoing 18 MV pelvic radiotherapy originates from photoneutrons by 67% [12]. Additionally, it has been shown that for glioma and hepatocellular patients undergoing 18 MV radiotherapy, secondary fatal cancer risk due to neutron contamination can reach 281 and 844 persons per one million persons, respectively [7,9]. For prostate patients undergoing 18 MV radiotherapy, it was reported that the total risk of secondary cancer in eye lenses, thyroid, and chiasma is 870 persons per one million persons [8].…”