Brass‐mesh‐bolus–Radiation safety analysis and activation characterization following high energy photon treatment

TL;DR

This study investigates the radiation risks of using brass-mesh bolus in high-energy photon treatments, finding that photo-neutron activation poses a greater exposure risk than previously assumed neutron-capture activation.

Contribution

The study introduces photo-neutron activation as a novel and dominant source of radiation exposure from brass-mesh bolus in high-energy treatments.

Findings

Photo-neutron activation produces radionuclides like Cu-62 and Zn-63, which emit positrons and annihilation photons.

Handling brass-mesh bolus for 30 seconds per treatment could expose therapists to 0.106 mrem per course.

Positron emission from photo-neutron activation has significant penetration due to annihilation photons.

Abstract

Brass‐mesh‐bolus (BMB) has been proposed as an alternative to water‐equivalent bolus due to its ease of setup and conformality to patient contour. In high‐energy beams, BMB can become radioactive and pose a potential exposure risk to therapists from frequent exposure while handling. Very little research has been performed on the method of activation and dose from clinically realistic activation of BMB, necessitating the need for further investigation.

To determine the expected activation of the BMB, products via neutron‐capture were calculated using thermal‐neutron cross‐section tables, as has been performed in previous literature. A novel consideration is to include the photo‐neutron activation components. Measurements were performed using an NaI‐scintillator after BMB irradiation, assessing its activity for both in‐field and out‐of‐field activation. The collected gamma spectrum was compared to expected peaks based on differing activation modes. Energy peaks are isolated for half‐life measurements to find relative ratios and potential long‐term activation risk. Potential exposure to workers after multiple irradiation events was measured using a Ludlum 9DP and BeO OSLDs.

Expected radionuclide products are predicted depending on activation mode: Cu‐64, Cu‐66, Zn‐65, and Zn‐69 via neutron capture and Cu‐62, Cu‐64, Zn‐63, and Zn‐65 via photo‐neutron activation. Gamma‐spectrum analysis shows that photo‐neutron activation is the dominant source of exposure with decay via positron emission. The effective half‐life of the 511 keV positron annihilation peak is measured as 12.16 min, mostly from Cu‐62 and Zn‐63. Survey measurements indicate the dose to workers of the BMB to be 0.106 mrem per course of a 25 fx, 400 MU/fx, 15 MV treatment if the therapist handles the BMB every fx for 30 s. Using the OSLDs, under the same conditions but handling the bolus for 1 min per fraction, skin dose was estimated as 9.23 mrem per course.

The dominant source of exposure from BMB is the result of photo‐neutron activation rather than neutron‐capture, as was historically assumed. Penetration of the resultant positron emission is considerable given the presence of annihilation photons compared to previously assumed beta decay and should be considered for exposure to workers.

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