Abstract
The shielding calculations for high energy (> 10 MV) linear accelerators must include the photoneutron production within the head of the accelerator. Procedures have been described to calculate the treatment room door shielding based on the neutron source strength (Q value) for a specific accelerator and energy combination. Unfortunately, there is currently little data in the literature stating the neutron source strengths for the most widely used linear accelerators. In this study, the neutron fluence for 36 linear accelerators, including models from Varian, Siemens, Elekta/Philips, and General Electric, was measured using gold‐foil activation. Several of the models and energy combinations had multiple measurements. The neutron fluence measured in the patient plane was independent of the surface area of the room, suggesting that neutron fluence is more dependent on the direct neutron fluence from the head of the accelerator than from room scatter. Neutron source strength, Q, was determined from the measured neutron fluences. As expected, Q increased with increasing photon energy. The Q values ranged from 0.02 for a 10 MV beam to 1.44(×1012) neutrons per photon Gy for a 25 MV beam. The most comprehensive set of neutron source strength values, Q, for the current accelerators in clinical use are presented for use in calculating room shielding.PACS number(s): 87.53.–j, 87.52.–g
Highlights
In most radiotherapy facilities, the older low-energy linear acceleratorslinacsare being replaced with new dual-energy linacs with photon beams у10 MV; as well, new treatment rooms are being built to accommodate the new dual-energy linacs
In addition to the Q values for the new linac and energy combinations, five linac/energy combinations already had Q values listed in the literature
Apparent exceptions to the good agreement of the results are the relationship between the Q values for the Elekta SL25 22 and 25 MV beams, which are 2.37ϫ1012 and 1.44ϫ1012, respectively, and the Philips SL20 17 and 18 MV beam’s Q values that are 0.69ϫ1012 and 0.46ϫ1012, respectively
Summary
The older low-energy linear acceleratorslinacsare being replaced with new dual-energy linacs with photon beams у10 MV; as well, new treatment rooms are being built to accommodate the new dual-energy linacs. The number of linacs with high-energy photon beamsу MVis increasing. These new linacs have the capacity to produce photoneutrons in the target, flattening filters and collimating devices if operated at energies above 10 MeV. The neutron component in treatment rooms where photon energies у15 MV are produced is significant and, as such, extra shielding is required. The data required to perform the shielding calculations for the neutrons, especially the shielding in the treatment door, are lacking. McGinley described a method to calculate the amount of shielding needed in the treatment door to account for photoneutron production using what he referred to as the neutron source strength for a specific linac.
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