Quantification and reduction of peripheral dose from leakage radiation on Siemens primus accelerators in electron therapy mode

Collins Yeboah,Dylan Hunt,Alex Karotki,Rick Holly

doi:10.1120/jacmp.v11i3.3105

Collins Yeboah, Dylan Hunt + Show 2 more

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https://doi.org/10.1120/jacmp.v11i3.3105

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Abstract

In this work, leakage radiation from EA200 series electron applicators on Siemens Primus accelerators is quantified, and its penetration ability in water and/or the shielding material Xenolite‐NL established. Initially, measurement of leakage from 10×10−25×25 cm2 applicators was performed as a function of height along applicator and of lateral distance from applicator body. Relative to central‐axis ionization maximum in solid water, the maximum leakage in air observed with a cylindrical ion chamber with 1 cm solid water buildup cap at a lateral distance of 2 cm from the front and right sidewalls of applicators were 17% and 14%, respectively; these maxima were recorded for 18 MeV electron beams and applicator sizes of ≥20×20 cm2. In the patient plane, the applicator leakage gave rise to a broad peripheral dose off‐axis distance peak that shifted closer to the field edge as the electron energy increases. The maximum peripheral dose from normally incident primary electron beams at a depth of 1 cm in a water phantom was observed to be equal to 5% of the central‐axis dose maximum and as high as 9% for obliquely incident beams with angles of obliquity ≤ 40°. Measured depth‐peripheral dose curves showed that the “practical range” of the leakage electrons in water varies from approximately 1.4 to 5.7 cm as the primary electron beam energy is raised from 6 to 18 MeV. Next, transmission measurements of leakage radiation through the shielding material Xenolite‐NL showed a 4 mm thick sheet of this material is required to attenuate the leakage from 9 MeV beams by two‐thirds, and that for every additional 3 MeV increase in the primary electron beam energy, an additional Xenolite‐NL thickness of roughly 2 mm is needed to achieve the aforementioned attenuation level. Finally, attachment of a 1 mm thick sheet of lead to the outer surface of applicator sidewalls resulted in a reduction of the peripheral dose by up to 80% and 74% for 9 and 18 MeV beams, respectively. This sidewall modification had an insignificant effect on the clinical depth dose, cross‐axis beam profiles, and output factors.PACS numbers: 87.53.Bn, 87.56.bd, 87.56.J‐

Highlights

155 Yeboah et al.: Quantification of peripheral dose lateral spread of the beam that occurs in the intervening air volume from inside the accelerator head to the patient surface
As the incident electron beam energy increases, the leakage detected around the top region of applicator sidewall increases to a maximum at 9 MeV and thereafter decreases with increasing incident energy (Fig 4(a-b))
It follows that leakage radiation in the region surrounding the lower half of applicator body will increase with incident electron energy

Summary

Introduction

155 Yeboah et al.: Quantification of peripheral dose lateral spread of the beam that occurs in the intervening air volume from inside the accelerator head to the patient surface. The goal of this work was to characterize the leakage radiation from the EA200 series electron applicators on Siemens Primus accelerators, quantify the peripheral dose due to the leakage radiation in the patient plane, and find strategies for minimizing the leakage radiation, such as ascertaining whether placement of bolus and/or an attenuating material outside the field on a patient’s skin (e.g. over the contra-lateral breast in IMC treatments) reduces the dose to the skin from leakage radiation. The results of measurements of leakage radiation in the vicinity of applicator body for various combinations of electron energies and applicator sizes are presented and discussed This is followed by a presentation and discussion of peripheral doses measured in the patient plane as a function of off-axis distance from field edge, angle of obliquity of incident beams, and depth in a water phantom. An investigation was conducted to ascertain whether or not the applicator leakage and/or the resulting peripheral dose to a patient could be reduced by modification of applicator sidewalls

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