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

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‐

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

“…Because the dose is deposited by primary electrons, they only penetrate a few centimeters. The dose decreases rapidly, such that by ~2–6 cm depth, there is only residual bremsstrahlung dose (which is about ~10% of the superficial electron dose) …”

“…In general, the out‐of‐field dose from electron therapy increases with treatment energy. It also increases with obliquity (i.e., with the gantry tilted toward the measurement point) but varies little with applicator size …”

“…The addition of lead shielding around electron applicators has also been suggested, and it has been shown to substantially reduce secondary radiation associated with electron therapy. Even 1 mm of lead can reduce the out‐of‐field dose by 70–80%, particularly near the field edge . Potential scattering from such shielding back into the primary treatment field, thereby affecting in‐field characteristics such as output and profile shape, has been raised as a concern with changing the nature of the applicator design …”

“…The dose decreases rapidly, such that by~2-6 cm depth, there is only residual bremsstrahlung dose (which is about~10% of the superficial electron dose). 154 Particularly for the Siemens accelerator and the Elekta Versa HD, out-of-field doses from electron therapy are relatively high compared with doses from photon therapy (even IMRT). In general, the out-of-field dose from electron FIG.…”

Management of radiotherapy patients with implanted cardiac pacemakers and defibrillators: A Report of the AAPM TG‐203†

“…Out‐of‐field radiation is electrons, and therefore not very penetrative, so the dose is predominantly limited to the patient surface on which the electron beam is incident (i.e., for an anterior electron field, the anterior patient surface receives the bulk of the out‐of‐field dose). However, CIEDs are superficially implanted, and the out‐of‐field electron dose has been measured to be as high as 3% of the given dose at dmax, extending at this level to beyond 30 cm from the field edge . The cumulative doses for superficial locations in the patient could potentially reach as much as 2 Gy.…”