Levels of leakage radiation from electron collimators of a linear accelerator

Out‐of‐field doses from radiotherapy can cause harmful side effects or eventually lead to secondary cancers. Scattered doses outside the applicator field, neutron source strength values, and neutron dose equivalents have not been broadly investigated for high‐energy electron beams. To better understand the extent of these exposures, we measured out‐of‐field dose characteristics of electron applicators for high‐energy electron beams on two Varian 21iXs, a Varian TrueBeam, and an Elekta Versa HD operating at various energy levels. Out‐of‐field dose profiles and percent depth‐dose curves were measured in a Wellhofer water phantom using a Farmer ion chamber. Neutron dose was assessed using a combination of moderator buckets and gold activation foils placed on the treatment couch at various locations in the patient plane on both the Varian 21iX and Elekta Versa HD linear accelerators. Our findings showed that out‐of‐field electron doses were highest for the highest electron energies. These doses typically decreased with increasing distance from the field edge but showed substantial increases over some distance ranges. The Elekta linear accelerator had higher electron out‐of‐field doses than the Varian units examined, and the Elekta dose profiles exhibited a second dose peak about 20 to 30 cm from central‐axis, which was found to be higher than typical out‐of‐field doses from photon beams. Electron doses decreased sharply with depth before becoming nearly constant; the dose was found to decrease to a depth of approximately E(MeV)/4 in cm. With respect to neutron dosimetry, Q values and neutron dose equivalents increased with electron beam energy. Neutron contamination from electron beams was found to be much lower than that from photon beams. Even though the neutron dose equivalent for electron beams represented a small portion of neutron doses observed under photon beams, neutron doses from electron beams may need to be considered for special cases.PACS number(s): 87.55.N‐, 87.55.ne, 87.56.bd, 87.56.jf

Section: Introductionmentioning

confidence: 99%

Out‐of‐field doses and neutron dose equivalents for electron beams from modern Varian and Elekta linear accelerators

Cárdenas

Nitsch

Kudchadker

et al. 2016

“…The applicators are designed to allow scattered electrons and transmission radiations to escape outside the treatment beam as little as possible. However, it is well known that a non‐ignorable fraction of scattered and transmission radiations can escape from the applicator and induce unintended doses outside the treatment field 6 , 7 , 8 , 9 , 10 , 11 , 12 …”

Section: Introductionmentioning

confidence: 99%

“…However, it is well known that a non-ignorable fraction of scattered and transmission radiations can escape from the applicator and induce unintended doses outside the treatment field. (6)(7)(8)(9)(10)(11)(12) The two main components of out-of-field doses outside the applicator are bremsstrahlung photon contamination and scattered electrons. (11,13) The bremsstrahlung photons can be produced in different structures of the accelerator head and in the patient irradiated volume.…”

Section: Introductionmentioning

confidence: 99%

“…(11) Several experimental investigations have studied the dose and the leakage levels outside the applicator for older electron applicators. (6)(7)(8)(9)(10) Another study has been performed on the peripheral dose for a modern Varian-type applicator. (11) Those authors found, for 4 MeV electron beams, a peak dose appearing at about 7 cm from the beam edge outward.…”

Section: Introductionmentioning

confidence: 99%

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Experimental assessment of out‐of‐field dose components in high energy electron beams used in external beam radiotherapy

Alabdoaburas

Mège

Chavaudra

et al. 2015

The purpose of this work was to experimentally investigate the out‐of‐field dose in a water phantom, with several high energy electron beams used in external beam radiotherapy (RT). The study was carried out for 6, 9, 12, and 18 MeV electron beams, on three different linear accelerators, each equipped with a specific applicator. Measurements were performed in a water phantom, at different depths, for different applicator sizes, and off‐axis distances up to 70 cm from beam central axis (CAX). Thermoluminescent powder dosimeters (TLD‐700) were used. For given cases, TLD measurements were compared to EBT3 films and parallel‐plane ionization chamber measurements. Also, out‐of‐field doses at 10 cm depth, with and without applicator, were evaluated. With the Siemens applicators, a peak dose appears at about 12–15 cm out of the field edge, at 1 cm depth, for all field sizes and energies. For the Siemens Primus, with a 10×10cm2 applicator, this peak reaches 2.3%, 1%, 0.9% and 1.3% of the maximum central axis dose (Dmax) for 6, 9, 12 and 18 MeV electron beams, respectively. For the Siemens Oncor, with a 10×10cm2 applicator, this peak dose reaches 0.8%, 1%, 1.4%, and 1.6% of Dmax for 6, 9, 12, and 14 MeV, respectively, and these values increase with applicator size. For the Varian 2300C/D, the doses at 12.5 cm out of the field edge are 0.3%, 0.6%, 0.5%, and 1.1% of Dmax for 6, 9, 12, and 18 MeV, respectively, and increase with applicator size. No peak dose is evidenced for the Varian applicator for these energies. In summary, the out‐of‐field dose from electron beams increases with the beam energy and the applicator size, and decreases with the distance from the beam central axis and the depth in water. It also considerably depends on the applicator types. Our results can be of interest for the dose estimations delivered in healthy tissues outside the treatment field for the RT patient, as well as in studies exploring RT long‐term effects.PACS number(s): 87.53.Bn, 87.56.bd, 87.56.J‐

“…Previous studies have shown that the leakage radiation on the outside surface of an applicator sidewall or just outside the applicator body can be prohibitively high. ( 1 – 5 ) This, in turn, leads to unacceptably high peripheral doses in the patient plane ( 1 – 3 ) and can be detrimental to the patient, especially when sensitive structures and/or the patient's skin is in close proximity to the applicator sidewall – as in the treatment of breast cancer involving the internal mammary lymph node chain (IMC) using a combination of parallel‐opposed photon tangents and an oblique “electron patch”. ( 6 , 7 ) Other clinical situations where the applicator leakage poses a significant risk to patients include exposure of the shoulders to leakage radiation when treating neck nodes at extended SSD, and exposure of the legs to leakage during testicular boosts for TBI.…”

Section: Introductionmentioning

confidence: 99%

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

Yeboah¹,

Karotki²,

Hunt³

et al. 2010

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‐