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 x 10 - 25 x 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 >or=20 x 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 <or=40 degrees . 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 18MeV beams, respectively. This sidewall modification had an insignificant effect on the clinical depth dose, cross-axis beam profiles, and output factors.

Figure 1

An EA220 ($20\times 20\hspace{0.17em}{\text{cm}}^2$) electron applicator used with Siemens Primus accelerators.

Figure 2

Schematic of the setup employed for measuring leakage radiation in the vicinity of applicator body.

Figure 3

Schematic of the experimental setup used for measuring peripheral doses. Measurements performed as a function of angle of beam obliquity, Θ, had isocenter‐to‐phantom surface distance, t, held constant at 5 cm.

Figure 4

Leakage at a lateral distance of 2 cm from applicator body (relative to central‐axis ionization maximum in a water‐equivalent phantom) as a function of vertical position along applicator: (a) front side of $10\times 10\hspace{0.17em}{\text{cm}}^2$ applicator; (b) front side of $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicator.

Figure 5

Dependence of leakage in the vicinity of applicator sidewall on lateral distance from applicator body. The presented data, acquired at the front side of a $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicator, are normalized to the central‐axis ionization maximum in a water‐equivalent phantom.

Figure 6

Dependence of peripheral dose in the patient plane on off‐axis distance from the field edge for (a) $10\hspace{0.17em}\times \hspace{0.17em}10$, (b) $15\hspace{0.17em}\times \hspace{0.17em}15$, (c) $20\hspace{0.17em}\times \hspace{0.17em}20$, and (d) $25\times 25\hspace{0.17em}{\text{cm}}^2$ applicators. Measurements were performed at a depth of 1 cm in a water phantom, at 100 cm SSD.

Figure 7

Dependence of peripheral dose on angle of beam obliquity for $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicator. Measurement was performed at the right side of applicator at a depth of 1 cm in water using isocenter‐to‐phantom surface distance, $\mathrm{t}=5\hspace{0.17em}\text{cm}$. Data obtained for normal beam incidence are compared to those acquired for a 40° obliquely incident beam.

Figure 8

Dependence of peripheral dose on angle of beam obliquity for (a) $10\times 10\hspace{0.17em}{\text{cm}}^2$, (b) $15\times 15\hspace{0.17em}{\text{cm}}^2$, (c) $20\times 20\hspace{0.17em}{\text{cm}}^2$, and (d) $25\times 25\hspace{0.17em}{\text{cm}}^2$ applicators. Using a constant isocenter‐to‐phantom surface distance, $\mathrm{t}=5\hspace{0.17em}\text{cm}$, measurements were performed at 1 cm depth in water at off‐axis distances of 14 cm and 10 cm from the field edge for electron beam energies of $\le \hspace{0.17em}12\hspace{0.17em}\text{MeV}$ and $\ge \hspace{0.17em}15\hspace{0.17em}\text{MeV}$, respectively.

Figure 9

Depth‐peripheral dose curves in water due to leakage radiation from $10\times 10\hspace{0.17em}{\text{cm}}^2$ applicator. These data were acquired at the front side of applicator, in the patient plane, along an axis parallel to the central axis but located at a lateral distance of 17 cm from it.

Figure 10

Depth‐peripheral dose curves in water due to leakage radiation from $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicator. These data were acquired at the front side of applicator, in the patient plane, along an axis parallel to the central axis but located at a lateral distance of 22 cm from it.

Figure 11

Transmission of applicator leakage radiation from $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicator through Xenolite‐NL for normally incident primary electron beams. The presented data are expressed as percentages of central‐axis “dose” maxima in solid water. The measurements were performed in solid water at depth of 0.2 cm and at a lateral distance of 22 cm from the central axis.

Figure 12

Ratio of transmitted leakage from $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicator through a given thickness of Xenolite‐NL, measured at a depth of 0.2 cm of solid water, to the leakage at the same depth of solid water in the absence of Xenolite‐NL attenuating material.

Figure 13

Comparison of peripheral‐dose distance curves measured at a depth of 1 cm in water on the right side of modified sidewall and standard $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicators. An oblique incident angle of 40° and a collimator angle of 0° were employed.

Figure 14

Depth‐peripheral doses measured on the front side of modified sidewall and standard $20\times 20\hspace{0.17em}{\text{cm}}^2$ applicators. The depth was measured along an axis perpendicular to the phantom surface and intersecting the surface at a lateral distance of 10 cm from the field edge. The setup utilized a gantry angle of 40° and a collimator angle of 90°.