Calculating percent depth dose with the electron pencil‐beam redefinition algorithm

Abstract

In the present work, we investigated the accuracy of the electron pencil‐beam redefinition algorithm (PBRA) in calculating central‐axis percent depth dose in water for rectangular fields. The PBRA energy correction factor C(E) was determined so that PBRA‐calculated percent depth dose best matched the percent depth dose measured in water. The hypothesis tested was that a method can be implemented into the PBRA that will enable the algorithm to calculate central‐axis percent depth dose in water at a 100‐cm source‐to‐surface distance (SSD) with an accuracy of 2% or 1‐mm distance to agreement for rectangular field sizes ≥2×2 cm. Preliminary investigations showed that C(E), determined using a single percent depth dose for a large field (that is, having side‐scatter equilibrium), was insufficient for the PBRA to accurately calculate percent depth dose for all square fields ≥2×2 cm. Therefore, two alternative methods for determining C(E) were investigated. In Method 1, C(E), modeled as a polynomial in energy, was determined by fitting the PBRA calculations to individual rectangular‐field percent depth doses. In Method 2, C(E) for square fields, described by a polynomial in both energy and side of square W [that is, C=C(E,W)], was determined by fitting the PBRA calculations to measured percent depth dose for a small number of square fields. Using the function C(E, W), C(E) for other square fields was determined, and C(E) for rectangular field sizes was determined using the geometric mean of C(E) for the two measured square fields of the dimension of the rectangle (square root method). Using both methods, PBRA calculations were evaluated by comparison with measured square‐field and derived rectangular‐field percent depth doses at 100‐cm SSD for the Siemens Primus radiotherapy accelerator equipped with a 25×25‐cm applicator at 10 MeV and 15 MeV. To improve the fit of C(E) and C(E, W) to the electron component of percent depth dose, it was necessary to modify the PBRA's photon depth dose model to include dose buildup. Results showed that, using both methods, the PBRA was able to predict percent depth dose within criteria for all square and rectangular fields. Results showed that second‐ or third‐order polynomials in energy (Methods 1 and 2) and in field size (Method 2) were typically required. Although the time for dose calculation using Method 1 is approximately twice that using Method 2, we recommend that Method 1 be used for clinical implementation of the PBRA because it is more accurate (most measured depth doses predicted within approximately 1%) and simpler to implement.PACS number: 87.53.Fs

Lineer Hızlandırıcı Cihazında (Linak) Alınan Rölatif Doz Ölçümlerinde ≤ 3 mm Geometrik Hataların Yüzde Derin Doz (%DD) ve Işın Profil (IP) Parametreleri Üzerine Etkisi

“…Tüm hızlandırıcılarda, enerji spektrumunu değiştirmek düzleştirici filtreler ile sağlanır. Bu etkinin büyüklüğü kullanılan malzemeye ve kalınlığına bağlıdır [2,3]. Bu faktörlere ek olarak, ikincil radyasyonların spektrumunu ve büyüklüğünü değiştiren kolimatör sisteminin tasarımı gibi başka faktörler de vardır [4].…”

Validation of the final aperture superposition technique to calculate electron output factors and depth dose curves

“…Several analytical methods have been proposed to calculate the PDD and ROF, alleviating the need to perform a time-consuming measurement for each patient insert. [1][2][3][4][5][6][7] These techniques utilize measured depth dose curves and output factors tabulated for several different field sizes. Alternately, Monte Carlo calculations can be used to simulate the required measured data.…”

“…Alternately, Monte Carlo calculations can be used to simulate the required measured data. 7 The sector integration, 2 pencil beam redefinition algorithm, 3 and nth root 4 methods require extensive tabulated data from which the dose from an irregularly shaped field is extrapolated. The two-source 1 and the lateral buildup ratio [5][6][7] methods use smaller sets of measured data to determine model parameters.…”

Estudo de dosimetria gel polimérica em radioterapia com feixes de elétrons utilizando ressonância magnética e simulação Monte Carlo

“…A figura 3, mostra as curvas de isodoses para um feixe de 9 MeV obtidos com campo de irradiação de 10 x 10 cm 2 e de distância fonte superfície de 100 cm. (BOYD et al, 2001;DING et al, 2005;CHI et al, 2006;PRICE et al, 2007).…”