Investigation of photon beam models in heterogeneous media of modern radiotherapy

Abstract

This study investigates the performance of photon beam models in dose calculations involving heterogeneous media in modern radiotherapy. Three dose calculation algorithms implemented in the CMS FOCUS treatment planning system have been assessed and validated using ionization chambers, thermoluminescent dosimeters (TLDs) and film. The algorithms include the multigrid superposition (MGS) algorithm, fast Fourier Transform Convolution (FFTC) algorithm and Clarkson algorithm. Heterogeneous phantoms used in the study consist of air cavities, lung analogue and an anthropomorphic phantom. Depth dose distributions along the central beam axis for 6 MV and 10 MV photon beams with field sizes of 5 cm x 5 cm and 10 cm x 10 cm were measured in the air cavity phantoms and lung analogue phantom. Point dose measurements were performed in the anthropomorphic phantom. Calculated results with three dose calculation algorithms were compared with measured results. In the air cavity phantoms, the maximum dose differences between the algorithms and the measurements were found at the distal surface of the air cavity with a 10 MV photon beam and a 5 cm x 5 cm field size. The differences were 3.8%. 24.9% and 27.7% for the MGS. FFTC and Clarkson algorithms. respectively. Experimental measurements of secondary electron build-up range beyond the air cavity showed an increase with decreasing field size, increasing energy and increasing air cavity thickness. The maximum dose differences in the lung analogue with 5 cm x 5 cm field size were found to be 0.3%. 4.9% and 6.9% for the MGS. FFTC and Clarkson algorithms with a 6 MV photon beam and 0.4%. 6.3% and 9.1% with a 10 MV photon beam, respectively. In the anthropomorphic phantom, the dose differences between calculations using the MGS algorithm and measurements with TLD rods were less than +/-4.5% for 6 MV and 10 MV photon beams with 10 cm x 10 cm field size and 6 MV photon beam with 5 cm x 5 cm field size, and within +/-7.5% for 10 MV with 5 cm x 5 cm field size, respectively. The FFTC and Clarkson algorithms overestimate doses at all dose points in the lung of the anthropomorphic phantom. In conclusion, the MGS is the most accurate dose calculation algorithm of investigated photon beam models. It is strongly recommended for implementation in modern radiotherapy with multiple small fields when heterogeneous media are in the treatment fields.

The accuracy of the pencil beam convolution and anisotropic analytical algorithms in predicting the dose effects due to attenuation from immobilization devices and large air gaps

“…Examples of the type of air gap investigated in these studies were ͑i͒ small rectangular, 5-10 ͑ii͒ triangular, 10 and ͑iii͒ cylindrical 11 cavities or channels. Investigations into slab air gaps of up to 5 cm thick have also been reported in several studies 7,9,11,12 and up to 10 cm in one study. 13 However, the dose at only one point beyond the air gap was investigated in the latter case.…”

“…Previous investigations into the dose beyond air gaps have only extended to a depth of 4 cm beyond the air gap. [5][6][7][8][9][10][11][12][13][14] II. METHODS…”

Dose prediction accuracy of collapsed cone convolution superposition algorithm in a multi-layer inhomogenous phantom

“…3,4 Several authors have conducted the evaluation of dose calculation algorithms for external beam radiation therapy. [5][6][7][8][9][10][11][12][13][14][15][16] Rana et al 16 investigated the dose prediction accuracy of Acuros XB algorithm and anisotropic analytical algorithm (AAA) for different field sizes and air gap thicknesses. The results from that study 16 revealed that dose predictions errors up to 3.8% for Acuros XB and up to 10.9% for AAA could occur during radiation treatment.…”

Dosimetry of a Small Air Cavity for Clinical Electron Beams: A Monte Carlo Study

“…9 -12 For photon radiotherapy in particular, there exists a large body of work evaluating the effect of an air cavity on the dose distribution in various inhomogeneous phantoms. [13][14][15][16][17] Dosimetric issues such as small photon fields for intensitymodulated radiotherapy and verifications/comparisons of dose calculation algorithms using Monte Carlo simulations have been studied thoroughly. 18 -21 This is not the case for electron radiotherapy.…”

“…PDD curves for different widths (w) of air cavities (thickness ϭ 1 cm) using electron beams with energies of (a) 6, (b) 9, and (c)16 MeV. The depths of air cavities are located at 2, 3, and 5 cm for the 6-, 9-, and 16-MeV electron beams, respectively.…”