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Variations of lung density and geometry on inhomogeneity correction algorithms: A Monte Carlo dosimetric evaluation
Author(s) -
Chow James C. L.,
Leung Michael K. K.,
Van Dyk Jake
Publication year - 2009
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3168966
Subject(s) - monte carlo method , algorithm , computer science , geometry , mathematics , medical physics , physics , statistics
This work contributed the following new information to the study of inhomogeneity correction algorithm: (1) Evaluation of lung dose calculation methods as a function of lung relative electron density( ρ e , lung)and treatment geometry and (2) comparison of doses calculated using the collapsed cone convolution (CCC) and adaptive convolution (AC) in lung using the Monte Carlo (MC) simulation with the EGSnrc‐based code. The variations ofρ e , lungand geometry such as the position and dimension of the lung were studied with different photon beam energies and field sizes. Three groups of inhomogeneous lung phantoms, namely, “slab,” “column,” and “cube,” with different positions, volumes, and shapes of lung in water as well as clinical computed tomography lung images were used. Theρ e , lungin each group of phantoms vary from 0.05 to 0.7. 6 and 18 MV photon beams with small( 4 × 4cm 2 )and medium( 10 × 10cm 2 )field sizes produced by a Varian 21 EX linear accelerator were used. This study reveals that doses in the inhomogeneous lung calculated by the CCC match well with those by AC within ±1%, indicating that the AC, with an advantage of shorter computing times (three to four times shorter than CCC), is a good substitute for CCC. Comparing the CCC and AC to MC in general, significant dose deviations are found when theρ e , lungis ≤ 0.3 . The degree of deviation depends on the photon beam energy and field size and is relatively large when high‐energy photon beams with small fields are used. For penumbra widths (20%–80%), the CCC and AC agree well with MC for the slab and cube phantoms with the lung volumes at the central beam axis (CAX). However, deviations( > 2 mm )occur in the column phantoms, with two lung volumes separated by a unit density column along the CAX in the middle using the 18 MV beam with 4 × 4cm 2field forρ e , lung ≤ 0.1 . This study provides new dosimetric data to evaluate the impact of the variations ofρ e , lungand geometry on dose calculations in inhomogeneous media using CCC and AC.