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A CT‐based analytical dose calculation method for HDR I 192 r brachytherapy
Author(s) -
Poon Emily,
Verhaegen Frank
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 - 10.1118/1.3184695
Subject(s) - imaging phantom , brachytherapy , monte carlo method , dosimetry , voxel , nuclear medicine , photon , attenuation , physics , radiation treatment planning , computational physics , materials science , optics , mathematics , radiation therapy , medicine , radiology , statistics
Purpose: This article presents an analytical dose calculation method for high‐dose‐rate I192 r brachytherapy, taking into account the effects of inhomogeneities and reduced photon backscatter near the skin. The adequacy of the Task Group 43 (TG‐43) two‐dimensional formalism for treatment planning is also assessed. Methods: The proposed method uses material composition and density data derived from computed tomography images. The primary and scatter dose distributions for each dwell position are calculated first as if the patient is an infinite water phantom. This is done using either TG‐43 or a database of Monte Carlo (MC) dose distributions. The latter can be used to account for the effects of shielding in water. Subsequently, corrections for photon attenuation, scatter, and spectral variations along medium‐ or low‐ Z inhomogeneities are made according to the radiological paths determined by ray tracing. The scatter dose is then scaled by a correction factor that depends on the distances between the point of interest, the body contour, and the source position. Dose calculations are done for phantoms with tissue and lead inserts, as well as patient plans for head‐and‐neck, esophagus, and MammoSite balloon breast brachytherapy treatments. Gamma indices are evaluated using a dose‐difference criterion of 3% and a distance‐to‐agreement criterion of 2 mm. PTRAN_CT MC calculations are used as the reference dose distributions. Results: For the phantom with tissue and lead inserts, the percentages of the voxels of interest passing the gamma criteria ( P γ ≥ 1)are 100% for the analytical calculation and 91% for TG‐43. For the breast patient plan, TG‐43 overestimates the target volume receiving the prescribed dose by 4% and the dose to the hottest 0.1cm 3of the skin by 9%, whereas the analytical and MC results agree within 0.4%.P γ ≥ 1are 100% and 48% for the analytical and TG‐43 calculations, respectively. For the head‐and‐neck and esophagus patient plans,P γ ≥ 1are ≥ 99 % for both calculation methods. Conclusions: A correction‐based dose calculation method has been validated for HDR I192 r brachytherapy. Its high calculation efficiency makes it feasible for use in treatment planning. Because tissue inhomogeneity effects are small and primary dose predominates in the near‐source region, TG‐43 is adequate for target dose estimation provided shielding and contrast solution are not used.