Abstract
To investigate the sensitivity of Monte Carlo (MC) calculated lung dose distributions to lung tissue characterization in external beam radiotherapy of breast cancer under Deep Inspiration Breath Hold (DIBH). EGSnrc based MC software was employed. Mean lung densities for one hundred patients were analysed. CT number frequency and clinical dose distributions were calculated for 15 patients with mean lung density below 0.14g/cm3. Lung volume with a pre-defined CT numbers was also considered. Lung tissue was characterized by applying different CT calibrations in the low-density region and air-lung tissue thresholds. Dose impact was estimated by Dose Volume Histogram (DVH) parameters. Mean lung densities below 0.14g/cm3 were found in 10% of the patients. CT numbers below -960 HU dominated the CT frequency distributions with a high rate of CT numbers at -990 HU. Mass density conversion approach influenced the DVH shape. V4Gy and V8Gy varied by 7% and 5% for the selected patients and by 9% and 3.5% for the pre-defined lung volume. V16Gy and V20Gy, were within 2.5%. Regions above 20Gy were affected. Variations in air- lung tissue differentiation resulted in DVH parameters within 1%. Threshold at -990 HU was confirmed by the CT number frequency distributions. Lung dose distributions were more sensitive to variations in the CT calibration curve below lung (inhale) density than to air-lung tissue differentiation. Low dose regions were mostly affected. The dosimetry effects were found to be potentially important to 10% of the patients treated under DIBH.
Highlights
An accurate description of the patient geometry in terms of tissue composition and mass density is essential when using the Monte Carlo (MC) method to calculate the absorbed dose to the irradiated volume in radiotherapy
Variations in the conversion of computer to mography (CT) numbers to mass density below lung density were seen to potentially influence the MC dose distributions in the ipsilateral lung for 10% of the patients treated under Deep Inspiration Breath Hold (DIBH)
MC dose distributions in lung volumes with mean density below 0.14 g/cm3 were sensitive to differences in the conversions of CT number to mass density in the air-lung inhale region
Summary
An accurate description of the patient geometry in terms of tissue composition and mass density is essential when using the Monte Carlo (MC) method to calculate the absorbed dose to the irradiated volume in radiotherapy. The curve cor relates CT numbers, expressed in Hounsfield Units (HU), to mass density relative to that of water. It is usually a bilinear function fitting CT numbers obtained after scanning a phantom with inserts of well-known density. The CT numbers stored in the patient scan are translated to mass density and material information and patient specific 3D phantoms created. The CT calibration curve and the material assignment scheme, which are not unique defined, may influence the dose calculations
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