CT NUMBER AND ELECTRON DENSITY RELATIONSHIP FOR PHOTON DOSE CALCULATION IN HEAD AND NECK, CHEST, SPINE AND PELVIC REGIONS

Abstract

CT images supply the HU information that relates to electron density (ρe). The ρe is important for tissue inhomogeneity correction for accurate dose calculation. The purpose of this study was to evaluate the dosimetric effect on CT images on different kVp setting using single HU-ρe curve. Anthropomorphic phantom was scanned using Siemens-SOMATOM Definition AS with 120 kVp setting and used as a reference image set. The procedures were repeated with DECT option (80/140 kVp), 80 and 140 kVp. The 120 kVp HU-ρe relationship curve from Gammex 467 tissue characterization phantom was created and applied into Eclipse treatment planning system. The five clinical plans in each 3D, IMRT and VMAT techniques in four regions (head and neck, chest, spine and pelvic regions) were exported and recalculated in Anthropomorphic phantom images for all kVp setting images. Moreover, the amalgam was inserted in Anthropomorphic phantom at teeth location and the radiation dose in head and neck region was evaluated. The isocenter dose and gamma index from SNC patient software with criteria 1%/1mm. (γ1%/1mm.) were selected for plan evaluation. According to treatment regions, the average point dose differences of DECT, 80 and 140 kVp images to reference image were 0.7%, 0.4% and 0.3% for H&N, 0.5%, 0.3% and 0.2% for chest, 0.6%, 0.5% and 0.1% for spine, 0.9%, 0.2% and 0.2% for pelvic region and 0.1%, 0.4% and 0.1% for H&N with amalgam, respectively. For gamma index, the average γ1%/1mm. passing rate in all regions and all techniques of DECT, 80 and 140 kVp images were 95.9%, 100% and 100%, respectively. From the results, the different kVp images gave the different HU value and it has impacted to dose distribution. The results of isocenter dose differences showed the good agreement with gamma index results. DECT images presented the highest average dose differences from other energies, especially in large area of high density. Pelvic region displayed the highest average dose differences than other regions because it consists of the large bone area in pelvic bone. IMRT and VMAT technique showed the highest average dose difference that might be due to the high MU and complicated leaf movements. In conclusion, it is possible to apply single 120 kVp HU-ρe relationship curve into all tube voltage scanning images with the highest average point dose difference of 0.6% in DECT and lowest average γ1%/1mm. passing rate of 95.9%.