Tolerance levels of CT number to electron density table for photon beam in radiotherapy treatment planning system

Abstract

The accuracy of computed tomography number to electron density (CT‐ED) calibration is a key component for dose calculations in an inhomogeneous medium. In a previous work, it was shown that the tolerance levels of CT‐ED calibration became stricter with an increase in tissue thickness and decrease in the effective energy of a photon beam. For the last decade, a low effective energy photon beam (e.g., flattening‐filter‐free (FFF)) has been used in clinical sites. However, its tolerance level has not been established yet. We established a relative electron density (ED) tolerance level for each tissue type with an FFF beam. The tolerance levels were calculated using the tissue maximum ratio (TMR) and each corresponding maximum tissue thickness. To determine the relative ED tolerance level, TMR data from a Varian accelerator and the adult reference computational phantom data in the International Commission on Radiological Protection publication 110 (ICRP‐110 phantom) were used in this study. The 52 tissue components of the ICRP‐110 phantom were classified by mass density as five tissues groups including lung, adipose/muscle, cartilage/spongy‐bone, cortical bone, and tooth tissue. In addition, the relative ED tolerance level of each tissue group was calculated when the relative dose error to local dose reached 2%. The relative ED tolerances of a 6 MVFFF beam for lung, adipose/muscle, and cartilage/spongy‐bone were ±0.044, ±0.022, and ±0.044, respectively. The thicknesses of the cortical bone and tooth groups were too small to define the tolerance levels. Because the tolerance levels of CT‐ED calibration are stricter with a decrease in the effective energy of the photon beam, the tolerance levels are determined by the lowest effective energy in useable beams for radiotherapy treatment planning systems.

Tolerance levels of mass density for CT number calibration in photon radiation therapy

“…The tolerance levels for cortical bone and tooth were not estimated because cortical bone and tooth were too thin to cause 2% dose error. 5 3.2%, respectively. The ranges of CVs for adipose/muscle at depths of 10 and 20 cm were 0.4%-0.7% and 1.3%-1.8%, respectively.…”

“…2,5 The RED tolerance levels were established to cause 2% dose error at effective tissue thickness with a 10 cm × 10 cm field. 2,5 The RED tolerance levels were established to cause 2% dose error at effective tissue thickness with a 10 cm × 10 cm field.…”

“…The RED tolerance level based on TMR and effective tissue thickness were shown in previous study. 2,5 The RED tolerance levels were established to cause 2% dose error at effective tissue thickness with a 10 cm × 10 cm field. The tolerance levels are determined by the minimum usable energy in a RTPS because the tolerance levels are more strict with lower beam energy.…”

“…The tolerance levels of CT-RED calibration were determined by classifying standard tissues to five tissue groups including lung, adipose/muscle, cartilage/ spongy-bone, cortical bone, and tooth tissue. 5 When the mass density (MD) is converted from RED within a tissue group, the conversion of pure organ is simple because MD and elemental composition are fixed.…”

“…MD tolerance levels are converted from RED tolerance levels with conversion factors. D err, RED and D err, MD are defined in eq (5)…”