The Influence of CT-to-Electron Density Calibration Models on the Accuracy of Radiotherapy Dose Calculation Bayu Ahmad Fauzan(1), Akbar Azzi(2), Aloysius Mario Yudi Putranto(3), Dewa Ngurah Yudhi Prasada(4)
(1,2) Faculty of Mathematics and Natural Sciences, Universitas Indonesia
(3) MRCCC Siloam Hospitals Semanggi
(4) Faculty of Mathematics and Natural Sciences, Universitas Udayana
Abstract
Radiotherapy requires accurate dose calculation in heterogeneous tissues, and the conversion from CT number to electron density remains an important source of uncertainty in treatment planning system (TPS) calculations. This study evaluated the influence of four CT-to-electron density (CT-ED) calibration models, namely the native Somatom curve, the stoichiometric Schneider model, the empirical Kumamoto model, and the Nakao three-parameter model, on TPS point-dose accuracy in a heterogeneous thorax phantom. Experimental measurements were performed using a 6 MV photon beam from a Varian Clinac iX linear accelerator and a Farmer-type ionization chamber in a CIRS thorax phantom. For the primary analysis, dose was calculated on a detector-free reference CT, while detector regions of interest were delineated on detector-insert CT images and registered to the reference CT for dose extraction. TPS-calculated mean doses were compared with measured doses at five audit points representing soft tissue, lung, and bone-equivalent regions. The relative deviations for the Somatom model were 0.57%, 0.58%, 0.82%, 0.80%, and 1.00% at Points 1, 3, 5, 9, and 10, respectively. The corresponding deviations were 0.47%, 1.08%, 1.37%, 1.90%, and 1.35% for Schneider- 0.17%, 0.28%, 0.52%, 0.35%, and 1.00% for Kumamoto- and 0.32%, 0.53%, 1.07%, 0.55%, and 1.45% for Nakao 3p. All evaluated models satisfied the IAEA TECDOC 1583 acceptance criteria. Kumamoto yielded the lowest overall mean deviation, followed by Somatom and Nakao 3p, while Schneider showed the largest deviation among the primary models. These findings indicate that TPS dose calculation remained robust against CT-ED calibration variation under thorax phantom audit conditions, although small model-dependent differences persisted in heterogeneous media.
Keywords: CT-to-Electron Density, Calibration Models, Hounsfield Unit, Radiotherapy Dosimetry
