Effect of computed tomography value error on dose calculation in adaptive radiotherapy with Elekta X‐ray volume imaging cone beam computed tomography

Abstract

Purpose We evaluated the effect of changing the scan mode of the Elekta X‐ray volume imaging cone beam computed tomography (CBCT) on the accuracy of dose calculation, which may be affected by computed tomography (CT) value errors in three dimensions. Methods We used the electron density phantom and measured the CT values in three dimensions. CT values were compared with planning computed tomography (pCT) values for various materials. The evaluated scan modes were for head and neck (S‐scan), chest (M‐scan), and pelvis (L‐scan) with various collimators and filter systems. To evaluate the effects of the CT value error of the CBCT on dose error, Monte Carlo calculations of dosimetry were performed using pCT and CBCT images. Results The L‐scan had a CT value error of approximately 800 HU at the isocenter compared with the pCT. Furthermore, inhomogeneity in the longitudinal CT value profile was observed in the bone material. The dose error for ±100 HU difference in CT values for the S‐scan and M‐scan was within ±2%. The center of the L‐scan had a CT error of approximately 800 HU and a dose error of approximately 6%. The dose error of the L‐scan occurred in the beam path in the case of both single field and two parallel opposed fields, and the maximum error occurred at the center of the phantom in the case of both the 4‐field box and single‐arc techniques. Conclusions We demonstrated the three‐dimensional CT value characteristics of the CBCT by evaluating the CT value error obtained under various imaging conditions. It was found that the L‐scan is considerably affected by not having a unique bowtie filter, and the S‐scan without the bowtie filter causes CT value errors in the longitudinal direction. Moreover, the CBCT dose errors for the 4‐field box and single‐arc irradiation techniques converge to the isocenter.

Effect of the Small Field of View and Imaging Parameters to Image Quality and Dose Calculation in Adaptive Radiotherapy

“…Rando Phantom is divided into 35 separate slices with a thickness of 2.5 cm. 13 Another phantom also used is Catphan 503 phantom to evaluate the CBCT image quality. Catphan phantom contains several modules which are used to measure various image quality tests; it has a diameter of 15 cm.…”

“…The following challenges come from the Hounsfield Unit (HU) in CBCT images that rely on many parameters such as tube voltage, filters, electric current (mAs), and reconstruction algorithm, just like in CT Simulator. 11- 13 However, CBCT uses a conical geometry beam and a flat panel detector to obtain the entire patient volume during a single linac rotation. 12,14- 16 This mechanism is more prone to additional artefacts, such as scatter and noise which are caused by the beam divergence.…”

“…17 Even so, previous studies have shown that CBCT images are capable of being used in dose calculation. 13,18 In dose calculation, the difference in Hounsfield Unit between CBCT and CT simulator images is bound to happen. There have been several attempts to decrease this difference in previous studies, for instance by subtracting the images using ImageJ Software (National Institutes of Health).…”

“…There have been several attempts to decrease this difference in previous studies, for instance by subtracting the images using ImageJ Software (National Institutes of Health). 13,18 If the difference is more than 100 HU, it could lead to a dose calculation error of up to 2%. 13 Consequently, it is necessary to use a suitable CBCT pre-set that produces an appropriate calibration curve.…”

“…13,18 If the difference is more than 100 HU, it could lead to a dose calculation error of up to 2%. 13 Consequently, it is necessary to use a suitable CBCT pre-set that produces an appropriate calibration curve.…”