Evaluation of on-board kV cone beam CT (CBCT)-based dose calculation

Abstract

On-board CBCT images are used to generate patient geometric models to assist patient setup. The image data can also, potentially, be used for dose reconstruction in combination with the fluence maps from treatment plan. Here we evaluate the achievable accuracy in using a kV CBCT for dose calculation. Relative electron density as a function of HU was obtained for both planning CT (pCT) and CBCT using a Catphan-600 calibration phantom. The CBCT calibration stability was monitored weekly for 8 consecutive weeks. A clinical treatment planning system was employed for pCT- and CBCT-based dose calculations and subsequent comparisons. Phantom and patient studies were carried out. In the former study, both Catphan-600 and pelvic phantoms were employed to evaluate the dosimetric performance of the full-fan and half-fan scanning modes. To evaluate the dosimetric influence of motion artefacts commonly seen in CBCT images, the Catphan-600 phantom was scanned with and without cyclic motion using the pCT and CBCT scanners. The doses computed based on the four sets of CT images (pCT and CBCT with/without motion) were compared quantitatively. The patient studies included a lung case and three prostate cases. The lung case was employed to further assess the adverse effect of intra-scan organ motion. Unlike the phantom study, the pCT of a patient is generally acquired at the time of simulation and the anatomy may be different from that of CBCT acquired at the time of treatment delivery because of organ deformation. To tackle the problem, we introduced a set of modified CBCT images (mCBCT) for each patient, which possesses the geometric information of the CBCT but the electronic density distribution mapped from the pCT with the help of a BSpline deformable image registration software. In the patient study, the dose computed with the mCBCT was used as a surrogate of the 'ground truth'. We found that the CBCT electron density calibration curve differs moderately from that of pCT. No significant fluctuation was observed in the calibration over the period of 8 weeks. For the static phantom, the doses computed based on pCT and CBCT agreed to within 1%. A notable difference in CBCT- and pCT-based dose distributions was found for the motion phantom due to the motion artefacts which appeared in the CBCT images (the maximum discrepancy was found to be approximately 3.0% in the high dose region). The motion artefacts-induced dosimetric inaccuracy was also observed in the lung patient study. For the prostate cases, the mCBCT- and CBCT-based dose calculations yielded very close results (<2%). Coupled with the phantom data, it is concluded that the CBCT can be employed directly for dose calculation for a disease site such as the prostate, where there is little motion artefact. In the prostate case study, we also noted a large discrepancy between the original treatment plan and the CBCT (or mCBCT)-based calculation, suggesting the importance of inter-fractional organ movement and the need for adaptive therapy to compensate for the anatomical chan...

Feasibility of CBCT‐based dose with a patient‐specific stepwise HU‐to‐density curve to determine time of replanning

“…Although many researchers7, 8, 9, 10, 11, 12, 13 have investigated CBCT‐based dose calculations and their use in replanning or online adaptive planning, clinical implementation has remained challenging because of the complexity of the technique or inability to achieve the accuracy required by treatment planning. In this study, we assessed the accuracy of the CBCT‐based dose calculations using patient‐specific stepwise HU‐to‐density curves and investigated the feasibility of using this method to determine replanning time.…”

“…Onboard kV cone‐beam CT (CBCT) is now widely available, and CBCT‐based dose calculation makes it possible to evaluate dosimetric change during the course of treatment. Although kV CBCT technology is mainly used to set up patients and localize anatomy, its potential for use in dose calculation has been recognized and reported 7, 8, 9, 10. Dose calculation accuracy using CBCT images has been evaluated by investigators 8, 9, 11, 12, 13.…”

“…Although kV CBCT technology is mainly used to set up patients and localize anatomy, its potential for use in dose calculation has been recognized and reported 7, 8, 9, 10. Dose calculation accuracy using CBCT images has been evaluated by investigators 8, 9, 11, 12, 13. The main source of dosimetric error stemming from CBCT‐based dose calculations (relative to fan‐beam‐based CT simulation) comes from the uncertainty in Hounsfield unit (HU)‐to‐electron density conversion of CBCT images.…”

“…The main source of dosimetric error stemming from CBCT‐based dose calculations (relative to fan‐beam‐based CT simulation) comes from the uncertainty in Hounsfield unit (HU)‐to‐electron density conversion of CBCT images. As a more direct approach, phantom‐based calibration of the HU‐to‐electron density curve was investigated 7, 8, 9, 13. Unlike fan‐beam CT, kV CBCT suffers from scatter, which results in greater HU uncertainty 13, 14, 15, 16.…”

“…Moreover, HUs are dependent on patient size,17, 18 making this method even more challenging. An alternative method that relies on mapping of electron density values from planning CT to CBCT images was introduced 8, 19, 20, 21. Because of the complexity of these methods, CBCT‐based dose is not commonly used in routine clinical practice.…”