In modern radiotherapy, kilovoltage computed tomography (kV-CT) is an essential imaging system for radiation treatment planning (RTP). kV-CT images are used for calculating therapeutic radiation doses absorbed by the patient’s body and for delineating the target volume and the volume in the surrounding normal organs. However, kV-CT images are significantly distorted by image artifacts caused by the presence of metal prostheses, such as dental fillings or artificial knee joints in a patient’s body. Thus, the accuracies of target delineation and treatment dose calculation are inevitably lowered. Many studies on metal artifact reduction (MAR) in kV-CT images have been conducted since Glover and Pelc [1] first reported this issue in 1981. Most studies on MAR suggested similar procedures: (1) identification of metal inserts in raw projection data or reconstructed CT images; and (2) replacement of the identified data with the neighboring data through various interpolation techniques [2-8]. Recently, alternative trials based on iterative methods have been suggested [9-11]. Although the effects of these MAR studies were significant, the original pixel values of CT images near metal prostheses could not be restored because incident X-ray photons were fully absorbed or scattered during penetration. This means that the treatment dose values cannot be calculated accurately. However, megavoltage CT (MV-CT) [12,13] uses higher X-ray energy beams that can penetrate high-density materials and produces CT images with rare metal artifacts and accurate pixel values of metal prostheses. MV-CT images present higher noise and lower image contrast due to the high transmission and scattering properties of megavoltage X-ray energies, which may preclude precise image reading or identification of regions of interest.

In 2015, we proposed a novel method to generate hybrid CT images utilizing the advantages of kV-CT and MV-CT and demonstrated its benefits in terms of image quality using a cylindrical phantom in three clinical cases [14]. We showed that, like MV-CT, hybrid CT also successfully removed artifacts due to metal prostheses while maintaining high image quality compared to that of kV-CT. As a follow-up study, this study evaluates the accuracy of the calculated treatment dose when the calculation was implemented using hybrid CT and compares it with that of the original kV-CT. MV-CT was excluded from this comparative study owing to its inherently low image quality. We expect that the feasibility of hybrid CT as an imaging technique for accurate radiotherapeutic dose calculation would be considered.

We obtained two dose distributions calculated using kV-CT and hybrid CT, and one dose distribution measured using a film per patient. Fig. 5 shows the three-dose distributions in patient 1. Comparing Fig. 5a and 5b, different dose distribution patterns (see black arrows) are observed. The pattern in hybrid CT is closer to the measured pattern than that observed in kV-CT.

Table 1 presents the evaluated gamma pass rates between the calculated and measured dose distributions, and the plots are shown in Fig. 6. The gamma pass rates decreased as the gamma criteria were strengthened in all patients. The pass rate dropped significantly at the 1%/1 mm criterion. The pass rate of hybrid CT was higher than that of kV-CT in all cases, which remained above 80% even at the criterion of 1%/1 mm.

Although the 3%/3 mm gamma criterion has been widely used for the verification of RTP, it is recommended to use stricter criteria because of the development of RTP techniques that require high accuracy and precision, such as intensity-modulated radiation treatment or stereotactic body radiation treatment [19,20]. In this study, all pass rates obtained using the 3%/3 mm gamma criterion, which was 99.0% (kV-CT) and 99.9% (Hybrid CT) on average, were sufficient to be accepted for treatment. When the 2%/2 mm criterion was used, the hybrid CT showed significant superiority over kV-CT (Fig. 6). When the 1%/1 mm criterion was used, the difference in gamma pass rates between them was about 13%p. These results indicate that hybrid CT, which can reduce artifacts due to metal prostheses, can aid accurate dose calculation for RTP. For instance, severe metal artifacts are frequently observed in head and neck CT images owing to metallic dental prostheses. Because the important organs at risk, such as the brain stem, optic nerves, and parotid glands, are located close to each other in the head and neck, an accurate and precise RTP is necessary. Thus, hybrid CT plays a significant role in making RTP for the head and neck.

Fig. 7 shows the gamma evaluation results of patient 1. Comparing Fig. 4 and 7, using kV-CT, we observe that the distribution of gamma failures where gamma values are greater than 1 approximately coincides with the distribution of metal artifacts (see black dotted boxes in Fig. 4a and 7). However, using hybrid CT, it can be visually verified that both image artifacts and gamma failures are significantly reduced.

Established MAR techniques that could help only in the reduction of image artifacts but not in image restoration, like hybrid CT, were not considered since the restored images were more helpful for target/organ identification and contouring. In our previous study [14], the hybrid CT was compared with three well-known MAR techniques, such as linear interpolation, Lagrange interpolation with high-order polynomials, and normalization prior to interpolation (NI) with the best performance among them. We compared the hybrid CT performance to NI. The difference in their dose calculation seems to be worthy for further study to evaluate the effects of secondary artifacts that may occur during the hybrid CT generation.

In this study, the hybrid CT performance was first verified for the head and neck. The same verification will be implemented for other sites where metallic prostheses can be used, such as artificial hip joints or surgical clips. Besides, verification using a phantom that more closely mimics human anatomy instead of using a cylindrical phantom may be necessary.

RTP should be carefully constructed when metal artifacts are present in kV-CT images considering their non-negligible effect on dose calculation. We expect that using hybrid CT can be a suitable approach to avoid the effect of severe metal artifacts on dose calculation and contouring.