Patient radiation dose in the 2-phase computed tomography imaging of the parathyroid glands

• Main Author: Δημητρούκας, Χρήστος
• Other Authors: Dimitroukas, Christos
• Language: English
• Published: 2023
• Subjects: Computed tomography; Diagnostic reference levels; 2-phases parathyroid imaging; Organ doses; Effective dose; Υπολογιστική τομογραφία; Διαγνωστικά επίπεδα αναφοράς; Απεικόνιση παραθυρεοειδών με 2 φάσεις; Δόσεις οργάνων; Ενεργός δόση

A large dose audit was initially performed to evaluate the radiation dose to 573 adult patients, who underwent eleven common CT examinations in two different scanners: one with 16 detector rows (GE Light Speed) and one with 80 detector rows (Toshiba Aquilion Prime). The CTDIvol, DLP, as well as patients’ demographic and technical data were recorded. The 75th percentiles of the distribution of the CTDIvol and DLP values were proposed as IDRLs and were compared to corresponding NDRLs and international values. All dose values are based on the 32-cm body phantom, except for the head, sinuses and auditory canal examinations in Toshiba system, as well as the head and neck examinations in GE system where the 16-cm head phantom is used. A reduction of up to 67% and 64% (Mann-Whitney test p<0.05) was reported for the mean CTDIvol and DLP values obtained with the Toshiba system compared to the GE system. The IDRLs for the Toshiba system were lower up to 71% and 68% compared to the corresponding NDRLs in terms of CTDIvol and DLP values, respectively. For the GE system, the IDRLs regarding the CTDIvol values were lower up to 23% and 39% compared to the corresponding NDRLs, except for the skull base examination that exceeded up to 112%, as well as for the head and lumbar spine examinations that were exceeded 10%, regarding the DLP values. The ED was also calculated based on the recorded DLP values and suitable conversion coefficients. Focusing on the neck region, the mean ED values were 2.95 mSv for the GE system and 2.02 mSv for the Toshiba system. These values were lower than those reported in most of the international studies previously published. A following study aimed to evaluate the radiation dose during five CTA examinations (brain, carotids, thoracic aorta, abdomen aorta, pulmonary arteries) performed with the above CT systems, and to establish IDRLs. The CTDIvol, DLP per examination, patients’ demographic and anatomical data, and technical data were retrospectively recorded for 240 adult patients. The 75th percentiles of the distribution of the CTDIvol and DLP values were again proposed as IDRLs. The radiation dose values were compared to corresponding previously published international data. Focusing on the neck region (carotids), the IDRLs were 6.5 mGy and 280 mGycm, in terms of CTDIvol and DLP values, respectively, for the Toshiba system. The respective mean ED value was 2.5 mSv. The reductions of the obtained CTDIvol and DLP values obtained with the Toshiba compared to the GE system were associated with the implementation of dose-saving technologies, such as lowered tube voltage and IR algorithm. The Toshiba system’s IDRLs and ED values were comparable or lower than those reported in most of the previously published studies. There are significant variations in patient doses during CTA examinations, due to the CT systems’ differences, scanning parameters and departmental CTA protocols utilised. These two introductory studies were focused on the neck region, i.e. standard neck examinations (one phase) and carotids’ CTAs (two phases), respectively. Patient dose surveys could contribute towards optimising radiation protection for patients; therefore, highlighting the necessity to increase the awareness and knowledge of the radiation dose levels during common CT or specific CTA examinations. The patient radiation dose from the two-phase parathyroid protocols of the two different CT systems applied in our Hospital was then evaluated and compared with that delivered by the other similar protocols previously published. 214 patients with PHPT were included in the study with a two-phase CT scan by using the above two scanners. The standard ‘neck’ or a modified ‘parathyroid’ protocol was used. The patient dose was again evaluated in terms of CTDIvol, DLP and ED per acquisition protocol and CT system. CTDIvol and DLP were recorded retrospectively, while the ED was calculated based on DLP and an appropriate conversion coefficient. Comparisons of patient dose between the two protocols and two CT systems and the corresponding published values were established. A significantly lower patient dose (40.2–43.2%) than the GE system (p<0.0001) resulted from the Toshiba system. The ‘parathyroid’ protocol resulted in a 6.5–9.6% lower patient dose than the standard ‘neck’ protocol. Compared with the literature, the lowest ED value (3.6 mSv) was observed since this protocol consists of a lowered tube voltage of 100 kVp, a reduced scan length for the pre-contrast phase and implementation of an IR algorithm. The objective of the next study was to estimate organs’ absorbed dose from the two-phase CT of parathyroid glands and ED based on three different methods, and to compare the dose values with those reported by other published protocols. CTDIvol, DLP, and the corresponding scan length during each phase of the modified parathyroid protocol were recorded, for 76 patients. One k-factor, and two different k-factors for the neck and chest area were used to estimate the ED from DLP. A Monte Carlo software, VirtualDoseCT, was also used for the estimation of organs’ absorbed dose and ED. Two-phase parathyroid CT resulted in a mean ED of 3.93 mSv, 4.29 mSv and 4.21 mSv according to the one k-factor, two k-factors, and VirtualDoseCT methods, respectively. The two k-factors method resulted in a slight overestimation of 1.9% in total ED compared to VirtualDoseCT. No statistically significant difference was found in ED values between these methods (Wilcoxon test, p>0.05), except for female patients in the pre-contrast phase. The organs inside the SFOV received the following doses: thymus 23.3 mGy, lungs 11.5 mGy, oesophagus 9.2 mGy, thyroid 6.9 mGy, and breast 6.3 mGy. The ED and organs’ dose were significantly lower in the pre-contrast than in the arterial phase (Wilcoxon test, p<0.001). A statistically significant difference was observed between male and female patients for the pre-contrast phase (Mann-Whitney test, p<0.05), regarding the ED values obtained with the two k-factors method and VirtualDoseCT software. The two k-factors method could be considered as safe for the ED estimation in clinical practice, if appropriate software is not available. A wide range of ED values was found in the literature, mainly depending on the acquisition protocol parameters and the estimation method. At our Hospital, the ED is reduced by decreasing the tube voltage, scan length and number of phases. Another study aimed to assess the influence of overranging on patient ED and radiosensitive organs' dose (OD) of adult patients undergoing two-phase parathyroid CT examinations utilising Monte Carlo simulation and propose methods for elimination of its impact in clinical practice. The CTDIvol, and DLP per examination, demographic and anatomical data, as well as technical data were retrospectively recorded for seventy-six patients. The ED and OD values were calculated for both sexes, and phases and as a total, with and without considering overranging, using VirtualDoseCT software incorporating a Toshiba Aquilion Prime 80 CT scanner and the recorded parameters. A significant reduction of about 30% (Wilcoxon test p<0.0001) was found in total ED values obtained without considering overranging (3.2 vs 4.2 mSv). A significant decrease was also found in total ODs without considering overranging for females (except for the thyroid), males and patients’ total sample (Wilcoxon test, p<0.05). The percentage differences in total ODs between the two methods were 2.1% for thyroid, 14.1% for oesophagus, 14.6% for thymus, 19.0% for eye lenses, 26.3% for lungs, and 83.5% for breasts. A non-significant decrease of 4.6% (Mann-Whitney test, p=0.259) was found in total ED values obtained without considering overranging for males compared to females (3.12 vs 3.26 mSv). In addition, a significant reduction was found in total ODs for males compared to females for thyroid, oesophagus, and eye lenses (Mann-Whitney test, p<0.0001), but not for breasts (Mann-Whitney test, p=0.061), and a non-significant increase for the lungs and thymus (Mann-Whitney test, p>0.05). Our results highlight that overranging should be of particular concern when optimisation strategies are planned during parathyroid CT imaging, since there is the potential to significantly reduce ED and OD absorbed by radiosensitive organs inside or adjacent to the scanning volume. Further work needs to be conducted, combining physical and technical parameters affecting overranging, to increase the awareness and understanding of the unnecessary patient dose during CT imaging, and motivate CT facilities to implement dose optimisation strategies. The purpose of the final study was to review the published literature and compare the different MDCT protocols used for PGs’ imaging. This was accomplished in terms of clinical and technical parameters of the applied protocols, the diagnostic efficacy and patient dose. An extended discussion was performed regarding the discrepancy in the appropriate combination of phases, clinical and technical parameters in relation to the opposing demands of high image quality and reasonably low patient radiation dose.