Radiation dose differences between thoracic radiotherapy planning CT and thoracic diagnostic CT scans

Abstract

Purpose: To compare the absorbed dose from computed tomography (CT) in radiotherapy planning (RP-CT) against those from diagnostic CT (DG-CT) examinations and to explore the possible reasons for any dose differences.Method: Two groups of patients underwent CT-scans of the thorax with either DG-CT (n=55) or RP-CT (n=55). Patients from each group had similar weight and body mass index (BMI) and were divided into low (<25) and high BMI (>25). Parameters including CTDIvol, DLP and scan-length were compared. Results:The mean CTDIvol and DLP values from RP-CT (38.1 mGy, 1472 mGy·cm) are approximately four times higher than for DG-CT (9.63 mGy, 376.5 mGy·cm). For low BMI group, the CTDIvol in the RP-CT scans (36.4 mGy) is 6.3 times higher than the one in the DG-CT scans (5.8 mGy). For high BMI group, the CTDIvol in the RP-CT (39.6 mGy) is 2.5 times higher than the one in the DG-CT scans (15.8 mGy). In the DG-CT scans a strong negative linear correlation between noise index (NI) and mean CTDIvol was observed (r =-0.954, p=0.004); the higher NI, the lower CTDIvol. This was not the case in the RP-DG scans. Conclusion:The absorbed radiation dose is significantly higher and less BMI dependent for RP-CT scans compared to DG-CT. Image quality requirements of the examinations should be researched to ensure that radiation doses are not unnecessarily high.

Establishment of national diagnostic reference levels (DRLs) for radiotherapy localisation computer tomography of the head and neck

“…11 A study comparing DI and RT CT imaging in the thorax found that radiation dose in RT was four times higher than DI CTs. 12 The variation in this study could suggest that the RT scan length is longer than that of the DI protocols. The majority of the RT departments scanned superiorly to clear the skull, as opposed to the level of the temporomandibular joint in the case of DI neck imaging margins.…”

“…13 As scan length is linked to DLP, decreasing scan length could decrease the radiation dose. 25 Sanderud et al 12 suggested that the lack of patient body mass index (BMI) specific modified CT protocols in thoracic RT planning was the main reason for higher patient dose in the RT setting when compared to DI. The departments in our survey were asked to provide data on average-sized patient, with no specific BMI criteria given.…”

“…RT dose surveys of thorax and breast CT localisation imaging has indicated wide variations in dose. 11,12 Such variations are unjustified given the same clinical requirements of the images. 16 This study demonstrated that similar dose variation occurs between RT Departments for HN CT localisation imaging, therefore establishing the need for DRLs and dose optimisation.…”

“…Diagnostic DRLs exist in many countries and there is some literature investigating DRLs in RT thorax and breast imaging 11,12 but no attempts have been made to introduce dose audit in HN RT CT imaging. Different scanning volumes, protocols and image quality requirements between diagnostic and RT CT do not support the use of diagnostic imaging (DI) DRLs in RT practice.…”

Establishment of national diagnostic reference levels for breast cancer CT protocols in radiation therapy

“…With this method of delivery, accurate delineation is critical to the success of the treatment owing to the steep dose gradients between target volumes and normal tissue. Sanderud et al 22 compared thoracic RT CT and diagnostic CT and reported that RT CT scans were performed with a very low noise index when compared with the diagnostic image, Sanderud et al compared thoracic RT CT and diagnostic CT and reported that RT CT scans were performed with a very low noise index when compared to the diagnostic scans, they stated that this was in an effort to met the image quality requirements for RT (22). This consequently results in very high mAs and, in turn, approximately 4 times higher CT dose to patients undergoing RT.…”