1. Introduction of a low dose protocol for thoracic ablation under CT guidance Thea Buchan, Jason Godbold, Dr Steve Bandula Interventional Oncology Service, University College London Hospitals
Background
Complex CT guided procedure, such as tumour ablation, often require multiple CT acquisitions with the potential for significant x-ray exposure to both the patient and practitioner (1,2,3). The use of general anaesthesia also increases the number of staff who may be exposed. High doses associated with CT guided intervention have been reported in the literature (1,2) with effective doses for ablation as high as 119mSv (5). With decreasing age of patients undergoing these therapies, the cumulative effect of radiation exposure must be considered (4,6).
Interventional CT teams must work to minimise x-ray dose to patients and staff, while maintaining clinically acceptable image quality. We proposed an optimised CT protocol for thoracic tumour ablation at our centre with the aim of reduced X-ray exposure.
Results
Introduction of a low dose protocol for CT imaging during thoracic tumour ablation resulted in a 42% reduction in average dose - DLP fell from to mGycm2 or a reduction in effective dose from 7.93 to 3.35 mSv (P = ) using the thoracic correction factor of
Chart 1: Mean and range of dose in DLP before and after implementation of the lower dose protocol.
There was no significant difference in signal to noise ratio between scans in the two groups (P = 0.002). Figure 1 shows a single patient who underwent ablative therapy before and after introduction of the low dose protocol. Images illustrates equivalent CT image quality during the two treatments.
Method
Scanner calculated patient dose collected as dose length product (DLP) was recorded for CT guided thoracic tumour ablation procedures using a dedicated interventional scanner (Prime, Toshiba Medical Systems, Japan) at a single tertiary Interventional Oncology centre. Protocols for patient imaging were reviewed by the Radiology team - including Radiographers and Radiologists. For all helical scans, tube voltage was reduced from 120 to 100kV, with tube current set to modulate from the planning scanograms.
During CT fluoroscopy, tube voltage was reduced from 120 to 100kV and tube current reduced to between 10mA (<85kg) and 30mA (>85kg) with a 0.5 second rotation time depending on patient size weight. Image quality was assessed in patients who had undergone procedures under both protocols by measuring the image noise (signal to noise ratio in a 2cm2 region of interest draw in air).
Dose data was collected from 20 consecutive cases prior to, and 20 cases following introduction of the optimised CT protocol. Effective dose was calculated from the DLP and the 2 groups compared using an unpaired t-tests.
Image 1. Comparison of imaging
Image 1. Standard dose CT fluoroscopy protocol Image 2. Reduced dose CT fluoroscopy protocol
Discussion
Radiation dose delivered during CT guided intervention deserves specific attention to reduced exposure to patients and practitioners. Utilisation of a low dose protocol for CT guided thoracic ablations can lead to significant dose savings.
Radiographers working within the interventional setting are key to the design, audit and implementation of these protocols, and should acknowledge the distinct requirements of interventional, as compared with diagnostic imaging. Radiographers should work closely with their teams to ensure that image quality requirements are understood and that they are able to manipulate protocols to balance dose against image quality.
Rathmann, N, Haeusler, U, Diezler, P, Weiss, C, Kostrzewa, M, Sadick, M, Schoenberg, S, & Diehl, S 2015, 'Evaluation of Radiation Exposure of Medical Staff During CT-Guided Interventions', Journal Of The American College Of Radiology, 12, 1, pp
Kloeckner, R, Santos, D, Schneider, J, Kara, L, Dueber, C, & Pitton, M 2013, 'Radiation exposure in CT-guided interventions', European Journal Of Radiology, 82, 12, pp
Chang, D, Hiss, S, Mueller, D, Hellmich, M, Borggrefe, J, Bunck, A, Maintz, D, & Hackenbroch, M 2015, 'Radiation Dose Reduction in Computed Tomography-Guided Lung Interventions using an Iterative Reconstruction Technique', Röfo - Fortschritte Auf Dem Gebiet Der Röntgenstrahlen Und Der Bildgebenden Verfahren
4. Eisenberg, J, Gervais, D, Singh, S, Kalra, M, Sabir, S, Paul, A, & Pandharipande, P 2015, 'Radiation exposure from CT-guided ablation of renal masses: effects on life expectancy', AJR. American Journal Of Roentgenology, 204, 2, pp. 335-
5. Leng, S, Christner, J, Carlson, S, Jacobsen, M, Vrieze, T, Atwell, T, & McCollough, C 2011, 'Radiation dose levels for interventional CT procedures', AJR. American Journal Of Roentgenology, 197, 1, pp. W97-W103, MEDLINE Complete, EBSCOhost, viewed 28 February 2017.
6. Stewart, J, Looney, C, Anderson-Evans, C, Toncheva, G, Sopko, D, Kim, C, Yoshizumi, T, Nelson, R, Stewart, J, Looney, C, Anderson-Evans, C, Toncheva, G, Sopko, D, Kim, C, Yoshizumi, T, & Nelson, R 2015, 'Percutaneous cryoablation of renal masses under CT fluoroscopy: radiation doses to the patient and interventionalist', Abdominal Imaging, 40, 7, pp , Academic Search Complete, EBSCOhost, viewed 28 February 2017.
342, MEDLINE Complete, EBSCOhost, viewed 28 February 2017.
References
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