The feasibility of patient size‐corrected, scanner‐independent organ dose estimates for abdominal CT exams

Abstract

Purpose:A recent work has demonstrated the feasibility of estimating the dose to individual organs from multidetector CT exams using patient-specific, scanner-independent CTDI vol -to-organ-dose conversion coefficients. However, the previous study only investigated organ dose to a single patient model from a full-body helical CT scan. The purpose of this work was to extend the validity of this dose estimation technique to patients of any size undergoing a common clinical exam. This was done by determining the influence of patient size on organ dose conversion coefficients generated for typical abdominal CT exams. Methods: Monte Carlo simulations of abdominal exams were performed using models of 64-slice MDCT scanners from each of the four major manufacturers to obtain dose to radiosensitive organs for eight patient models of varying size, age, and gender. The scanner-specific organ doses were normalized by corresponding CTDI vol values and averaged across scanners to obtain scannerindependent CTDI vol -to-organ-dose conversion coefficients for each patient model. In order to obtain a metric for patient size, the outer perimeter of each patient was measured at the central slice of the abdominal scan region. Then, the relationship between CTDI vol -to-organ-dose conversion coefficients and patient perimeter was investigated for organs that were directly irradiated by the abdominal scan. These included organs that were either completely ͑"fully irradiated"͒ or partly ͑"partially irradiated"͒ contained within the abdominal exam region. Finally, dose to organs that were not at all contained within the scan region ͑"nonirradiated"͒ were compared to the doses delivered to fully irradiated organs. Results: CTDI vol -to-organ-dose conversion coefficients for fully irradiated abdominal organs had a strong exponential correlation with patient perimeter. Conversely, partially irradiated organs did not have a strong dependence on patient perimeter. In almost all cases, the doses delivered to nonirradiated organs were less than 5%, on average across patient models, of the mean dose of the fully irradiated organs. Conclusions: This work demonstrates the feasibility of calculating patient-specific, scannerindependent CTDI vol -to-organ-dose conversion coefficients for fully irradiated organs in patients undergoing typical abdominal CT exams. A method to calculate patient-specific, scanner-specific, and exam-specific organ dose estimates that requires only knowledge of the CTDI vol for the scan protocol and the patient's perimeter is thus possible. This method will have to be extended in future 820 820 Med. Phys. 38 "2…,

Radiation Exposure from CT Scans: How to Close Our Knowledge Gaps, Monitor and Safeguard Exposure—Proceedings and Recommendations of the Radiation Dose Summit, Sponsored by NIBIB, February 24–25, 2011

“…Several studies have shown, for the same scanner output (ie, the same CTDI vol ), that smaller patients actually absorb more radiation dose than larger patients (15,16). Therefore, if two patients of different size are scanned with the same technical factors, then the scanner would report an identical CTDI vol value, but the actual absorbed dose would be higher for a smaller patient than a larger patient.…”

A Computed Tomography (CT) wish list, from a medical physicist’s perspective

“…This approach could be particularly useful in light of the current CT technologies where vendors are varying pitches on the fly throughout an exam or modulating and even switching off the X-ray beam over high risk organs. Further, variants such as the size-specific dose estimate (SSDE) recommended by the American Association of Medical Physicists (AAPM) [6] or even size-specific organ absorbed dose estimates based on the CTDI vol [7] may be useful additions to the dose information displayed on the scanner. An automatically calculated lateral and anteriorposterior dimension (recognising skin edge) for each patient would also assist in size-specific dosimetry.…”

Effect of Body Habitus on Radiation Dose during CT Fluoroscopy-Guided Spine Injections

“…Although the relationship between body habitus and radiation dose in diagnostic CT imaging has been extensively investigated [9][10][11][12][13][14][15][16] , dose estimation in CTF procedures is more compli- Figure 1 Scout CT image of prone phantom demonstrating inner (*) and outer (**) layers of 4.5-cm-thick circumferential adipose-equivalent material. A single layer was applied to simulate average body habitus, and two layers were applied to simulate oversize habitus.…”