Determining organ dose: the holy grail

Abstract

Among the various metrics to quantify CT radiation dose, organ dose is generally regarded as one of the best to reflect patient radiation burden. Organ dose is dependent on two main factors, namely patient anatomy and irradiation field. An accurate estimation of organ dose requires detailed modeling of both factors. The modeling of patient anatomy needs to reflect the anatomical diversity and complexity across the population so that the attributes of a given clinical patient can be properly accounted for. The modeling of the irradiation field needs to accurately reflect the CT system condition, especially the tube current modulation (TCM) technique. We present an atlas-based method to model patient anatomy via a library of computational phantoms with representative ages, sizes and genders. A clinical patient is matched with a corresponding computational phantom to obtain a representation of patient anatomy. The irradiation field of the CT system is modeled using a validated Monte Carlo simulation program. The tube current modulation profiles are simulated using a manufacturer-generalizable ray-tracing algorithm. Combining the patient model, Monte Carlo results, and TCM profile, organ doses are obtained by multiplying organ dose values from a fixed mA scan (normalized to CTDIvol-normalized, denoted as h organ ) and an adjustment factor that reflects the specific irradiation of each organ. The accuracy of the proposed method was quantified by simulating clinical abdominopelvic examinations of 58 patients. The predicted organ doses showed good agreement with simulated organ dose across all organs and modulation schemes. For an average CTDIvol of a CT exam of 10 mGy, the absolute median error across all organs was 0.64 mGy (-0.21 and 0.97 for 25th and 75th percentiles, respectively). The percentage differences were within 15%. The study demonstrates that it is feasible to estimate organ doses in clinical CT examinations for protocols without and with tube current modulation. The methodology can be used for both prospective and retrospective estimation of organ dose.

Patient organ and effective dose estimation in CT: comparison of four software applications

“…However, the dose to the organs is not an immediate information easy to be obtained. Samei et al [13] defined its determination as a Holy Grail [13]. In the 1990s, the European Commission (Council Recommendation 1999/519/EC) encouraged the research of new methods to estimate the patient dose in CT.…”

Vertical off‐centering affects organ dose in chest CT: Evidence from Monte Carlo simulations in anthropomorphic phantoms

“…Moreover, in CT with tube current modulation, the CTDI displayed at the end of the scan is averaged over the entire scanned length and does not reflect the changes of exposure associated with TCM . In contrast to CTDI, absorbed dose per organ addresses both the effects of scanner output and the patient characteristics, being more relevant for determining potential stochastic risks and deterministic effects from ionizing radiation . The study published in 2013 by Kaasalainen et al described the effect of patient off‐centering on absorbed dose in pediatric CTs.…”

Effect of reduced z-axis scan coverage on diagnostic performance and radiation dose of neck computed tomography in patients with suspected cervical abscess

“…Nevertheless, this approach also lacks the information of the cumulative effective dose and organ doses, which can only be approximated via conversion factors and with a remaining uncertainty about the actual dose delivered [30]. However, reliable results, especially of organ doses, would be of interest as they offer the most detailed insight of the radiation burden [18]. In this context, workflow integrated dose management platforms may be helpful, to facilitate a quicker and easier overview for detailed changes in dose estimates of different examination protocols and alterations of protocol setting.…”

“…However, these dose values only insufficiently account for size-specific patient characteristics [16,17]. Moreover, the resulting differences in effective dose and specific organ doses can only be approximated in complex calculations or phantom studies, which is not feasible for clinical routine [18,19]. In this context, workflow integrated dose measurement platforms may be helpful to improve patient safety by providing an individual, more detailed dose report, including effective dose and organ doses.…”