NEED FOR INDIVIDUAL CANCER RISK ESTIMATES IN X-RAY AND NUCLEAR MEDICINE IMAGING

Patterns and trends of population radiation exposure and projected the risk of exposure-induced death from gamma camera examinations in Yazd Province

INTRODUCTION

The use of nuclear medicine examinations as imaging modalities has recently increased. This study aimed to assess the radiation dose delivered to patients from common diagnostic nuclear medicine examinations and estimate the risk of exposure-induced death (REID). In addition, the frequency of nuclear medicine procedures were evaluated from 2015-2018.

MATERIALS AND METHODS

The data were collected from adult patients who had undergone cardiac, skeletal, renal, lacrimal, and thyroid imaging. For each patient, the effective dose was calculated using dose conversion factors, and REID was estimated using PCXMC. The frequency of examinations between 2015 and 2018 was obtained from the hospital information system (HIS).

RESULTS

The highest estimated effective dose was attributed to the scans of myocardial stress (8.09 ± 1.28 mSv), myocardial rest (5.59 ± 1.27 mSv), and thyroid imaging (3.93 ± 0.55 mSv). In addition, cardiac stress examination had the highest REID values for solid cancers (212.5 ± 67.5) and bone scans had the highest REID values for leukemia (11.5 ± 2.5). A large increase in the number of myocardial perfusion scans was a significant contributor to an increase in collective effective dose from 23.37 man-Sv in 2015 to 49.47 man-Sv in 2018, a compound annual growth rate (CAGR) of 26%.

CONCLUSION

Although the annual frequency and per capita effective dose of nuclear medicine procedures in Yazd Province increased continuously, they were comparably lower than those reported for other countries. Despite this, the cancer risks of nuclear medicine scans at the individual level are negligible (around 0.01% excess fatal cancer risk), yet the increasing tendency for these examinations could be of concern.

XDose: toward online cross-validation of experimental and computational X-ray dose estimation

PURPOSE

As the spectrum of X-ray procedures has increased both for diagnostic and for interventional cases, more attention is paid to X-ray dose management. While the medical benefit to the patient outweighs the risk of radiation injuries in almost all cases, reproducible studies on organ dose values help to plan preventive measures helping both patient as well as staff. Dose studies are either carried out retrospectively, experimentally using anthropomorphic phantoms, or computationally. When performed experimentally, it is helpful to combine them with simulations validating the measurements. In this paper, we show how such a dose simulation method, carried out together with actual X-ray experiments, can be realized to obtain reliable organ dose values efficiently.

METHODS

A Monte Carlo simulation technique was developed combining down-sampling and super-resolution techniques for accelerated processing accompanying X-ray dose measurements. The target volume is down-sampled using the statistical mode first. The estimated dose distribution is then up-sampled using guided filtering and the high-resolution target volume as guidance image. Second, we present a comparison of dose estimates calculated with our Monte Carlo code experimentally obtained values for an anthropomorphic phantom using metal oxide semiconductor field effect transistor dosimeters.

RESULTS

We reconstructed high-resolution dose distributions from coarse ones (down-sampling factor 2 to 16) with error rates ranging from 1.62 % to 4.91 %. Using down-sampled target volumes further reduced the computation time by 30 % to 60 %. Comparison of measured results to simulated dose values demonstrated high agreement with an average percentage error of under [Formula: see text] for all measurement points.

CONCLUSIONS

Our results indicate that Monte Carlo methods can be accelerated hardware-independently and still yield reliable results. This facilitates empirical dose studies that make use of online Monte Carlo simulations to easily cross-validate dose estimates on-site.

Toward automated and personalized organ dose determination in CT examinations - A comparison of two tissue characterization models for Monte Carlo organ dose calculation with a Therapy Planning System

PURPOSE

Computed tomography (CT) is a versatile tool in diagnostic radiology with rapidly increasing number of examinations per year globally. Routine adaption of the exposure level for patient anatomy and examination protocol cause the patients' exposures to become diversified and harder to predict by simple methods. To facilitate individualized organ dose estimates, we explore the possibility to automate organ dose calculations using a radiotherapy treatment planning system (TPS). In particular, the mapping of CT number to elemental composition for Monte Carlo (MC) dose calculations is investigated.

METHODS

Organ dose calculations were done for a female thorax examination test case with a TPS (Raystation™, Raysearch Laboratories AB, Stockholm, Sweden) utilizing a MC dose engine with a CT source model presented in a previous study. The TPS's inherent tissue characterization model for mapping of CT number to elemental composition of the tissues was calibrated using a phantom with known elemental compositions and validated through comparison of MC calculated dose with dose measured with Thermo Luminescence Dosimeters (TLD) in an anthropomorphic phantom. Given the segmentation tools of the TPS, organ segmentation strategies suitable for automation were analyzed for high contrast organs, utilizing CT number thresholding and model-based segmentation, and for low contrast organs utilizing water replacements in larger tissue volumes. Organ doses calculated with a selection of organ segmentation methods in combination with mapping of CT numbers to elemental composition (RT model), normally used in radiotherapy, were compared to a tissue characterization model with organ segmentation and elemental compositions defined by replacement materials [International Commission on Radiological Protection (ICRP) model], frequently favored in imaging dosimetry.

RESULTS

The results of the validation with the anthropomorphic phantom yielded mean deviations from the dose to water calculated with the RT and ICRP model as measured with TLD of 1.1% and 1.5% with maximum deviations of 6.1% and 8.7% respectively over all locations in the phantom. A strategy for automated organ segmentation was evaluated for two different risk organ groups, that is, low contrast soft organs and high contrast organs. The relative deviation between organ doses calculated with the RT model and with the ICRP model varied between 0% and 20% for the thorax/upper abdomen risk organs.

CONCLUSIONS

After calibration, the RT model in the TPS provides accurate MC dose results as compared to measurements with TLD and the ICRP model. Dosimetric feasible segmentation of the risk organs for a female thorax demonstrates a possibility for automation using the segmentation tool available in a TPS for high contrast organs. Low contrast soft organs can be represented by water volumes, but organ dose to the esophagus and thyroid must be determined using standardized organ shapes. The uncertainties of the organ doses are small compared to the overall uncertainty, at least an order of magnitude larger, in the estimates of lifetime attributable risk (LAR) based on organ doses. Large-scale and automated individual organ dose calculations could provide an improvement in cancer incidence estimates from epidemiological studies.

Radiation dose from X-ray examinations of impacted canines: cone beam CT vs two-dimensional imaging

OBJECTIVES

To compare the radiation dose to children examined for impacted canines, using two-dimensional (2D) examinations (panoramic and periapical radiographs) and cone beam CT (CBCT).

METHODS

Organ doses were determined using an anthropomorphic 10-year-old child phantom. Two CBCT devices, a ProMax3D and a NewTom5G, were examined using thermoluminescent dosimeters. For the panoramic radiograph, a Promax device was used and for periapical radiographs, a Prostyle device with a ProSensor digital sensor was used. Both the panoramic and the intraoral devices were examined using Gafchromic-QR2 dosimetric film placed between the phantom slices.

RESULTS

ProMax3D and NewTom5G resulted in an effective dose of 88 µSv and 170 µSv respectively. A panoramic radiograph resulted in an effective dose of 4.1 µSv, while a periapical radiograph resulted in an effective dose of 0.6 µSv and 0.7 µSv using a maxillary lateral projection and central maxillary incisor projection respectively.

CONCLUSIONS

The effective dose from CBCT ranged from 140 times higher dose (NewTom5G compared to two periapical radiographs) to 15 times higher dose (ProMax3D compared to three periapical and one panoramic radiograph) than a 2D examination.

A COMPARATIVE STUDY OF ORGAN DOSES ASSESSMENT FOR PATIENTS UNDERGOING CONVENTIONAL X-RAY EXAMINATIONS: PHANTOM EXPERIMENTS VS. CALCULATIONS

A series of phantom experiments were performed with the aim of estimating organ doses for patients undergoing conventional X-ray chest and pelvis examinations. The experiments were performed using physical phantoms corresponding to an adult and a 5-year-old child. Mean organ doses and entrance surface dose were measured using TL-dosemeters. The measured organ doses were compared with the data obtained by calculations using available software tools (EDEREX and PCXMC 2.0) based on the computational MIRD-5 stylized models. The differences between calculated and measured doses for organs located fully or partly in the primary radiation beam did not exceed ±33% with the probability of 95% for the tube voltage 60-140 kV both for an adult and a 5-year-old child phantom. This study suggests that EDEREX and PCXMC 2.0 can be used to estimate organ and effective dose for adult as well as pediatric patients undergoing conventional X-ray examinations.

Lifetime attributable risk as an alternative to effective dose to describe the risk of cancer for patients in diagnostic and therapeutic nuclear medicine

The aim of this study is to implement lifetime attributable risk (LAR) predictions of cancer for patients of various age and gender, undergoing diagnostic investigations or treatments in nuclear medicine and to compare the outcome with a population risk estimate using effective dose and the International Commission on Radiological Protection risk coefficients. The radiation induced risk of cancer occurrence (incidence) or death from four nuclear medicine procedures are estimated for both male and female between 0 and 120 years. Estimations of cancer risk are performed using recommended administered activities for two diagnostic (¹⁸F-FDG and ^99mTc-phosphonate complex) and two therapeutic (¹³¹I-iodide and ²²³Ra-dichloride) radiopharmaceuticals to illustrate the use of cancer risk estimations in nuclear medicine. For ¹⁸F-FDG, the cancer incidence for a male of 5, 25, 50 and 75 years at exposure is 0.0021, 0.0010, 0.0008 and 0.0003, respectively. For ^99mTc phosphonates complex the corresponding values are 0.000 59, 0.000 34, 0.000 27 and 0.000 13, respectively. For an ¹³¹I-iodide treatment with 3.7 GBq and 1% uptake 24 h after administration, the cancer incidence for a male of 25, 50 and 75 years at exposure is 0.041, 0.029 and 0.012, respectively. For ²²³Ra-dichloride with an administration of 21.9 MBq the cancer incidence for a male of 25, 50 and 75 years is 0.31, 0.21 and 0.09, respectively. The LAR estimations are more suitable in health care situations involving individual patients or specific groups of patients than the health detriment based on effective dose, which represents a population average. The detriment consideration in effective dose adjusts the cancer incidence for suffering of non-lethal cancers while LAR predicts morbidity (incidence) or mortality (cancer). The advantages of these LARs are that they are gender and age specific, allowing risk estimations for specific patients or subgroups thus better representing individuals in health care than effective dose.