EFFECTIVE DOSE ESTIMATION FROM ORGAN DOSE MEASUREMENTS IN FAST-kV SWITCH DUAL ENERGY COMPUTED TOMOGRAPHY.

The purpose of this study was to validate a novel approach to estimating effective dose (E) in 'fast-kV switch dual energy computed tomography' using MOSFET detectors. The effective energy of the combined dual energy environment was characterized with the dual energy CT scanner and then MOSFETs were calibrated matching to the effective energy of the dual energy CT beam with a conventional CT beam. The calibration method was then experimentally validated by comparing the dose between MOSFET and an ion chamber (IC) using a standard CTDI body phantom. The measured doses of the MOSFET and IC were 17.1 mGy ± 3.8% and 17.1 mGy ± 0.4%, respectively. To measure organ doses, an adult anthropomorphic phantom loaded with 18 MOSFET detectors was scanned using a standard fast-kV switch dual energy abdomen/pelvis CT protocol. E was calculated by applying ICRP 103 tissue weighting factors as well as partial volume correction factors for organs that were not completely covered by the protocol field-of-view. E from the dual energy abdomen/pelvis CT was calculated to be 17.8 mSv ± 11.6%. This calculation was then compared to E from dose length product method, which yielded 14.62 mSv.

Patient Dose Comparison for Intraoperative Imaging Devices Used in Orthopaedic Lumbar Spinal Surgery.

BACKGROUND: The aim of this study was to determine the amount of radiation exposure from intraoperative imaging during two-level and four-level lumbar fusions. METHODS: Five imaging systems were studied: multidetector CT (MDCT) scanner (CT A); two mobile CT units (CT B and CT C); a C-arm (D); and fluoroscopy (E). Metal oxide semiconductor field effect transistor dosimeters measured doses at 25 organ locations using an anthropomorphic phantom. A fat-equivalent phantom was used to simulate an obese body mass index (BMI). RESULTS: The effective dose (ED) for C-arm D was estimated using commercial software. The ED for others was computed from the measured mean organ doses. EDs for a normal BMI patient, receiving a four-level fusion, are as follows: CT A (12.00 ± 0.30 mSv), CT B (5.90 ± 0.25 mSv), CT C (2.35 ± 0.44 mSv), C-arm D (0.44 mSv), and fluoroscopy E (0.30 ± 0.3 mSv). The rankings are consistent across all three BMI values except CT C and fluoroscopy E, which peaked in the overweight size because of system limitations. The other machines' ED trended with patient BMI. CONCLUSION: The dose reduction protocols were confirmed according to the manufacturer's specifications. The results of this study emphasize the need for the appropriate selection of the imaging system, especially because the type of device could have a substantial effect on patient radiation risk.

Real-time dose-rate monitoring with gynecologic brachytherapy: Results of an initial clinical trial.

PURPOSE: A nanoscintillator-based fiber-optic dosimeter (nanoFOD) was developed to measure real-time dose rate during high-dose-rate (HDR) brachytherapy. A trial was designed to prospectively test clinical feasibility in gynecologic implants. METHODS AND MATERIALS: A clinical trial enrolled women undergoing vaginal cylinder HDR brachytherapy. The nanoFOD was fixed to the cylinder alongside two thermoluminescent dosimeters (TLDs). Treatment was delivered and real-time dose rates captured by the nanoFOD. The nanoFOD and TLD positions were identified in CT images and used to extract the treatment planning system (TPS) calculated dose. The nanoFOD and TLD cumulative doses were compared with the TPS. RESULTS: Nine women were enrolled for 30 fractions, and real-time data were available in 27 treatments. The median ratio of nanoFOD/TPS dose was 1.00 (IQR 0.94-1.02), with a TLD/TPS ratio of 1.01 (IQR 0.98-1.04). Of the nanoFOD dose measurements, 63% were within 5% of the TPS, 26% between 5 and 10% of the TPS, and the remaining 11% between 10 and 20% of the TPS dose. Of the TLD measurements, 70% were within 5% of the TPS, 22% between 5 and 10% of the TPS, and 7% between 10 and 20% of the TPS dose. CONCLUSIONS: Real-time dose-rate measurements during HDR brachytherapy were feasible using the nanoFOD and cumulative dose per fraction showed reasonable agreement to TLD and TPS doses. Additional studies to determine dose thresholds that would yield a low false alarm rate and ongoing device development efforts to improve localization of the scintillator in CT images are needed before this detector should be used to inform clinical decisions.

Evaluating Lens Dose Reduction in Pediatric Neuroradiology Examinations Using Automated Kilovoltage Selection Software.

OBJECTIVE: The purpose of this study is to evaluate the potential of an automated kilo-voltage selection software for the reduction of lens dose in pediatric CT scans. MATERIALS AND METHODS: Two metal oxide semiconductor field effect transistor (MOSFET) detectors measured the lens dose in two anthropomorphic 1- and 5-year-old phantoms. These phantoms were scanned using a clinical pediatric brain protocol at 120 kVp as a control with the MDCT scanner. Scans were then repeated using automated kilovoltage software. The automated kilovoltage was set to operate at tube potentials of 120, 110, and 100 kVp. Dose savings were compared with the average lens dose of both eyes between automated kilovoltage and the control setting. Image quality was studied by contrast-to-noise ratios (CNRs) for each setting. RESULTS: The mean (± SD) lens dose from the routine brain scan without automated kilovoltage was 0.92 ± 0.03 cGy and 0.81 ± 0.03 cGy for the 1- and 5-year-old phantoms, respectively. Use of the automated kilovoltage software at 120 kVp, 110 kVp, and 100 kVp resulted in dose reductions of 9.8%, 17.4%, and 19.6%, respectively, for the 1-year-old phantom and 1.2%, 8.6%, and 17.3%, respectively, for the 5-year-old phantom. The CNR for all automated kilovoltage scans was within 11% of the control scans for the 1-year-old and within 6% for the 5-year-old phantom. CONCLUSION: Our results show that automated kilovoltage software is effective for reducing the radiation dose to the lens of the eye in pediatric patients. Furthermore, the image quality by CNR remained acceptable within 11% of the baseline for all kilovoltage settings used.

Comparison of Radiation Exposure from Fixed Table Fluoroscopy to a Portable C-Arm During Ureteroscopy.

INTRODUCTION: Current treatment practices within the field of endourology require the routine use of radiation exposure to provide adequate imaging during urologic procedures. One such procedure requiring repeated radiation exposure during treatment is ureteroscopy. We set out to compare estimated fluoroscopic radiation exposures employing fixed table and portable C-arm fluoroscopy. MATERIALS AND METHODS: A cross-sectional dosimetry phantom model was placed supine on both fixed fluoroscopy and standard operating room tables. The models were then exposed to three separate 5-minute runs of fluoroscopic exposure. Metal oxide semiconductor field effect transistor dosimeters were utilized in organ-specific locations to determine specific radiation exposure dosages. Absorbed radiation was determined for each organ location for both fluoroscopy units. Organ dose volumetric corrections were performed for skin and red bone marrow, to correct for the nonirradiated portion. Organ dose rate (ODR, mGy/s) and effective dose rate (EDR, mSv/s) were calculated, with values reported as mean ± standard deviation. RESULTS: There were found to be statistically significant elevations for both total EDR and organ-specific dose rates with the use of fixed table fluoroscopy compared with C-arm fluoroscopy. EDR was found to be 0.0240 ± 0.0019 mSv/s for the fixed table unit and 0.0029 ± 0.0005 mSv/s for the C-arm unit (p = 0.0024). Internal organs exposed to the most radiation during fixed table fluoroscopy included the gall bladder and stomach in comparison to C-arm fluoroscopy, which found elevated exposure in the kidneys, pancreas, and spleen. CONCLUSION: The routine use of fixed table fluoroscopy results in significantly elevated estimated organ doses and EDR when directly compared with C-arm fluoroscopy in model trials. This difference should be taken into consideration by practicing urologists when patient treatment requires the use of fluoroscopy to maintain radiation exposure as low as reasonably achievable.

Impact of imaging approach on radiation dose and associated cancer risk in children undergoing cardiac catheterization.

OBJECTIVES: To quantify the impact of image optimization on absorbed radiation dose and associated risk in children undergoing cardiac catheterization. BACKGROUND: Various imaging and fluoroscopy system technical parameters including camera magnification, source-to-image distance, collimation, antiscatter grids, beam quality, and pulse rates, all affect radiation dose but have not been well studied in younger children. METHODS: We used anthropomorphic phantoms (ages: newborn and 5 years old) to measure surface radiation exposure from various imaging approaches and estimated absorbed organ doses and effective doses (ED) using Monte Carlo simulations. Models developed in the National Academies' Biological Effects of Ionizing Radiation VII report were used to compare an imaging protocol optimized for dose reduction versus suboptimal imaging (+20 cm source-to-image-distance, +1 magnification setting, no collimation) on lifetime attributable risk (LAR) of cancer. RESULTS: For the newborn and 5-year-old phantoms, respectively ED changes were as follows: +157% and +232% for an increase from 6-inch to 10-inch camera magnification; +61% and +59% for a 20 cm increase in source-to-image-distance; -42% and -48% with addition of 1-inch periphery collimation; -31% and -46% with removal of the antiscatter grid. Compared with an optimized protocol, suboptimal imaging increased ED by 2.75-fold (newborn) and fourfold (5 years old). Estimated cancer LAR from 30-min of posteroanterior fluoroscopy using optimized versus suboptimal imaging, respectively was 0.42% versus 1.23% (newborn female), 0.20% versus 0.53% (newborn male), 0.47% versus 1.70% (5-year-old female) and 0.16% versus 0.69% (5-year-old male). CONCLUSIONS: Radiation-related risks to children undergoing cardiac catheterization can be substantial but are markedly reduced with an optimized imaging approach. © 2016 Wiley Periodicals, Inc.

Radiation Dosimetry for Ureteroscopy Patients: A Phantom Study Comparing the Standard and Obese Patient Models.

PURPOSE: To determine the effect of obesity on radiation exposure during simulated ureteroscopy. METHODS: A validated anthropomorphic adult male phantom with a body mass index (BMI) of approximately 24 kg/m(2), was positioned to simulate ureteroscopy. Padding with radiographic characteristics of human fat was placed around the phantom to create an obese model with BMI of 30 kg/m(2). Metal oxide semiconductor field effect transistor (MOSFET) dosimeters were placed at 20 organ locations in both models to measure organ dosages. A portable C-arm was used to provide fluoroscopic x-ray radiation to simulate ureteroscopy. Organ dose rates were calculated by dividing organ dose by fluoroscopy time. Effective dose rate (EDR, mSv/sec) was calculated as the sum of organ dose rates multiplied by corresponding ICRP 103 tissue weighting factors. RESULTS: The mean EDR was significantly increased during left ureteroscopy in the obese model at 0.0092 ± 0.0004 mSv/sec compared with 0.0041 ± 0.0003 mSv/sec in the nonobese model (P < 0.01), as well as during right ureteroscopy at 0.0061 ± 0.0002 and 0.0036 ± 0.0007 mSv/sec in the obese and nonobese model, respectively (P < 0.01). EDR during left ureteroscopy was significantly greater than right ureteroscopy in the obese model (P = 0.02). CONCLUSIONS: Fluoroscopy during ureteroscopy contributes to the overall radiation dose for patients being treated for nephrolithiasis. Obese patients are at even higher risk because of increased exposure rates during fluoroscopy. Every effort should be made to minimize the amount of fluoroscopy used during ureteroscopy, especially with obese patients.

Characterization of MOSFET Dosimeter Angular Response Using a Spherical Phantom for Fluoroscopic Dosimetry.

Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) dosimeters, placed in anthropomorphic phantoms, are a standard method for organ dosimetry in medical x-ray imaging applications. However, many x-ray applications, particularly fluoroscopy procedures, use variable projection angles. During dosimetry, the MOSFET detector active area may not always be perpendicular to the x-ray beam. The goal of this study was to characterize the dosimeter's angular response in the fluoroscopic irradiation involved in pediatric cardiac catheterization procedures, during which a considerable amount of fluoroscopic x-ray irradiation is often applied from various projection angles. A biological x-ray irradiator was used to simulate the beam quality of a biplane fluoroscopy imaging system. A custom-designed acrylic spherical scatter phantom was fabricated to measure dosimeter response (in mV) in two rotational axes, axial (ψ) and normal-to-axial (θ), in 30° increments, as well as four common oblique angles used in cardiac catheterization: a) 90° Left Anterior Oblique (LAO); b) 70° LAO/ 20° Cranial; c) 20° LAO/ 15° Cranial; and d) 30° Right Anterior Oblique (RAO). All results were normalized to the angle where the dosimeter epoxy is perpendicular to the beam or the Posterior-Anterior projection angle in the clinical setup. The relative response in the axial rotation was isotropic (within ± 10% deviation); that in the normal-to-axial rotation was isotropic in all angles except the ψ = 270° angle, where the relative response was 83 ± 9%. No significant deviation in detector response was observed in the four common oblique angles, with their relative responses being: a) 102 ± 3%; b) 90 ± 3%; c) 92 ± 3%; and d) 95 ± 3%, respectively. These angular correction factors will be used in future dosimetry studies for fluoroscopy. The spherical phantom may be useful for other applications, as it allows the measurement of dosimeter response in virtually all angles in the 3-dimensional spherical coordinates.

A NOVEL APPROACH FOR EFFECTIVE DOSE MEASUREMENTS IN DUAL-ENERGY CT.

A novel method was presented for the effective dose (ED) measurement with metal-oxide-semiconductor field-effect transistor (MOSFET) detectors in dual-energy (DE) dual-source (DS) computed tomography (CT) scanner. This study demonstrated that the mean energy of the combined spectrum in dual-source computed tomography can be used to measure the ED. For validation, the MOSFET dose at the centre cavity of a CT dose index (CTDI) body phantom was compared with the dose measured with an energy-independent ion chamber (IC). A clinical abdomen/pelvis scan was performed with an adult anthropomorphic phantom, and ED was compared between the MOSFET method and the dose length product (DLP) method. The tissue doses in the CTDI phantom were 2.08±(2.70 %) with IC and 2.20±(4.82 %) cGy with MOSFET; the per cent difference was 5.91 %, and the t-test showed that there was no statistically significant difference. EDs for the abdomen/pelvis scans were 5.01±(2.34 %) mSv with MOSFET and 5.56 mSv with the DLP method.

Microdosimetric and Biological Effects of Photon Irradiation at Different Energies in Bone Marrow.

To ensure reliability and reproducibility of radiobiological data, it is necessary to standardize dosimetry practices across all research institutions. The photoelectric effect predominates over other interactions at low energy and in high atomic number materials such as bone, which can lead to increased dose deposition in soft tissue adjacent to mineral bone due to secondary radiation particles. This may produce radiation effects that deviate from higher energy photon irradiation that best model exposure from clinical radiotherapy or nuclear incidences. Past theoretical considerations have indicated that this process should affect radiation exposure of neighboring bone marrow (BM) and account for reported differences in relative biological effectiveness (RBE) for hematopoietic failure in rodents. The studies described herein definitively estimate spatial dose distribution and biological effectiveness within the BM compartment for (137)Cs gamma rays and 320 kVp X rays at two levels of filtration: 1 and 4 mm Cu half-value layer (HVL). In these studies, we performed: 1. Monte Carlo simulations on a 5 µm resolution model of mouse vertebrae and femur derived from micro-CT images; 2. In vitro biological experiments irradiating BM cells plated directly on the surface of a bone-equivalent material (BEM); and 3. An in vivo study on BM cell survival in irradiated live mice. Simulation results showed that the relative dose increased in proximity to bone at the lower radiation energies and produced averaged values of relative dose over the entire BM volume within imaged trabecular bone of 1.17, 1.08 and 1.01 for beam qualities of 1 mm Cu HVL, 4 mm Cu HVL and (137)Cs, respectively. In accordance with Monte Carlo simulations, in vitro irradiation of BM cells located on BEM and in vivo whole-body irradiation at a prescribed dose to soft tissue of 6 Gy produced relative cell killing of hematopoietic progenitors (CFU-C) that significantly increased for the 1 mm Cu HVL X rays compared to radiation exposures of higher photon energies. Thus, we propose that X rays of the highest possible kVp and filtration be used to investigate radiation effects on the hematopoietic system, as this will allow for better comparisons with high-energy photon exposures applied in radiotherapy or as anticipated in a nuclear event.

Feasibility of using the computed tomography dose indices to estimate radiation dose to partially and fully irradiated brains in pediatric neuroradiology examinations.

The purpose of this study was two-fold: (a) to measure the dose to the brain using clinical protocols at our institution, and (b) to develop a scanner-independent dosimetry method to estimate brain dose. Radiation dose was measured with a pediatric anthropomorphic phantom and MOSFET detectors. Six current neuroradiology protocols were used: brain, sinuses, facial bones, orbits, temporal bones, and craniofacial areas. Two different CT vendor scanners (scanner A and B) were used. Partial volume correction factors (PVCFs) were determined for the brain to account for differences between point doses measured by the MOSFETs and average organ dose. The CTDIvol and DLP for each protocol were recorded. The dose to the brain (mGy) for scanners A and B was 10.7 and 10.0 for the brain protocol, 7.8 and 3.2 for the sinus, 10.2 and 8.6 for the facial bones, 7.4 and 4.7 for the orbits and 1.6 and 1.9 for the temporal bones, respectively. On scanner A, the craniofacial protocol included a standard and high dose option; the dose measured for these exams was 3.9 and 16.9 mGy, respectively. There was only one craniofacial protocol on scanner B; the brain dose measured on this exam was 4.8 mGy. A linear correlation was found between DLP and brain dose with the conversion factors: 0.049 (R(2) = 0.87), 0.046 (R(2) = 0.89) for scanner A and B, and 0.048 (R(2) = 0.89) for both scanners. The range of dose observed was between 1.8 and 16.9 mGy per scan. This suggests that brain dose estimates may be made from DLP.

Radiation Exposure during the Evaluation and Management of Nephrolithiasis.

PURPOSE: There is rising concern over the increasing amount of patient radiation exposure from diagnostic imaging and medical procedures. Patients with nephrolithiasis are at potentially significant risk for radiation exposure due to the need for imaging to manage recurrent stone disease. We reviewed the literature in an attempt to better characterize actual risks and discussed methods to reduce radiation exposure for adult patients with nephrolithiasis. MATERIALS AND METHODS: A PubMed search was performed using the key words nephrolithiasis, stones, radiation, fluoroscopy, ureteroscopy, percutaneous nephrolithotomy, computerized tomography and shock wave lithotripsy. Additional citations were identified by reviewing reference lists of pertinent articles. RESULTS: A total of 50 relevant articles were included in this review. Patients with a first time acute stone event are exposed to a significant amount of radiation. Most radiation is from computerized tomography. Patients undergoing percutaneous nephrolithotomy are exposed to an equal or greater amount of radiation than they received from computerized tomography. Risk factors for increased exposure during percutaneous nephrolithotomy include obesity, multiple tracts and a larger stone burden. Ureteroscopy exposes patients to approximately the same amount of radiation as plain x-ray of the kidneys, ureters and bladder. Risk factors for increased exposure during ureteroscopy include obesity and ureteral dilation. During shock wave lithotripsy the amount of radiation exposure is not well characterized. Interventions to reduce exposure to patients include using ultrasound when possible and implementing low dose computerized tomography protocols. The as low as reasonably achievable principle of radiation exposure should always be followed when fluoroscopy is performed. The use of an air retrograde pyelogram may also reduce exposure during percutaneous nephrolithotomy. Fluoroscopy time during ureteroscopy may be decreased by a laser guided C-arm, a dedicated C-arm technician, stent placement under direct vision and tactile feedback to help guide wire placement. CONCLUSIONS: Patients with nephrolithiasis are at significant risk for increased radiation exposure from the imaging and fluoroscopy used during treatment. The true risks of low radiation exposure remain uncertain. It is important to be aware of these risks to provide better counseling for patients. Urologists must also be familiar with techniques to decrease radiation exposure for patients with nephrolithiasis.

Percutaneous cryoablation of renal masses under CT fluoroscopy: radiation doses to the patient and interventionalist.

PURPOSE: Computed tomographic (CT) fluoroscopy-guided percutaneous cryoablation is an effective therapeutic method used to treat focal renal masses. The purpose of this study is to quantify the radiation dose to the patient and interventional radiologist during percutaneous cryoablation of renal masses using CT fluoroscopic guidance. METHODS: Over a 1-year period, the CT fluoroscopy time during percutaneous cryoablation of renal masses was recorded in 41 patients. The level of complexity of each procedure was designated as simple, intermediate, or complex. Patient organ radiation doses were estimated using an anthropomorphic model. Dose to the interventional radiologist was estimated using ion chamber survey meters. RESULTS: The average CT fluoroscopy time for technically simple cases was 47 s, 126 s for intermediate cases, and 264 s for complex cases. The relative risk of hematologic stomach and liver malignancy in patients undergoing this procedure was 1.003-1.074. The lifetime attributable risk of cancer ranged from 2 to 58, with the highest risk in younger patients for developing leukemia. The estimated radiation dose to the interventionalist without lead shielding was 390 mR (3.9 mGy) per year of cases. CONCLUSIONS: The radiation risk to the patient during CT fluoroscopy-guided percutaneous renal mass cryoablation is, as expected, related to procedure complexity. Quantification of patient organ radiation dose was estimated using an anthropomorphic model. This information, along with the associated relative risk of malignancy, may assist in evaluating risks of the procedure, particularly in younger patients. The radiation dose to the interventionist is low regardless of procedure complexity, but highlights the importance of lead shielding.

Lifetime Attributable Risk of Cancer From Radiation Exposure During Parathyroid Imaging: Comparison of 4D CT and Parathyroid Scintigraphy.

OBJECTIVE: The purpose of this study is to measure the organ doses and effective dose (ED) for parathyroid 4D CT and scintigraphy and to estimate the lifetime attributable risk of cancer incidence associated with imaging. MATERIALS AND METHODS: Organ radiation doses for 4D CT and scintigraphy were measured on the basis of imaging with our institution's protocols. An anthropomorphic phantom with metal oxide semiconductor field effect transistor detectors was scanned to measure CT organ dose. Organ doses from the radionuclide were based on International Commission for Radiological Protection report 80. ED was calculated for 4D CT and scintigraphy and was used to estimate the lifetime attributable risk of cancer incidence for patients differing in age and sex with the approach established by the Biologic Effects of Ionizing Radiation VII report. A 55-year-old woman was selected as the standard patient according to the demographics of patients with primary hyperparathyroidism. RESULTS: Organs receiving the highest radiation dose from 4D CT were the thyroid (150.6 mGy) and salivary glands (137.8 mGy). For scintigraphy, the highest organ doses were to the colon (41.5 mGy), gallbladder (39.8 mGy), and kidneys (32.3 mGy). The ED was 28 mSv for 4D CT, compared with 12 mSv for scintigraphy. In the exposed standard patient, the lifetime attributable risk for cancer incidence was 193 cancers/100,000 patients for 4D CT and 68 cancers/100,000 patients for scintigraphy. Given a baseline lifetime incidence of cancer of 46,300 cancers/100,000 patients, imaging results in an increase in lifetime incidence of cancer over baseline of 0.52% for 4D CT and 0.19% for scintigraphy. CONCLUSION: The ED of 4D CT is more than double that of scintigraphy, but both studies cause negligible increases in lifetime risk of cancer. Clinicians should not allow concern for radiation-induced cancer to influence decisions regarding workup in older patients.

Fiber-optic detector for real time dosimetry of a micro-planar x-ray beam.

PURPOSE: Here, the authors describe a dosimetry measurement technique for microbeam radiation therapy using a nanoparticle-terminated fiber-optic dosimeter (nano-FOD). METHODS: The nano-FOD was placed in the center of a 2 cm diameter mouse phantom to measure the deep tissue dose and lateral beam profile of a planar x-ray microbeam. RESULTS: The continuous dose rate at the x-ray microbeam peak measured with the nano-FOD was 1.91 ± 0.06 cGy s(-1), a value 2.7% higher than that determined via radiochromic film measurements (1.86 ± 0.15 cGy s(-1)). The nano-FOD-determined lateral beam full-width half max value of 420 µm exceeded that measured using radiochromic film (320 µm). Due to the 8° angle of the collimated microbeam and resulting volumetric effects within the scintillator, the profile measurements reported here are estimated to achieve a resolution of ∼0.1 mm; however, for a beam angle of 0°, the theoretical resolution would approach the thickness of the scintillator (∼0.01 mm). CONCLUSIONS: This work provides proof-of-concept data and demonstrates that the novel nano-FOD device can be used to perform real-time dosimetry in microbeam radiation therapy to measure the continuous dose rate at the x-ray microbeam peak as well as the lateral beam shape.

Investigating the accuracy of microstereotactic-body-radiotherapy utilizing anatomically accurate 3D printed rodent-morphic dosimeters.

PURPOSE: Sophisticated small animal irradiators, incorporating cone-beam-CT image-guidance, have recently been developed which enable exploration of the efficacy of advanced radiation treatments in the preclinical setting. Microstereotactic-body-radiation-therapy (microSBRT) is one technique of interest, utilizing field sizes in the range of 1-15 mm. Verification of the accuracy of microSBRT treatment delivery is challenging due to the lack of available methods to comprehensively measure dose distributions in representative phantoms with sufficiently high spatial resolution and in 3 dimensions (3D). This work introduces a potential solution in the form of anatomically accurate rodent-morphic 3D dosimeters compatible with ultrahigh resolution (0.3 mm(3)) optical computed tomography (optical-CT) dose read-out. METHODS: Rodent-morphic dosimeters were produced by 3D-printing molds of rodent anatomy directly from contours defined on x-ray CT data sets of rats and mice, and using these molds to create tissue-equivalent radiochromic 3D dosimeters from Presage. Anatomically accurate spines were incorporated into some dosimeters, by first 3D printing the spine mold, then forming a high-Z bone equivalent spine insert. This spine insert was then set inside the tissue equivalent body mold. The high-Z spinal insert enabled representative cone-beam CT IGRT targeting. On irradiation, a linear radiochromic change in optical-density occurs in the dosimeter, which is proportional to absorbed dose, and was read out using optical-CT in high-resolution (0.5 mm isotropic voxels). Optical-CT data were converted to absolute dose in two ways: (i) using a calibration curve derived from other Presage dosimeters from the same batch, and (ii) by independent measurement of calibrated dose at a point using a novel detector comprised of a yttrium oxide based nanocrystalline scintillator, with a submillimeter active length. A microSBRT spinal treatment was delivered consisting of a 180° continuous arc at 225 kVp with a 20 × 10 mm field size. Dose response was evaluated using both the Presage/optical-CT 3D dosimetry system described above, and independent verification in select planes using EBT2 radiochromic film placed inside rodent-morphic dosimeters that had been sectioned in half. RESULTS: Rodent-morphic 3D dosimeters were successfully produced from Presage radiochromic material by utilizing 3D printed molds of rat CT contours. The dosimeters were found to be compatible with optical-CT dose readout in high-resolution 3D (0.5 mm isotropic voxels) with minimal artifacts or noise. Cone-beam CT image guidance was possible with these dosimeters due to sufficient contrast between high-Z spinal inserts and tissue equivalent Presage material (CNR ∼10 on CBCT images). Dose at isocenter measured with optical-CT was found to agree with nanoscintillator measurement to within 2.8%. Maximum dose in line profiles taken through Presage and film dose slices agreed within 3%, with FWHM measurements through each profile found to agree within 2%. CONCLUSIONS: This work demonstrates the feasibility of using 3D printing technology to make anatomically accurate Presage rodent-morphic dosimeters incorporating spinal-mimicking inserts. High quality optical-CT 3D dosimetry is feasible on these dosimeters, despite the irregular surfaces and implanted inserts. The ability to measure dose distributions in anatomically accurate phantoms represents a powerful useful additional verification tool for preclinical microSBRT.

A novel method of estimating effective dose from the point dose method: a case study--parathyroid CT scans.

The purpose of this study was to validate a novel approach of applying a partial volume correction factor (PVCF) using a limited number of MOSFET detectors in the effective dose (E) calculation. The results of the proposed PVCF method were compared to the results from both the point dose (PD) method and a commercial CT dose estimation software (CT-Expo). To measure organ doses, an adult female anthropomorphic phantom was loaded with 20 MOSFET detectors and was scanned using the non-contrast and 2 phase contrast-enhanced parathyroid imaging protocols on a 64-slice multi-detector computed tomography scanner. E was computed by three methods: the PD method, the PVCF method, and the CT-Expo method. The E (in mSv) for the PD method, the PVCF method, and CT-Expo method was 2.6  ±  0.2, 1.3  ±  0.1, and 1.1 for the non-contrast scan, 21.9  ±  0.4, 13.9  ±  0.2, and 14.6 for the 1st phase of the contrast-enhanced scan, and 15.5  ±  0.3, 9.8  ±  0.1, and 10.4 for the 2nd phase of the contrast-enhanced scan, respectively. The E with the PD method differed from the PVCF method by 66.7% for the non-contrast scan, by 44.9% and by 45.5% respectively for the 1st and 2nd phases of the contrast-enhanced scan. The E with PVCF was comparable to the results from the CT-Expo method with percent differences of 15.8%, 5.0%, and 6.3% for the non-contrast scan and the 1st and 2nd phases of the contrast-enhanced scan, respectively. To conclude, the PVCF method estimated E within 16% difference as compared to 50-70% in the PD method. In addition, the results demonstrate that E can be estimated accurately from a limited number of detectors.

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

This study investigated the degree to which body habitus influences radiation dose during CT fluoroscopy (CTF)-guided lumbar epidural steroid injections (ESI). An anthropomorphic phantom containing metal oxide semiconductor field effect transistor (MOSFET) detectors was scanned at two transverse levels to simulate upper and lower lumbar CTF-guided ESI. Circumferential layers of adipose-equivalent material were sequentially added to model patients of three sizes: small (cross-sectional dimensions 25×30 cm), average (34×39 cm), and oversize (43×48 cm). Point dose rates to skin and internal organs within the CTF beam were measured. Scattered point dose rates 5 cm from the radiation beam were also measured. Direct point dose rates to the internal organs ranged from 0.05-0.11 mGy/10mAs in the oversized phantom, and from 0.18-0.43 mGy/10mAs in the small phantom. Skin direct point dose rates ranged from 0.69-0.71 mGy/10mAs in the oversized phantom and 0.88-0.94 mGy/10mAs in the small phantom. This represents a 180-310% increase in organ point dose rates and 24-36% increase in skin point dose rates in the small habitus compared with the oversize habitus. Scatter point dose rates increased by 83-117% for the small compared to the oversize phantom. Decreasing body habitus results in substantial increases in direct organ and skin point doses as well as scattered dose during simulated CTF-guided procedures. Failure to account for individual variations in body habitus will result in inaccurate dose estimation and inappropriate choice of tube current in CTF-guided procedures.

Verification of a novel method for tube voltage constancy measurement of orthovoltage x-ray irradiators.

PURPOSE: For orthovoltage x-ray irradiators, the tube voltage is one of the most fundamental system parameters as this directly relates to the dosimetry in radiation biology studies; however, to the best of our knowledge, there is no commercial portable quality assurance (QA) tool to directly test the constancy of the tube voltage greater than 160 kV. The purpose of this study is to establish the Beam Quality Index (BQI), a quantity strongly correlated to the tube voltage, as an alternative parameter for the verification of the tube voltage as part of the QA program of orthovoltage x-ray irradiators. METHODS: A multipurpose QA meter and its associated data acquisition software were used to customize the measurement parameters to measure the BQI and collect its time-plot. BQI measurements were performed at 320 kV with four filtration levels on three orthovoltage x-ray irradiators of the same model, one of which had been recently energy-calibrated at the factory. RESULTS: For each of the four filtration levels, the measured BQI values were in good agreement (<5%) between the three irradiators. BQI showed filtration-specificity, possibly due to the difference in beam quality. CONCLUSIONS: The BQI has been verified as a feasible alternative for monitoring the constancy of the tube voltage for orthovoltage irradiators. The time-plot of BQI offers information on the behavior of beam energy at different phases of the irradiation time line. In addition, this would provide power supply performance characteristics from initial ramp-up to plateau, and finally, the sharp drop-off at the end of the exposure.