Imaging Dosimetry Research Articles

SU‐E‐T‐294: Simulations to Investigate the Feasibility of ‘dry’ Optical‐CT Imaging for 3D Dosimetry

Purpose:To perform simulations investigating the feasibility of “dry” optical‐CT, and determine optimal design and scanning parameters for a novel dry tank telecentric optical‐CT 3D dosimetry system. Such a system would have important advantages in terms of practical convenience and reduced cost.Methods:A Matlab based ray‐tracing simulation platform, ScanSim, was used to model a telecentric system with a polyurethane dry tank, cylindrical dosimeter, and surrounding fluid. This program's capabilities were expanded for the geometry and physics of dry scanning. To categorize the effects of refractive index (RI) mismatches, simulations were run for several dosimeter (RI = 1.5−1.48) and fluid (RI = 1.55−1.33) combinations. Additional simulations examined the effect of increasing gap size (1–5mm) between the dosimeter and tank wall, and of changing the telecentric lens tolerance (0.5°−5°). The evaluation metric is the usable radius; the distance from the dosimeter center where the measured and true doses differ by less than 2%.Results:As the tank/dosimeter RI mismatch increases from 0–0.02, the usable radius decreases from 97.6% to 50.2%. The fluid RI for matching is lower than either the tank or dosimeter RI. Changing gap sizes has drastic effects on the usable radius, requiring more closely matched fluid at large gap sizes. Increasing the telecentric tolerance through a range from 0.5°–5.0° improved the usable radius for every combination of media.Conclusion:Dry optical‐CT with telecentric lenses is feasible when the dosimeter and tank RIs are closely matched (<0.01 difference), or when data in the periphery is not required. The ScanSim tool proved very useful in situations when the tank and dosimeter have slight differences in RI by enabling estimation of the optimal choice of RI of the small amount of fluid still required. Some spoiling of the telecentric beam and increasing the tolerance helps recover the usable radius.

SU-D-BRE-01: A Realistic Breathing Phantom of the Thorax for Testing New Motion Mitigation Techniques with Scanning Proton Therapy

Purpose: A prototype breathing phantom (named LuCa) has been developed which simulates the anatomy and motion of a patient thorax.In this work, we describe the results of the first commissioning tests with LuCa. Methods: The phantom provides a close representation of the human thorax. The lungs,contained within a tissue-equivalent ribcage and skin,are made from a polymer foam,which is inflated and deflated using a custommade ventilator. A tumor is simulated using a wooden ball with cutplanes for placing GafChromic films. The ventilator,controlled with Labview software,simulates a full range of breathing motion types.Commissioning tests were performed to assess its performance using imaging (CT and radiographic) and film dosimetry as follows:i)maximum Tumor excursion at acceptable pressure ranges, ii)tumor Motion repeatability between breathing periods,iii)reproducibility between measurement days,iv)tumor-to-surface motion correlation and v)reproducibility of film positioning in phantom. Results: The phantom can generate repeatable motion patterns with sin{sup 4},sin,breath-hold (tumor amplitude repeatability 3%. This work is funded by Swiss National Fund Grants 320030-127569 and 320030-1493942-1.« less

SU-E-T-364: 6X FFF and 10X FFF Portal Dosimetry Output Factor Verification: Application for SRS/SBRT

Purpose: To enhance portal dosimetry of high dose rate SRS/SBRT plan verifications with extensive imager measurement of output factors (OF). Methods: Electronic portal image dosimetry (EPID), implemented on the Varian Edge allows for acquisition of its two energies: 6X FFF and 10 FFF (1400 and 2400 MU/min, respectively) at source to imager distance (SID) =100cm without imager saturation. Square and rectangular aSi OF following EPID calibration were obtained. Data taken was similar to that obtained during beam commissioning (of almost all field sizes from 1×1 to 15×15 and 20×20 cm{sup 2}, [Trilogy] and [Edge], respectively) to construct a table using the OF tool for use in the Portal Dosimetry Prediction Algorithm (PDIP v11). The Trilogy 6x SRS 1000 MU/min EPID data were taken at 140 SID. The large number of OF were obtained for comparison to that obtained with diode detectors and ion chambers (cc13 for >3×3 field size). As Edge PDIP verification is currently ongoing, EPID measurements of three SRS/SBRT plans for the Trilogy were taken and compared to results obtained prior to these measurements. Results: The relative difference output factors of field sizes 2×2 and higher compared to commissioning data were (mean+/-SD, [range]): Edge 6X (−1.9+/−2.9%, [−5.9%,3.1%]), Edgemore » 10X (−0.7+/−1.2%, [− 3.3%,0.8%] and Trilogy (0.03+/−0.5%, [−1.4%,1.1%]) with EPID over predicting. The results for the 140 SID showed excellent agreement throughout except at the 1×1 to 1×15 and 15×1 field sizes where differences were: −10.6%, −6.0% and −5.8%. The differences were also most pronounced for the 1×1 at 100 SID. They were −7.4% and −11.5% for 6X and 10X, respectively. The Gamma (3%, 1mm) for three clinical plans improved by 8.7+/−1.8%. Conclusion: Results indicate that imager output factor measurements at any SID of high dose rate SRS/SBRT are quite reliable for portal dosimetry plan verification except for the smallest fields. This work was not funded by Varian Oncology Systems. Some authors have other work partly funded by Varian Oncology Systems.« less

SU‐E‐CAMPUS‐T‐02: Exploring Radiation Acoustics CT Dosimeter Design Aspects for Proton Therapy

Purpose:Investigate the design aspects and imaging dose capabilities of the Radiation Acoustics Computed Tomography (RA CT) dosimeter for Proton induced acoustics, with the objective to characterize a pulsed pencil proton beam. The focus includes scanner geometry, transducer array, and transducer bandwidth on image quality.Methods:The geometry of the dosimeter is a cylindrical water phantom (length 40cm, radius 15cm) with 71 ultrasound transducers placed along the length and end of the cylinder to achieve a weighted set of projections with spherical sampling. A 3D filtered backprojection algorithm was used to reconstruct the dosimetric images and compared to MC dose distribution. First, 3D Monte Carlo (MC) Dose distributions for proton beam energies (range of 12cm, 16cm, 20cm, and 27cm) were used to simulate the acoustic pressure signal within this scanner for a pulsed proton beam of 1.8×107 protons, with a pulse width of 1 microsecond and a rise time of 0.1 microseconds. Dose comparison within the Bragg peak and distal edge were compared to MC analysis, where the integrated Gaussian was used to locate the 50% dose of the distal edge. To evaluate spatial fidelity, a set of point sources within the scanner field of view (15×15×15cm3) were simulated implementing a low‐pass bandwidth response function (0 to 1MHz) equivalent to a multiple frequency transducer array, and the FWHM of the point‐spread‐function determined.Results:From the reconstructed images, RACT and MC range values are within 0.5mm, and the average variation of the dose within the Bragg peak are within 2%. The spatial resolution tracked with transducer bandwidth and projection angle sampling, and can be kept at 1.5mm.Conclusion:This design is ready for fabrication to start acquiring measurements. The 15 cm FOV is an optimum size for imaging dosimetry. Currently, simulations comparing transducer sensitivity, bandwidth, and proton beam parameters are being evaluated to assess signal‐to‐noise.

Weightbearing CBCT, MDCT, and 2D imaging dosimetry of the foot and ankle

Aims: This study compares effective doses associated with 2D lateral, oblique, and AP radiographs with 3D MDCT and Cone Beam CT images of the foot and ankle. Methods: An anthropomorphic phantom of the foot and ankle was constructed from an adult human skeleton and soft tissue equivalent material. Optical stimulated dosimeters were placed at 21 locations within and on tissues and anatomy of interest. Effective dose was calculated following 2007 ICRP recommendations. Three projections were exposed to simulate conventional 2D imaging. Standard and optimized dose MDCT scans were exposed to simulate typical CT options. Ten, 20, and 30cm fields of view, 100 and 120kVp and 4.5 and 6.8mAs exposures were tested using a PedCAT® CBCT device designed for weightbearing imaging. Dose was calculated for an adult as well as for a 5 and 10-year-old child to assess the impact of age on risk estimation. Results: Standard adult effective doses for single foot imaging were 0.6µSv for 2D, 3.8µSv for CBCT, and 25µSv for MDCT (p = 0.0013). Dose differed significantly with age (p = 0.0185). For a 5-year-old, doses rise to 0.8µSv for 2D, 18µSv for CBCT, and 200µSv for MDCT. Small and medium CBCT fields produced adult doses of 2.3 mSv and 0.9 mSv respectively. Conclusions: The effective dose for small FOV CBCT or conventional 2D series examinations is comparable to a few hours of equivalent background dose. Such doses are negligible; therefore, the dose of radiation should not be a concern when considering the use of CBCT imaging for foot/ankle examination.

A comparison of two practical approaches for cone beam computed tomography (CBCT) dosimetry using Monte Carlo technique

A comparison of two practical approaches for cone beam computed tomography (CBCT) dosimetry using Monte Carlo technique

Feasibility study of SPECT system for online dosimetry imaging in boron neutron capture therapy

A single collimator version of a proposed PG-SPECT system was manufactured and experimentally tested. Combining this experimental data with Monte Carlo simulation, the viability of Ge and CdTe semiconductors detectors was calculated. It was determined that the best detector of the ones compared would be a CdTe detector of 2–3mm, aided by the benefit of adding a Compton-suppression anti-coincidence timing detector.

MIRD Pamphlet No. 24: Guidelines for Quantitative 131I SPECT in Dosimetry Applications

The reliability of radiation dose estimates in internal radionuclide therapy is directly related to the accuracy of activity estimates obtained at each imaging time point. The recently published MIRD pamphlet no. 23 provided a general overview of quantitative SPECT imaging for dosimetry. The present document is the first in a series of isotope-specific guidelines that will follow MIRD 23 and focuses on one of the most commonly used therapeutic radionuclides, (131)I. The purpose of this document is to provide guidance on the development of protocols for quantitative (131)I SPECT in radionuclide therapy applications that require regional (normal organs, lesions) and 3-dimensional dosimetry.

Superficial dosimetry imaging based on Čerenkov emission for external beam radiotherapy with megavoltage x‐ray beam

Čerenkov radiation emission occurs in all tissue, when charged particles (either primary or secondary) travel at velocity above the threshold for the Čerenkov effect (about 220 KeV in tissue for electrons). This study presents the first examination of optical Čerenkov emission as a surrogate for the absorbed superficial dose for MV x-ray beams. In this study, Monte Carlo simulations of flat and curved surfaces were studied to analyze the energy spectra of charged particles produced in different regions near the surfaces when irradiated by MV x-ray beams. Čerenkov emission intensity and radiation dose were directly simulated in voxelized flat and cylindrical phantoms. The sampling region of superficial dosimetry based on Čerenkov radiation was simulated in layered skin models. Angular distributions of optical emission from the surfaces were investigated. Tissue mimicking phantoms with flat and curved surfaces were imaged with a time domain gating system. The beam field sizes (50 × 50-200 × 200 mm(2)), incident angles (0°-70°) and imaging regions were all varied. The entrance or exit region of the tissue has nearly homogeneous energy spectra across the beam, such that their Čerenkov emission is proportional to dose. Directly simulated local intensity of Čerenkov and radiation dose in voxelized flat and cylindrical phantoms further validate that this signal is proportional to radiation dose with absolute average discrepancy within 2%, and the largest within 5% typically at the beam edges. The effective sampling depth could be tuned from near 0 up to 6 mm by spectral filtering. The angular profiles near the theoretical Lambertian emission distribution for a perfect diffusive medium, suggesting that angular correction of Čerenkov images may not be required even for curved surface. The acquisition speed and signal to noise ratio of the time domain gating system were investigated for different acquisition procedures, and the results show there is good potential for real-time superficial dose monitoring. Dose imaging under normal ambient room lighting was validated, using gated detection and a breast phantom. This study indicates that Čerenkov emission imaging might provide a valuable way to superficial dosimetry imaging in real time for external beam radiotherapy with megavoltage x-ray beams.

Small field of view cone beam CT temporomandibular joint imaging dosimetry

Cone beam CT (CBCT) is generally accepted as the imaging modality of choice for visualisation of the osseous structures of the temporomandibular joint (TMJ). The purpose of this study was to compare the radiation dose of a protocol for CBCT TMJ imaging using a large field of view Hitachi CB MercuRay™ unit (Hitachi Medical Systems, Tokyo, Japan) with an alternative approach that utilizes two CBCT acquisitions of the right and left TMJs using the Kodak 9000(®) 3D system (Carestream, Rochester, NY). 25 optically stimulated luminescence dosemeters were placed in various locations of an anthropomorphic RANDO(®) Man phantom (Alderson Research Laboratories, Stanford, CT). Dosimetric measurements were performed for each technique, and effective doses were calculated using the 2007 International Commission on Radiological Protection tissue weighting factor recommendations for all protocols. The radiation effective dose for the CB MercuRay technique was 223.6 ± 1.1 μSv compared with 9.7 ± 0.1 μSv (child), 13.5 ± 0.9 μSv (adolescent/small adult) and 20.5 ± 1.3 μSv (adult) for the bilateral Kodak acquisitions. Acquisitions of individual right and left TMJ volumes using the Kodak 9000 3D CBCT imaging system resulted in a more than ten-fold reduction in the effective dose compared with the larger single field acquisition with the Hitachi CB MercuRay. This decrease is made even more significant when lower tube potential and tube current settings are used.

Superficial dosimetry imaging of Čerenkov emission in electron beam radiotherapy of phantoms

Čerenkov emission is generated from ionizing radiation in tissue above 264 keV energy. This study presents the first examination of this optical emission as a surrogate for the absorbed superficial dose. Čerenkov emission was imaged from the surface of flat tissue phantoms irradiated with electrons, using a range of field sizes from 6 cm × 6 cm to 20 cm × 20 cm, incident angles from 0° to 50°, and energies from 6 to 18 MeV. The Čerenkov images were compared with the estimated superficial dose in phantoms from direct diode measurements, as well as calculations by Monte Carlo and the treatment planning system. Intensity images showed outstanding linear agreement (R2 = 0.97) with reference data of the known dose for energies from 6 to 18 MeV. When orthogonal delivery was carried out, the in-plane and cross-plane dose distribution comparisons indicated very little difference (±2–4% differences) between the different methods of estimation as compared to Čerenkov light imaging. For an incident angle 50°, the Čerenkov images and Monte Carlo simulation show excellent agreement with the diode data, but the treatment planning system had a larger error (OPT = ±1∼2%, diode = ±2∼3%, TPS = ±6–8% differences) as would be expected. The sampling depth of superficial dosimetry based on Čerenkov radiation has been simulated in a layered skin model, showing the potential of sampling depth tuning by spectral filtering. Taken together, these measurements and simulations indicate that Čerenkov emission imaging might provide a valuable method of superficial dosimetry imaging from incident radiotherapy beams of electrons.

MO‐D‐134‐06: A Preliminary Method Correlating Patient‐Specific Parameters with Direct Measurements to Estimate Organ Doses in CT

Purpose: In the field of Computed Tomography (CT) dosimetry, there remains a need to accurately measure organ doses. Such measurements are only meaningful if they are performed under actual clinical scanning conditions, for this purpose, a cadaver can serve as the measurement subject that most closely mimics a living patient. Organ doses were measured in 7 adult female cadaveric subjects with varying body mass indices (BMIs) and for various CT protocols. Methods: A tube placement system allowed external access to internal organs, in which optically‐stimulated luminescent dosimeters (OSLDs) were placed and used to measure dose. Dosimeter placement and location was based on organ size and distribution. In order to determine organ doses of real patients, a correlation between various patient size parameters and the measured organ doses was explored. Only measurements that could be performed on a CT image or subject‐specific parameters or data which could otherwise be obtained for an actual patient were considered for this correlation. The size parameters that were examined included: body mass index (BMI), the AP and lateral dimensions of the patient, and patient perimeter. Results: The BMIs for the 7 subjects ranged from 16.6–43.9, spanning from underweight to extreme obesity. Overall average organ doses from a CAP exam for all subjects ranged from 11.8–24.4 mGy. Generally, organ doses were shown to increase with all size parameters examined. Conclusion: For the purpose of accuracy, the estimation of patient dose in CT must be based on actual physical measurements. A complete set of direct organ dose measurements for 7 adult female cadavers has been accomplished for common CT exams with this research. It has been shown before that patient size parameters can be indicative of patient dose. This work has shown further validation of this concept.

Validation of the delivered dose for gated rapidarc treatments using the electronic portal imaging device

Purpose Validate the dose for Gated RapidArc (RA) treatment technique using portal imaging dosimetry aSi1000. Methods and materials 5 cases of liver stereotactic gated treatments were selected. RA plans were optimized using the Eclipse TPS (v.10) and the PRO3 (v.) algorithm. Verification of treatment plans was performed using an Electronic Portal Imaging Dose (EPID). Portal Dose Prediction algorithm was used to predict the dose at the isocenter. Gated RA treatments were performed using a Varian Novalis TrueBeam linear accelerator. The portal dose images were acquired with a portal imager aSi1000. A respiratory gating signal was generated using a phantom using infrared reflective markers providing a periodic movement. The measurements were performed with respiratory gating in the same conditions of treatment and compared with portal images acquired without gating and both were then compared to images of dose predicted by the TPS. Quantitative analysis of the consistency of dose distributions was performed using the gamma index. Criteria 2 mm in distance (DTA Distance To Agreement) and 2% in dose difference (DDmax) and 3 mm/3% were selected for the evaluation. Results The average gamma index <1 obtained for plans with Gated RA compared with images dose predicted by the TPS is 97.25% (±2.32) for 2 mm DTA criteria and 2% DD and 98.95% (±1.10) for 3 mm DTA criteria and 3% in DD. The average index for gamma RA plans without gating compared to images of dose predicted by the TPS is 97.16% (±2.46) for 2 mm DTA and DD 2% and 98.92% (±1.13) for 3 mm/ 3%.Comparison of images dose acquired with and without gating is 100% (±0) for 2 mm in DTA and 2% in DD as well as 3 mm/3%. Conclusion The dose for RA treatment technique with and without respiratory gating shows a very good agreement. The portal imager aSi1000 can be used as a tool for verification of Gated RA treatment plans. Validate the dose for Gated RapidArc (RA) treatment technique using portal imaging dosimetry aSi1000. 5 cases of liver stereotactic gated treatments were selected. RA plans were optimized using the Eclipse TPS (v.10) and the PRO3 (v.) algorithm. Verification of treatment plans was performed using an Electronic Portal Imaging Dose (EPID). Portal Dose Prediction algorithm was used to predict the dose at the isocenter. Gated RA treatments were performed using a Varian Novalis TrueBeam linear accelerator. The portal dose images were acquired with a portal imager aSi1000. A respiratory gating signal was generated using a phantom using infrared reflective markers providing a periodic movement. The measurements were performed with respiratory gating in the same conditions of treatment and compared with portal images acquired without gating and both were then compared to images of dose predicted by the TPS. Quantitative analysis of the consistency of dose distributions was performed using the gamma index. Criteria 2 mm in distance (DTA Distance To Agreement) and 2% in dose difference (DDmax) and 3 mm/3% were selected for the evaluation. The average gamma index <1 obtained for plans with Gated RA compared with images dose predicted by the TPS is 97.25% (±2.32) for 2 mm DTA criteria and 2% DD and 98.95% (±1.10) for 3 mm DTA criteria and 3% in DD. The average index for gamma RA plans without gating compared to images of dose predicted by the TPS is 97.16% (±2.46) for 2 mm DTA and DD 2% and 98.92% (±1.13) for 3 mm/ 3%.Comparison of images dose acquired with and without gating is 100% (±0) for 2 mm in DTA and 2% in DD as well as 3 mm/3%. The dose for RA treatment technique with and without respiratory gating shows a very good agreement. The portal imager aSi1000 can be used as a tool for verification of Gated RA treatment plans.

SU-E-T-107: Portal Dosimetry Output Factors for TrueBeam: A Comparison of Six Machines

Purpose: Available on the TrueBeam (Varian Medical Systems, Palo Alto, CA), portal dosimetry is a technique of fluence measurement capable of performing patient‐specific QA using an accelerator's onboard EPID device. Being fully‐integrated, this system is accurate and fast, however its time‐consuming commissioning requirements can steer clinics away from this solution. Through the comparison of portal dosimetry output factor tables measured on 6 TrueBeams, this study aims to determine if a set of predetermined data is a proper option for use in commissioning. The use of such data could preclude the necessity for machine‐specific measurement, reducing commissioning time by 0.5 and 1.0 days. Methods: The commissioning of the Portal Dosimetry Image Prediction algorithm in Varian's Eclipse Treatment Planning System requires generation of a relative output factor table of the mean central pixel value determined during EPID irradiation of various field sizes. These field sizes should cover a well‐sampled distribution of inplane and crossplane collimator‐width combinations. These values are used by the TPS to translate a field's fluence into a simulate EPID image and compared to actual fluences measured on the EPID. Data were acquired for 6 MV photon beams from 6 different TrueBeam accelerators. Results: Individually, each PDOF table allowed for accurate fluence prediction in their respective systems. An average PDOF table was acquired with a mean SD of 0.3%, a value comparable to the uncertainty in vendor‐provided relative scatter factors for the same energy. Conclusion: The data provided shows that the use of a single set of ideal portal dosimetry output factors, presented herein as the average of six TrueBeams, can expedite the commissioning process without introducing dosimetric error to portal dosimetry. The authors present these data for this purpose, while the use of such data should come at the discretion of the clinic and per vendor recommendation.

A Monte Carlo tool for evaluating VMAT and DIMRT treatment deliveries including planar detectors

The aim of this work is to describe and validate a new general research tool that performs Monte Carlo (MC) simulations for volumetric modulated arc therapy (VMAT) and dynamic intensity modulated radiation therapy (DIMRT), simultaneously tracking dose deposition in both the patient CT geometry and an arbitrary planar detector system. The tool is generalized to handle either entrance or exit detectors and provides the simulated dose for the individual control-points of the time-dependent VMAT and DIMRT deliveries. The MC simulation tool was developed with the EGSnrc radiation transport. For the individual control point simulation, we rotate the patient/phantom volume only (i.e. independent of the gantry and planar detector geometries) using the gantry angle in the treatment planning system (TPS) DICOM RP file such that each control point has its own unique phantom file. After MC simulation, we obtained the total dose to the phantom by summing dose contributions for all control points. Scored dose to the sensitive layer of the planar detector is available for each control point. To validate the tool, three clinical treatment plans were used including VMAT plans for a prostate case and a head-and-neck case, and a DIMRT plan for a head-and-neck case. An electronic portal imaging device operated in ‘movie’ mode was used with the VMAT plans delivered to cylindrical and anthropomorphic phantoms to validate the code using an exit detector. The DIMRT plan was delivered to a novel transmission detector, to validate the code using an entrance detector. The total MC 3D absolute doses in patient/phantom were compared with the TPS doses, while 2D MC doses were compared with planar detector doses for all individual control points, using the gamma evaluation test with 3%/3 mm criteria. The MC 3D absolute doses demonstrated excellent agreement with the TPS doses for all the tested plans, with about 95% of voxels having γ <1 for the plans. For planar dosimetry image comparisons, we defined an acceptable pass rate of >90% of percentage pixels with γ <1. We found that over 90% of control points in the plans passed this criterion. In general, our results indicate that the simulation tool is suitable for accurately calculating both patient/phantom doses and planar doses for VMAT dose delivery. The tool will be valuable to check performance and advance the development of in vivo planar detectors for use in measurement-based VMAT dose verification. In addition, the tool can be useful as an independent research tool for VMAT commissioning of the TPS and delivery system.

Practice-based evidence to evidence-based practice: building the National Radiation Oncology Registry.

The National Radiation Oncology Registry (NROR), sponsored by the Radiation Oncology Institute and the American Society for Radiation Oncology, is designed to collect standardized information on cancer care delivery among patients treated with radiotherapy in the United States and will focus on patients with prostate cancer. Stakeholders were engaged through a forum that emphasized the need for patient-centered outcomes, minimal data burden, and maximal connectivity to existing registries and databases. An electronic infrastructure is under development to provide connectivity across radiation oncology and hospital information systems. The NROR Gateway features automatic abstraction as well as aggregation of treatment and outcome data. The prostate cancer data dictionary provides standardized elements in four domains: facility, physician, patient, and treatment. The pilot phase will consist of clinical centers chosen to provide a representative mix of radiation treatment modalities, facility types, population-based settings, and regional locations. The initial set of radiation practice metrics includes physician board certification and maintenance, ordering of staging scans, active surveillance discussion, dose prescriptions for low-risk/high-risk disease, radiation fields for low-risk/high-risk disease, image-guided radiation therapy use, androgen deprivation therapy use, post-brachytherapy implant computed tomography dosimetry, collection of toxicity assessments, and longitudinal patient follow-up. The NROR pilot study will provide the framework for expansion to a nationwide electronic registry for radiation oncology.

Estimativa dos valores de MSAD em procedimentos de tomografia computadorizada utilizando filmes radiocrômicos

OBJETIVO: Verificar a viabilidade de filmes radiocrômicos como um dosímetro alternativo para estimativa da dose média em cortes múltiplos a partir dos perfis de kerma. MATERIAIS E MÉTODOS: Os filmes foram distribuídos em cilindros posicionados no centro e nas regiões periféricas de um objeto simulador padrão de abdome utilizado para dosimetria em tomografia computadorizada. RESULTADOS: Os valores de dose média em cortes múltiplos calculados foram 13,6 ± 0,7, 13,5 ± 0,7 e 18,7 ± 1,0 mGy para os valores de passo (pitch) de 0,75, 1,00 e 1,50, respectivamente. CONCLUSÃO: Apesar de os resultados mostrarem valores menores que o nível de referência de radiodiagnóstico de 25 mGy estabelecido pela legislação brasileira para exames de abdome, eles sugerem que há espaço para otimização dos procedimentos e uma revisão do valor para o nível de referência de radiodiagnóstico brasileiro.

PO-0982: Additional value of T2-weighted MR imaging for post-planning dosimetry after I-125 prostate brachytherapy

PO-0982: Additional value of T2-weighted MR imaging for post-planning dosimetry after I-125 prostate brachytherapy

Three-Dimensional Dosimetry Imaging of 125I Plaque Using Gafchromic Film for Eye Cancer Treatment

Three-Dimensional Dosimetry Imaging of 125I Plaque Using Gafchromic Film for Eye Cancer Treatment

Screening Computed Tomography Colonography With 256-Slice Scanning

The aim of this study was to determine the radiation burden and the lifetime attributable risk (LAR) of radiation-induced cancer in patients undergoing screening 256-slice computed tomography colonography (CTC) and compare CTC-related radiogenic risks to corresponding nominal lifetime intrinsic risk of cancer. A Monte Carlo simulation software dedicated for computed tomography (CT) dosimetry was used to determine absorbed doses to primarily exposed radiosensitive organs of 31 women and 29 men subjected to screening CTC on a 256-slice CT scanner. Effective dose was estimated from (a) organ dose data and (b) dose-length product. Organ-specific and total LARs of cancer were estimated using published risk factors. Cumulative LARs from repeated CTC studies on individuals participating in a colorectal cancer screening program were compared with corresponding lifetime intrinsic risks. The mean organ dose-derived effective dose was estimated to be 2.92 and 2.61 mSv for female and male individuals, respectively. The dose-length product method was found to overestimate effective dose from CTC by 26% and 13% in female and male individuals, respectively. Compared with previously published results for 64-slice CT scanners, 256-slice CTC was found to be associated with up to 45% less radiation burden. The cumulative LAR of radiation-induced cancer from repeated quinquennial screening CTC studies between the ages of 50 and 80 years was estimated to increase the lifetime intrinsic risk of cancer by less than 0.2%. The level of patient radiation burden and theoretical radiogenic cancer risks associated with screening CTC performed using modern low-dose protocols and techniques may not justify disapproval of CTC as a mass screening tool.