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A calculation of the relative biological effectiveness of 125I and 103Pd brachytherapy sources using the concept of proximity function

The clinical application of encapsulated radioactive sources in brachytherapy plays an important role in the treatment of malignancy. 125I and 103Pd sources have been widely used in the permanent implant of prostate cancer. An important consideration for the choice of brachytherapy sources is their relative biological effectiveness (RBE). Previous calculations of this quantity have used the dose-averaged lineal energy, yD, as a measure of biological effectiveness. In this approach, however, the selection of a relevant site size remains an open question. Here we avoid this problem by using the generalized theory of dual radiation action to calculate the initial slope, alpha, of the dose-effect curves using the proximity function, t(x), and the biological response function, gamma(x). At low doses and/or low dose rates (e.g., prostate implants) the parameter alpha determines the RBE. Proximity function, t(x), is the probability distribution function of distances between pairs of sublesions; and the biological function, gamma(x), is the probability that two sublesions at a distance x apart results in a lesion. Functions t(x) have been calculated for each source using the Monte Carlo transport codes PHOEL and PROTON5. The function gamma(x) has been taken from a published analysis. The RBE values thus obtained are: 1.5 for 125I and 1.6 for 103Pd. The question of whether an effective site size exists where yD approximates best the variation of alpha with radiation quality is also addressed.

AAPM TG 158: Measurement and calculation of doses outside the treated volume from external-beam radiation therapy

The introduction of advanced techniques and technology in radiotherapy has greatly improved our ability to deliver highly conformal tumor doses while minimizing the dose to adjacent organs at risk. Despite these tremendous improvements, there remains a general concern about doses to normal tissues that are not the target of the radiation treatment; any nontarget radiation should be minimized as it offers no therapeutic benefit. As patients live longer after treatment, there is increased opportunity for late effects including second cancers and cardiac toxicity to manifest. Complicating the management of these issues, there are unique challenges with measuring, calculating, reducing, and reporting nontarget doses that many medical physicists may have limited experience with. Treatment planning systems become dramatically inaccurate outside the treatment field, necessitating a measurement or some other means of assessing the dose. However, measurements are challenging because outside the treatment field, the radiation energy spectrum, dose rate, and general shape of the dose distribution (particularly the percent depth dose) are very different and often require special consideration. Neutron dosimetry is also particularly challenging, and common errors in methodology can easily manifest as errors of several orders of magnitude. Task Group 158 was, therefore, formed to provide guidance for physicists in terms of assessing and managing nontarget doses. In particular, the report: (a) highlights major concerns with nontarget radiation; (b) provides a rough estimate of doses associated with different treatment approaches in clinical practice; (c) discusses the uses of dosimeters for measuring photon, electron, and neutron doses; (d) discusses the use of calculation techniques for dosimetric evaluations; (e) highlights techniques that may be considered for reducing nontarget doses; (f) discusses dose reporting; and (g) makes recommendations for both clinical and research practice.

Rapid dose calculations for stereotactic radiosurgery

A three-dimensional dose calculation algorithm is described for stereotactic radiosurgery using multiple noncoplanar beam arcs. Precalculated dose libraries of 20-deg arc segments, or mini arcs, are stored in computer memory which permits rapid calculation of complete, high resolution, three-dimensional isodose distributions and dose volume histograms. Three-dimensional patient contours and target volumes are obtained from CT scans and angiographic x rays. Rapid dose calculations are made possible by the use of arc libraries and an improved algorithm for mapping beam doses to the dose calculation grid. This permits more flexibility in designing optimum treatment plans, as five-six complete plans can be generated in less than 1 h. Thus many possible treatment options can be tested in the 3-4-h time period typically available in stereotactic procedures between CT scanning and treatment.

Recommended ethics curriculum for medical physics graduate and residency programs: Report of Task Group 159

The AAPM Professional Council approved the formation of a task group in 2007, whose purpose is to develop recommendations for an ethics curriculum for medical physics graduate and residency programs. Existing programs ethics curricula range in scope and content considerably. It is desirable to have a more uniform baseline curriculum for all programs. Recommended subjects areas, suggested ethics references, and a sample curriculum are included. This report recommends a reasonable ethics course time to be 15-30 h while allowing each program the flexibility to design their course.

Effect of refraction on dose reconstruction in optical-CT gel dosimetry

We address the problem of dose reconstruction based on limited experimentally accessible data due to the effect of refraction in optical-CT gel dosimetry. The refractive index mismatch between the components of the optical-CT scanner result in light scattering and ultimately in the inability to capture parts of the projection datasets. We determine the maximum loss of data and the corresponding refractive index mismatch for which accurate dose reconstruction in the central part of the phantom is still possible. Also, a mathematical formalism that indicates how exact reconstructions can be obtained using a priori knowledge of the optical attenuation coefficient of the gel is presented. This study establishes rigorous design principles for accurate 3D dose reconstruction.

SU‐GG‐T‐188: A Field Size Specific Backscatter Correction Algorithm for Accurate EPID Dosimetry

PURPOSEnPortal dose images acquired with an amorphous silicon electronic portal imaging device (EPID) suffer from artifacts related to backscattered radiation. The backscatter signal varies as a function of field size (FS) and location on the EPID. Most current portal dosimetry algorithms fail to account for the FS dependence. The ramifications of this omission are investigated and solutions for correcting the measured dose images for FS specific backscatter are proposed.nnnMETHODSnA series of open field dose images were obtained for field sizes ranging from 2 x 2 to 30 x 40 cm2. Each image was analyzed to determine the amount of backscatter present. Two methods to account for the relationship between FS and backscatter are offered. These include the use of discrete FS specific correction matrices and the use of a single generalized equation. The efficacy of each approach was tested on the clinical dosimetric images for ten patients, 49 treatment fields. The fields were evaluated to determine whether there was an improvement in the dosimetric result over the commercial vendors current algorithm.nnnRESULTSnIt was found that backscatter manifests itself as an asymmetry in the measured signal primarily in the inplane direction. The maximum error is approximately 3.6% for 10 x 10 and 12.5 x 12.5 cm2 field sizes. The asymmetry decreased with increasing FS to approximately 0.6% for fields larger than 30 x 30 cm2. The dosimetric comparison between the measured and predicted dose images was significantly improved (p << .001) when a FS specific backscatter correction was applied. The average percentage of points passing a 2%, 2 mm gamma criteria increased from 90.6% to between 96.7% and 97.2% after the proposed methods were employed.nnnCONCLUSIONSnThe error observed in a measured portal dose image depends on how much its FS differs from the 30 x 40 cm2 calibration conditions. The proposed methods for correcting for FS specific backscatter effectively improved the ability of the EPID to perform dosimetric measurements. Correcting for FS specific backscatter is important for accurate EPID dosimetry and can be carried out using the methods presented within this investigation.

SU-FF-T-172: Verification of Whole-Body Dosimetry in An IMRT Treatment Planning System

Purpose:IMRT has been widely used in radiation therapy, since this technique shows potential for further improving the therapeutic ratio and reducing complications. On the other hand, it has been suggested that IMRT presents a potential impact on the induction of second malignancies, because it can result in a higher whole‐body dose due to leakage radiation. In the routine treatment planning process, complete information on the whole‐body dose‐volume histogram is not available due to the limited patient body volume imaged in the CTtreatment planning process. In addition, for IMRT, larger volumes of normal tissues are being exposed to low doses, and the dosimetric uncertainties of a treatment planning system at these doses are relatively large. In this study, whole‐body dosimetry calculated from the Eclipse‐Helios planning system was verified using a whole‐body anthropomorphic phantom and MOSFET detectors, as well as polymergels.Method and Materials: The “ATOM” whole‐body anthropomorphic phantom was CT‐ scanned into the Eclipse‐Helios system. An IMRT prostate plan was designed for the ATOM phantom. Each MOSFET detector was calibrated at various angles based on ion chamber dosimetry. The MOSFET detectors were precisely placed in relocatable dosimeter positions corresponding to various internal organs, allowing point‐dose measurements and comparison. BANG® polymergel, prepared in a cylindrical container, was placed at the phantom head position to measure the 3D dose distribution. The DVH in the gel cylinder, analyzed with an optical CT scanner, was compared with that from the planning system. Results: Preliminary results show that the agreement between the MOSFET measurements and the calculated results is within 5% for points within the target. At low‐dose regions (0.1–60%), discrepancies are larger but reasonable. DVH comparison between gels and the treatment planning will be presented. Conclusion: Anthropomorphic phantom with MOSFET detectors and polymergels can provide whole‐body dosimetry verification for IMRT.

SU‐E‐T‐68: Improvement of Optical CT Scanner for 3‐D Dosimetry

PURPOSEnThe first generation 3D optical CT scanner OCTOPUS has been modified. The modified scanner has a new developed user control panel written in Labview which provides more flexibility to optimize mechanical control and data acquisition technique. The total scan time has been significantly reduced from initial 10 hours to ∼ 4 hours.nnnMETHODSnThe modified scanner is developed from OCTOPUS by adding a 50/50 beam splitter, 4 broadband (400-750nm) dielectric mirrors and an identical photodetector as the original one. A 25mm diameter filter with central wavelength 630nm is attached to each of the photodetectors to remove ambient light. The laser source is HeNe Laser with wavelength 632.8 nm. All these optical components are aligned well and mounted on a single movable frame. The frame can move vertically along the water tank to allow laser beam covering the whole dosimeter. To test the modified scanner, the sinogram of a homemade phantom with two plastic rods (7 mm diameter) has been generated by combining two sets of projection data acquired from perpendicular photodetectors. We also studied a single field irradiation (4cm x 4cm) generated from a linear accelerator with 6 MV x-ray beam using an 11cm diameter and 8 cm height PRESAGE dosimeter.nnnRESULTSnThe sinogram shows that the combined data from two photodetectors match well. By setting the same window/level value using ImageJ, the optical density of irradiated square field obtained using the modified scanner shows comparable contrast, sharpness and uniformity as OCTPUS. Isodose distribution among OCTOPUS, the modified scanner and Eclipse plan agree well at the 90%, 75%, 50% isodose lines.nnnCONCLUSIONnBy comparing results obtained using the modified scanner and OCTOPUS, we have evaluated that the modified scanner can achieve comparable dose reconstruction quality as OCTOPUS while reducing total scan and reconstruction time to ∼4 hours.

SU-E-T-376: 3-D Commissioning for An Image-Guided Small Animal Micro- Irradiation Platform

PURPOSEnA 3-D radiochromic plastic dosimeter has been used to cross-test the isocentricity of a high resolution image-guided small animal microirradiation platform. In this platform, the mouse stage rotating for cone beam CT imaging is perpendicular to the gantry rotation for sub-millimeter radiation delivery. A 3-D dosimeter can be used to verify both imaging and irradiation coordinates.nnnMETHODSnA 3-D dosimeter and optical CT scanner were used in this study. In the platform, both mouse stage and gantry can rotate 360° with rotation axis perpendicular to each other. Isocentricity and coincidence of mouse stage and gantry rotations were evaluated using star patterns. A 3-D dosimeter was placed on mouse stage with center at platform isocenter approximately. For CBCT isocentricity, with gantry moved to 90°, the mouse stage rotated horizontally while the x-ray was delivered to the dosimeter at certain angles. For irradiation isocentricity, the gantry rotated 360° to deliver beams to the dosimeter at certain angles for star patterns. The uncertainties and agreement of both CBCT and irradiation isocenters can be determined from the star patterns. Both procedures were repeated 3 times using 3 dosimeters to determine short-term reproducibility. Finally, dosimeters were scanned using optical CT scanner to obtain the results.nnnRESULTSnThe gantry isocentricity is 0.9 ± 0.1 mm and mouse stage rotation isocentricity is about 0.91 ± 0.11 mm. Agreement between the measured isocenters of irradiation and imaging coordinates was determined. The short-term reproducibility test yielded 0.5 ± 0.1 mm between the imaging isocenter and the irradiation isocenter, with a maximum displacement of 0.7 ± 0.1 mm.nnnCONCLUSIONnThe 3-D dosimeter can be very useful in precise verification of targeting for a small animal irradiation research. In addition, a single 3-D dosimeter can provide information in both geometric and dosimetric uncertainty, which is crucial for translational studies.

SU‐E‐T‐281: Quantitative Evaluation of the MRI Image Distortion in Gamma Knife Radiosurgery

Purpose: To quantify the spatial distortion of the MRIimages in the Elekta Leksell GammaPlan version 9 planning system using different registration methods. Methods: The RPC SRS phantom was imaged using a GE Signa HDxt 1.5Tesla MR scanner and a GE LightSpeed VCT CTscanner. Both the T1 weighted MRIimages and the fast imaging employing steady state acquisition (FIESTA) MRIimages were acquired, along with the axial and helical CTimages. The skull shell and the target in the phantom were contoured in all the four series of images acquired. The MRIimages were registered in the planning system in three different ways: 1) using the fiduciary marks in the imaging box; 2) using a global co‐registration with FOV just covering skull shell; 3) using a local co‐registration to max 8.5cm FOV at image center. The target positions in all the four series of images were compared using the software tool in the planning system. Results: The target positions as obtained from the helical and the axial CTimages agree within 0.1mm. For the registration method the fiduciary marks, the T1 weighted MRIimages are shifted from the CTimages 0.9mm in the anterior‐posterior direction, and 0.5mm in both the superior‐inferior and the left‐right directions. The corresponding displacements for the FIESTA images are 0.6mm, 0.5mm and 0.5mm respectively. The shifts of the target positions are significantly reduced in the global co‐registration approach, and all less than 0.2mm in the local co‐registration approach. Conclusions: MRIimage distortion in GammaPlan version 9 is in the sub‐millimeter range for the central region of the MRIimage. The spatial distortion in the FIESTA images is smaller than that in the T1 weighted images. Co‐registration to the central region of images can reduce MRIimaging distortion effect significantly.