Y Cao

SU-F-T-56: Dosimetric Characterization of the INTRABEAM 50 KV X-Ray System with a Needle Applicator in Heterogeneous Tissues

PURPOSE We report the depth dose measurements in air, in solid water, and in bone materials for the Intrabeam 50 kV x-rays with a needle applicator. METHODS The absolute dose was measured using a PTW TN34013W soft x-ray ion chamber. Gammex tissue equivalent materials of solid water, inner bone, and cortical bone slabs (minimum thickness of 2 mm) were used. In addition, the PTW solid water slabs with a minimum thickness of 1 mm were used. The manufactory calibrated depth dose data in water were compared. The x-ray source together with a needle applicator was secured on an Intrabeam stand. The slabs lay on a 6 degrees of freedom treatment couch with a digitally controlled minimum step size of 0.1 mm. The depth of the source to the ion chamber was accurately and reproducibly adjusted by moving the couch up and down. RESULTS The depth dose measurements for the Intrabeam 50 kV x-rays with a needle applicator were conducted up to 20 mm in depth. The values for the PTW solid water were close to those for water. The Gammex solid water demonstrated lower values compared to water, consistent with the observation of its positive CT number. At a depth of 10 mm, the dose rates of the system are 29.6, 3.6, 1.2, and 0.24 Gy/min in air, in water, in inner bone, and in cortical bone, respectively. The 10 mm water equivalent depths in inner and cortical bone are about 6.4 and 4.1 mm. A function of power law combining exponential was used to fit and interpolate data well. CONCLUSION Direct depth dose measurements in different materials provide a basis for treatment calculation and planning taking into account the heterogeneous effect. The results can be used for verification of analytical and/or Monte Carlo dose calculation methods as well.

TH-CD-304-02: Clinical Uncertainty of in Vivo Dosimetry for Intensity-Modulated Radiation Therapy Using Optically-Stimulated Luminescent Dosimeters

Purpose: Several studies have reported the physical properties of optically-stimulated luminescent dosimeters (OSLDs) and suggest their efficacy for clinical in vivo dosimetry, but few publications have assessed the clinical uncertainties associated with OSLD-based in vivo dosimetry for conformal and intensity-modulated treatment. The purpose of the current work is to identify and characterize clinical uncertainties for OSLD-based in vivo dosimetry. Methods: OSLDs are placed in dosimetrically-appropriate locations on a weekly basis, covered with small 5 mm bolus squares, exposed during patient treatment, read, and compared with doses predicted from the treatment planning system. Six (6) parameters were identified as significant contributors to uncertainty in the process: Inherent physical OSLD uncertainty (σ_ OSLD), OSLD reader uncertainty (σ_reader), dose calculation uncertainty in the build-up region (σ _calc), uncertainty of depth for planned dose (σ_d), dosimetric uncertainty due to daily image-guided shifts (σ_ s), and placement uncertainty (σ_p). σ_d, σ_ s, and σ_p were estimated by analyzing clinical OSLD dosimetry for inverse-planned intensity-modulated treatment plans (prostate, lung, and head-and-neck) and field-in-field intensity-modulated treatment plans (breast). Total uncertainty was estimated by summing the 6 components in quadrature. Results: σ_OSLD was defined by the manufacturer (±3.0%). σ_calc was assumed to be approximately ±5.0% at 5 mm depth from other works in the literature. σ_reader and σ_s were measured to be ±1.0 and ±5.8% at 5 mm depth respectively. σ_p was found to be ±3.6% for breast, ±4.2% for prostate, ±4.4% for lung, and ±10.6% for head-and-neck. Total uncertainty was ±9.0% for breast, ±11.5% for prostate, ±13.2% for lung, and ±16.1% for head-and-neck. Conclusion: Site-specific clinical uncertainty for a limited selection of sites ranged from ±9.0% to ±16.1%. The largest components were image-guided shifts and estimated placement uncertainty. A wider selection of anatomical sites, site-specific correction factors, and clinical tolerance/action level guidelines will be presented.

SU-E-T-638: Evaluation and Comparison of Landauer Microstar (OSLD) Readers

PURPOSE To evaluate and compare characteristic performance of a new Landauer nanodot Reader with the previous model. METHODS In order to calibrate and test the reader, a set of nanodots were irradiated using a Varian Truebeam Linac. Solid water slabs and bolus were used in the process of irradiation. Calibration sets of nanodots were irradiated for radiation dose ranges: 0 to 10 and 20 to 1000 cGy, using 6MV photons. Additionally, three sets of nanodots were each irradiated using 6MV, 10MV and 15MV beams. For each beam energy, and selected dose in the range of 3 to 1000 cGy, a pair of nanodots was irradiated and three readings were obtained with both readers. RESULTS The analysis shows that for 3 photon beam energies and selected ranges of dose, the calculated absorbed dose agrees well with the expected value. The results illustrate that the new Microstar II reader is a highly consistent system and that the repeated readings provide results with a reasonably small standard deviation. For all practical purposes, the response of system is linear for all radiation beam energies. CONCLUSION The Microstar II nanodot reader is consistent, accurate, and reliable. The new hardware design and corresponding software contain several advantages over the previous model. The automatic repeat reading mechanism, that helps improve reproducibility and reduce processing time, and the smaller unit size that renders ease of transport, are two of such features. Present study shows that for high dose ranges a polynomial calibration equation provides more consistent results. A 3rd order polynomial calibration curve was used to analyze the readings of dosimeters exposed to high dose range radiation. It was observed that the results show less error compared to those calculated by using linear calibration curves, as provided by Landauer system software for all dose ranges.

SU‐F‐T‐52: Study of Energy Dependent Effect of Dosimetry Systems Used in Therapeutic Soft X‐Ray Energy Range

PURPOSE To investigate energy dependent effects of different dosimetry systems which can be used as in vivo dosimetry monitoring for intraoperative radiotherapy in therapeutic soft x-ray energy range. METHODS Three dosimetry systems were evaluated in therapeutic soft x-ray energy range: optically stimulated luminescent dosimeter (OSLD) nanoDots, radiochromic EBT2 and EBT3 films. The x-ray photons were produced by a Zeiss Intrabeam 50 kV x-ray radiotherapy system. Solid water and bolus slabs with different thicknesses were used in the process of irradiation. An aluminum filter set was used to measure HVLs of X-rays. Calibration curves were made at different depth of boluses. RESULTS Half Value Layers at depths of 0, 3, 10, and 20 mm of solid water were measured to represent the energy change versus depth, yielding 0.306, 0.482, 0.865 and 0.901 respectively and indicating nearly unchanged HVL beyond 1 cm depth. The responses of each system at different depths were normalized to the response at 2 cm depth. In film dosimetry, the response is calculated as optical density (OD). The results show that there is nearly the same energy dependence for EBT2 and EBT3. At a HVL of 0.482 mm Al, the relative responses of nanoDots and EBT3 are 0.85 ± 0.04 and 0.89 ± 0.03 compared to those at 0.901 mm Al HVL, respectively, indicating no obvious difference between those two systems within the measurement uncertainty. CONCLUSION It was observed that the studied dosimeter response increases about 13% from the x-ray energy of 0.48 mm Al to 0.90 mm Al. Therefore, caution should be exercised in using an appropriate calibration curve, and x-ray beam hardening effect has to be taken into account.

SU-F-T-654: Pacemaker Dose Estimate Using Optically Stimulated Luminescent Dosimeter for Left Breast Intraoperative Radiation Therapy

PURPOSE To assess and report the in vivo dose for a patient with a pacemaker being treated in left breast intraoperative radiation therapy (IORT). The ZEISS Intrabeam 50 kVp X-ray beam with a spherical applicator was used. METHODS The optically stimulated luminescent dosimeters (OSLDs) (Landauer nanoDots) were employed and calibrated under the conditions of the Intrabeam 50 kVp X-rays. The nanoDots were placed on the patient at approximately 15 cm away from the lumpectomy cavity both under and above a shield of lead equivalence 0.25 mm (RayShield X-Drape D-110) covering the pacemaker area during IORT with a 5 cm spherical applicator. RESULTS The skin surface dose near the pacemaker during the IORT with a prescription of 20 Gy was measured as 4.0±0.8 cGy. The dose behind the shield was 0.06±0.01 Gy, demonstrating more than 98% dose reduction. The in vivo skin surface doses during a typical breast IORT at a 4.5 cm spherical applicator surface were further measured at 5, 10, 15, and 20 cm away to be 159±11 cGy, 15±1 cGy, 6.6±0.5 cGy, and 1.8±0.1 cGy, respectively. A power law fit to the dose versus the distance z from the applicator surface yields the dose fall off at the skin surface following z^-2.5, which can be used to estimate skin doses in future cases. The comparison to an extrapolation of depth dose in water reveals an underestimate of far field dose using the manufactory provided data. CONCLUSION The study suggests the appropriateness of OSLD as an in vivo skin dosimeter in IORT using the Intrabeam system in a wide dose range. The pacemaker dose measured during the left breast IORT was within a safe limit.

SU-F-T-53: Treatment Planning with Inhomogeneity Correction for Intraoperative Radiotherapy Using KV X-Ray Beams

PURPOSE The current standard in dose calculation for intraoperative radiotherapy (IORT) using the ZEISS Intrabeam 50 kV x-ray system is based on depth dose measurements in water and no heterogeneous tissue effect has been taken into account. We propose an algorithm for pre-treatment planning including inhomogeneity correction based on data of depth dose measurements in various tissue phantoms for kV x-rays. METHODS Direct depth dose measurements were made in air, water, inner bone and cortical bone phantoms for the Intrabeam 50 kV x-rays with a needle applicator. The data were modelled by a function of power law combining exponential with different parameters. Those phantom slabs used in the measurements were scanned to obtain CT numbers. The x-ray beam initiated from the source isocenter is ray-traced through tissues. The corresponding doses will be deposited/assigned at different depths. On the boundary of tissue/organ changes, the x-ray beam will be re-traced in new tissue/organ starting at an equivalent depth with the same dose. In principle, a volumetric dose distribution can be generated if enough directional beams are traced. In practice, a several typical rays traced may be adequate in providing estimates of maximum dose to the organ at risk and minimum dose in the target volume. RESULTS Depth dose measurements and modeling are shown in Figure 1. The dose versus CT number is shown in Figure 2. A computer program has been written for Kypho-IORT planning using those data. A direct measurement through 2 mm solid water, 2 mm inner bone, and 1 mm solid water yields a dose rate of 7.7 Gy/min. Our calculation shows 8.1±0.4 Gy/min, consistent with the measurement within 5%. CONCLUSION The proposed method can be used to more accurately calculate the dose by taking into account the heterogeneous effect. The further validation includes comparison with Monte Carlo simulation.

SU‐E‐T‐102: Factors That Affect the Accuracy of IMRT Quality Assurance Measurements and Their Clinical Significance

Purpose: The purpose of this work was to perform error analysis on various processes that directly affect the accuracy of quality assurance (QA) measurements for intensity modulated radiation therapy(IMRT)delivery and to gauge their overall clinical impact for various clinical sites. Methods: In this study, we developed a numerical model based on the propagation of errors to quantify the complexity of treatment plans by the level of the intensity of modulation, and correlated the level of the modulation directly to the agreement of the IMRT QA results. We also provide detailed analysis of the uncertainties from IMRT QA results stratified by treatment site specific plans and fidelity in plan delivery. Results: Using a 5%/2mm criterion, it was found that the level of agreement of IMRT QA measurements was better for pelvic and chest plans than for head and neck, breast and brain plans. Errors tended to first increase with an increase in the number of Multi‐leaf Collimator(MLC) segments from 5 to 20 and then decrease with further increase in the number of segments. It was determined that other factors that impact the accuracy of the measurements include measurement device limitations, machine output variations, and phantom setup errors. Similarly, uncertainties in the beam data modeling and the calculation algorithm, as well as the initial beam commissioning data are main contributing factors to the accuracies of the treatment planning systems. Conclusions: The overall agreement of IMRT QA results is affected by both ‐ the accuracy of the measurement and that of the treatment planning system. We will demonstrate that the IMRT QA analysis criteria currently used in many clinics can be modified and improved to reflect the uncertainties of the process using the error propagation model developed in this study.

SU-E-T-430: Quantifying Alterations in Flattened and Flattening-Filter-Free Beam Characteristics from a Gantry Mounted, in Vivo Beam Delivery Verification System on a TrueBeam Linear Accelerator

Purpose: Planning and delivery of radiation therapy are becoming more complex with the wide use of intensity‐modulated radiation therapy(IMRT) and volumetric modulated arc therapy (VMAT). In vivo quality assurance of field modulation is possible using a novel multiwire “harp” ionization chamber array DAVID (PTW, Freiburg, Germany) positioned at the treatment head. The purpose of this work is to pre‐validate its use in vivo by characterizing potential dosimetric changes in the beam introduced by the device.Methods: Beam profiles for 6, 10, 15 MV with flattening filter and 6 MV flattening‐filter‐free (FFF) were acquired for square field sizes of 5, 10, 20, and 35 cm at 5.8 cm depth on a TrueBeam linear accelerator (Varian, Palo Alto, CA). Profiles were acquired in the target‐gun, left‐right, and diagonal directions using the StarCheck Maxi ion chamber array (PTW, Freiburg, Germany) with and without DAVID in place. Ion chamber spacing was 3 mm. Attenuation was measured by calculating the transmission factor at the central chamber with and without DAVID. Results: For beams with a flattening filter, transmission factors ranged from 95–96% with 15 MV to 92–94% with 6 MV. For the 6 MV FFF, the transmission factor was 91–92%. Beam profiles were minimally affected by DAVID, demonstrating at most 1–2% difference at 35 cm field size, for the higher energy beams. For 6 MV and 6 MV FFF and field sizes less than 20 cm, profiles agreed within 1% even in penumbral regions Conclusions: Except for a correctable transmission factor, the presence of DAVID minimally influences dosimetric profiles. Transmission factors range from 92%–96% for conventional beams, but can be lower for FFF beams. In future work, percent depth dose and electron contamination will be measured to examine spectral influences of DAVID on FFF beams

SU-GG-T-82: Clinical Experiences and Dosimetric Comparison of the Model 6711 and 9011 I-125 for Intra-Operative Prostate Implant

Purpose:This study compares the film dosimetric characteristics of seed model 9011 I‐125 (THIN) and model 6711 I‐125 (Both sources from Oncura). The THIN seed uses a smaller 20 gauge needle diameter compared to the 18 gauge standard used for model 6711. Method and Materials: Twenty patients (pts) underwent transperineal brachytherapy; 10 each with model 9011 and model 6711 seeds, respectively. Both groups were implanted with dynamic intra‐operative technique using the MICK applicator. The source activity range was 0.45 −0.5 mCi. Three weeks after the implant, a chest x‐ray, pelvic x‐ray and pelvic CT were obtained. The intra‐operative and post‐implant evaluations were done with Variseed 8.0.1. Additionally, Model 9011 and 6711 seeds were exposed to Gafchromic EBT2 films and scanned with the Vida Pro Advantage film scanner. The dosimetric properties of the two models were analyzed and compared using RIT software. Results: Film dosimetry demonstrated that the dosimetric parameters for THIN seed model 9011 were similar to those of model 6711. Anisotropy and radial dose function were slightly different, within 1% for r>1cm. Intra‐operatively, the thin needles were better‐visualized and easier to track on ultrasoundimages. Visualization of individual sources on pelvic X‐ray films for model 6711 and 9011 were similar. There was no seed migration to the lung for either group. For post‐implant evaluation model 9011 seeds were more difficult to identify on pelvic CTimages using Seed Finder module of Veriseed software. Conclusion: The dosimetric characteristics of THIN seed model 9011 and model 6711 are similar. Intra‐operative visualization of the THIN seed improved source capture on the intra‐operative planning system. Post implant source localization was more difficult on CTimages.

SU‐GG‐T‐94: An Automated Tool for Determining Output Factor for Electron CutOuts

Purpose: To provide streamlined methods for quickly and accurately determining the output factor for irregularly shaped electron cutouts. Measurement on a linear accelerator of output factor for electron cutouts is a time consuming physics task. Existing methods that calculate output factor for shaped apertures are approximations based on cutout length and width, or require a printed version of a cutout shape for either manual measurement or scanning and digitizing. Methods and Materials: An integrated spreadsheet and database provides several options for output factor determination: a measured value or an approximation based on cutout length and width may be used, an image based database containing previously measured cutouts can be searched, or the electron cutout shape contour can be imported electronically from the Philips Pinnacle3treatment planning system, and the output factor calculated without further user interaction. For the last option, a weighted sector‐integration based algorithm is used that employs polar coordinates and does not require either the calculation of the intersection of equispaced vectors with the block contour or the repeated rotation of the contour. A set of measurements taken for a variety of energies, electron cone sizes, and circular cut out diameters is used to generate a standard output factor table for the sector integration approach. Results: The output factor calculations were tested on a set of cutouts for electron boost treatments. Comparison of measured and calculated output factors are presented. Conclusion: In a busy clinic environment, measurement of output factors for electron cutouts is time burden on physics staff, so automated methods of calculation that involve a minimum of staff time while preserving accuracy have proved valuable in our clinics.