Optimizing the response, precision, and cost of a DNA double-strand break dosimeter.

We developed a dosimeter that measures biological damage following delivery of therapeutic beams in the form of double-strand breaks (DSBs) to DNA. The dosimeter contains DNA strands that are labeled on one end with biotin and on the other with fluorescein and attached to magnetic microbeads. Following irradiation, a magnet is used to separate broken from unbroken DNA strands. Then, fluorescence is utilized to measure the relative amount of broken DNA and determine the probability for DSB. The long-term goal for this research is to evaluate whether this type of biologically based dosimeter holds any advantages over the conventional techniques. The purpose of this work was to optimize the dosimeter fabrication and usage to enable higher precision for the long-term research goal. More specifically, the goal was to optimize the DNA dosimeter using three metrics: the response, precision, and cost per dosimeter. Six aspects of the dosimeter fabrication and usage were varied and evaluated for their effect on the metrics: (1) the type of magnetic microbeads, (2) the microbead to DNA mass ratio at attachment, (3) the type of suspension buffer used during irradiation, (4) the concentration of the DNA dosimeter during irradiation, (5) the time waited between fabrication and irradiation of the dosimeter, and (6) the time waited between irradiation and read out of the response. In brief, the best results were achieved with the dosimeter when attaching 4.2 µg of DNA with 1 mg of MyOne T1 microbeads and by suspending the microbead-connected DNA strands with 200 µl of phosphate-buffered saline for irradiation. Also, better results were achieved when waiting a day after fabrication before irradiating the dosimeter and also waiting an hour after irradiation to measure the response. This manuscript is meant to serve as guide for others who would like to replicate this DNA dose measurement technique.

Evaluation and comparison of second-check monitor unit calculation software with Pinnacle3 treatment planning system.

The purpose of this study was to evaluate and compare the accuracy of dose calculations in second check softwares (Diamond, IMSure, MuCheck, and RadCalc) against the Phillips Pinnacle3 treatment planning system. Eighteen previously treated patients' treatment planning files consisting of a total of 204 beams were exported from the Pinnacle3 TPS to each of the four second check software. Of these beams, 145 of the beams used were IMRT plans while 59 were VMAT arcs. The values were represented as a percent difference between primary and secondary calculations and used for statistical analysis. Box plots, Pearson Correlation, and Bland-Altman analysis were performed in MedCalc. The mean percent difference in calculated dose for Diamond, IMSure, MuCheck, and RadCalc from Pinnacle3 were -0.67%, 0.31%, 1.51% and -0.36%, respectively. The corresponding variances were calculated to be 0.07%, 0.13%, 0.08%, and 0.03%; and the largest percent differences were -7.9%, 9.70%, 9.39%, and 5.45%. The dose differences of each of the second check software in this study can vary considerably and VMAT plans have larger differences than IMRT. Among the four second check softwares, RadCalc values has shown a high agreement on average with low variation, and had the smallest percent range from Pinnacle3 values. The closest in average percent difference from the Pinnacle3data was the IMSure software, but suffered from significantly larger variance and percent range. The values reported by Diamond and MuCheck had significantly high percent differences with TPS values.

Pediatric Cranio-spinal Axis Irradiation: Comparison of Radiation-induced Secondary Malignancy Estimations Based on Three Methods of Analysis for Three Different Treatment Modalities.

Pediatric cranio-spinal axis irradiation (CSI) is a valuable treatment for many central nervous system (CNS) diseases, but due to the life expectancies and quality of life expectations for children, the minimization of the risk for radiation-induced secondary malignancies must be a high priority. This study compared the estimated CSI-induced secondary malignancy risks of three radiation therapy modalities using three different models. Twenty-four (n = 24) pediatric patients previously treated with CSI for tumors of the CNS were planned using three different treatment modalities: three-dimensional conformal radiation therapy (3D-CRT), volume modulated arc therapy (VMAT), and Tomotherapy. Each plan was designed to deliver 23.4 Gy (1.8 Gy/fraction) to the target which was defined as the entire brain and spinal column with a 0.7 cm expansion. The mean doses as well as the dose volume histograms (DVH) of specific organs were analyzed for secondary malignancy risk according to three different methods: the effective dose equivalent (EDE), the excess relative risk (ERR), and the linear quadratic (LQ) models. Using the EDE model, the average secondary risk was highest for the 3D-CRT plans (37.60%), compared to VMAT (28.05%) and Tomotherapy (27.90%). The ERR model showed similarly that the 3D-CRT plans had considerably higher risk (10.84%) than VMAT and Tomotherapy, which showed almost equal risks (7.05 and 7.07%, respectively). The LQ model requires organ-specific cell survival parameters, which for the lungs, heart, and breast relevant values were found and applied. The lung risk for secondary malignancy was found to be 1.00, 1.96, and 2.07% for 3D-CRT, VMAT, and Tomotherapy, respectively. The secondary cancer risk for breast was estimated to be 0.09, 0.21, and 0.27% and for heart it was 9.75, 6.02 and 6.29% for 3D-CRT, VMAT, and Tomotherapy, respectively. Based on three methods of secondary malignancy estimation, the 3D-CRT plans produced highest radiation-induced secondary malignancy risk, and the VMAT and Tomotherapy plans had nearly equal risk. Pediatric patients must be treated with reducing long term sequelae as a priority.

SU-E-T-562: Reducing the Arc Span for CSA VMAT Delivery.

PURPOSE: To dosimetrically evaluate advantages and disadvantages of using multiple, shorter, sub-arcs versus full arc deliveries for treatment of cranio-spinal axis (CSA) irradiation. METHODS: Five (n=5) cranio-spinal axis irradiation patients were planned using 2 complete arcs, one superior and one inferior; with gantry rotations from 1 to 359 degrees. Due to supine patient setup, each original full arc was then replanned split into two sub arcs with gantry rotations from 1 to 100 and 260 to 359 degrees creating 4 smaller arcs. The PTV was normalized such that 95% received at least 23.4 Gy in 13 fractions. The PTV was evaluated based on conformity number and homogeneity index. The normal structures were evaluated based on maximum and mean doses. Beam on times and monitor units were compared. RESULTS: Averaged over all patients, conformity number was calculated to be approximately 0.86 and 0.82 for full arc and sub arc plans respectively. The homogeneity index was approximately 1.07 and 1.06 for full arc and sub arc plans. This indicates better target conformity but less homogeneous dose distribution for full arc plans as compared with sub arc plans. With the exception of the eyes, each normal structure evaluated had lower maximum doses with subarc plans. All normal structures, with the exception of the left kidney, had lower mean doses using sub arc deliveries. Beam on times were shorter on average for full arcs, but the monitor units were lower on average for sub arcs. CONCLUSIONS: Overall, CSA patients would benefit from the use of sub arc treatment deliveries versus full arc deliveries. Nearly all normal structure doses were lower for sub arcs, while the PTV was still adequately covered and beam on times and monitor units were similar.

SU-E-T-601: Dosimetric Evaluation of the Parameter Variation with Varying Calculation Grid Size in the IMRT Cases.

PURPOSE: The aim of this study is to compare the plan results that are obtained by using different calculation grid sizes ranging from 0.15 to 0.50 cm, and the same dose calculation algorithm (Superposition), in Intensity Modulated Radiotherapy (IMRT) for different treatment sites. Results are then used to study the suitability of dose grid size with respect to site. METHODS: For each of the calculation grid sizes, three different sites; namely, Lung, Prostate, and Head and Neck were analyzed. Treatment plans were created using 6MV photon beam quality and IMRT technique on the CMS XiO (Computerized Medical System, St.Louis, MO) treatment planning system. Dose volume histograms were generated for each of the cases and statistical analysis performed included mean relative difference and Homogeneity Index for target structures. Comparison was done first by using 0.30 cm calculation grid as a golden standard and keeping the same number of monitor units (MUs) per beam for each grid size, then the second part involved renormalizing plans to have the same target coverage (100% of the prescription dose covering at least 95% of the target volume) for each grid size used. Future study plans include treatment plans delivery on Varian 21 EX linear accelerator with Millennium (120) MLC and their verification with the Sun Nuclear Mapcheck 2D array. To increase the diode array resolution, 2D array will be shifted in 1 mm increments in x and y direction. Measured fields will be merged using Sun Nuclear Files Combined function and compared with intensity maps exported from the CMS XiO treatment planning system calculated with minimum segment size of 1 cm. RESULTS: The maximum percentage of variation recorded between calculation grid sizes used was in the case of the Head and Neck treatments. For the lung and prostate cases there was little variation in the results based on the calculation grid size chosen, specifically between 0.30, 0.20 and 0.15 cm. However head and neck and prostate cases with nodal involvement showed significant variation in the dosimetric results based on the grid size chosen. Overall results vary from case to case and also depend on the plan complexity. For larger treatment areas calculating with the grid size smaller than 0.30 cm may be impossible as time needed for calculation rises exponentially with the field size involved. CONCLUSIONS: IMRT places a higher requirement on dose grid resolution than conventional radiation therapy. While 0.30-0.40 cm grid was assumed adequate for conformal treatment planning, smaller dose grid is required at least in the areas of high dose. In the cases where steep dose gradients exist smaller grid size should be used while calculating and evaluating treatment plans, as the choice of the calculation grid size may in certain cases even influence clinical results.

SU-E-T-387: Validation of a New System for Patient Specific IMRT QA and Comparison with Other Commerical Systems.

PURPOSE: The focus of this project is to compare the Octavius 4D with current commercial available dose validation systems: MatriXX MultiCube and Delta4. METHODS AND MATERIALS: Many challenges are faced with properly measuring Intensity Modulated Radiotherapy (IMRT). It has become common practice for clinics to use film, arrays, or multiple detectors to validate dose measurements pretreatment for static and dynamic treatments. IMRT QAs for various treatment sites were measured for patients using three different dose validation systems. All measurements were taken on a Varian CLinac 2100 C/D, SN-757, 80 MLC with 6MV. The treatment plans evaluated were Step-N-Shoot. Data analysis was performed using the software provided with each dose validation system. Detailed information was gathered from each system with their perspective advantages. The latest system, Octavius 4D, allows one to calculate the Gamma Index for Coronal, Sagittal, and Transversal views for every slice included in the measurement along with the traditional data analysis provided; histograms, horizontal and vertical profiles, DTA. RESULTS AND DISCUSSION: The Gamma Index values were observed using the MatriXX Multicube, Delta4, and Octavius 4D. The treatment plan included five fields at various gantry angles. Also the gamma index and profiles were calculated for various treatment sites. Delta 4 and the Octavius 4D appears to be quite comparable. Each device has the ability to allow one to verify segmented and composite fields, measure dose profiles and analysis using the Gamma Method. ConclusionsSimilar IMRT QA measurements will be made for more Step-N-Shoot cases with the addition of SmartArcs. The limitations of each system will be determined for each system using the Gamma Index as a reference while varying the Region of Interest, Threshold, and Gamma Method (local, normalization, and maximum dose), as well as the 2D- profiles for these cases.

SU-E-T-465: Exploring the Dosimetric and Tumor Control Consequences of Prostate Seed Loss and Migration.

PURPOSE: Once implanted, prostate brachytherapy seeds are vulnerable to loss and movement. A general estimation of these effects may be useful for making patient care decisions when seeds are lost after the post-implant scan. The goal of this work was to explore the dosimetric and radiobiological effects of the types of seed loss and migration common in prostate brachytherapy. METHODS: This study evaluates five patients. For each, three treatment plans were created using Iodine-125, Palladium-103 and Cesium-131. The three seeds closest to the urethra were identified and modeled as seeds lost through the urethra. The three seeds closest to the exterior of prostatic capsule were identified and modeled as those lost from the prostate periphery. The seed locations and organ contours were exported from Prowess and used by in-house software to perform the dosimetric and radiobiological evaluation. The radiobiological evaluation was based on the linear-quadratic model. Seed loss was simulated by removing 1, 2 or 3 seeds near the urethra 0, 2 or 4 days after the implant or removing seeds near the exterior of the prostate 14, 21 or 28 days after the implant. RESULTS: Loss of 1, 2 or 3 seeds through the urethra resulted in D90 reduction of 2%, 5% and 7% loss respectively. Due to delayed loss of peripheral seeds, effects were less severe than for loss through the urethra. However, while the dose reduction is modest for multiple lost seeds, the reduction in tumor control probability was minimal. CONCLUSIONS: The goal of this work was to explore the dosimetric and radiobiological effects of the types of seed loss and migration commonly seen in prostate brachytherapy. The results presented show that loss of multiple seeds can cause a substantial reduction of D90 coverage. However, the dose reduction was not seen to significantly reduce tumor control probability.

SU-E-T-458: Radiobiological Comparison of Single and Dual-Isotope Prostate Seed Implants.

PURPOSE: Several isotopes are available for low dose-rate brachytherapy of the prostate. Currently, most implants use a single isotope. However, the use of dual-isotope implants may yield an advantageous combination of characteristics such as half-life and relative biological effectiveness. However, the use of dual-isotope implants complicates treatment planning and quality assurance. Do the benefits of dual-isotope implants outweigh the added difficulty? The goal of this work was to use a linear-quadratic model to compare single and dual-isotope implants. METHODS: Ten patients were evaluated in this study. For each patient, six treatment plans were created with single or dual-isotope combinations of 1251, 103Pd and 131Cs. For each plan the prostate, urethra, rectum and bladder were contoured by a physician. The biologically effective dose was used to determine the tumor control probability and normal tissue complication probabilities for each plan. Each plan was evaluated using favorable, intermediate and unfavorable radiobiological parameters. The results of the radiobiological analysis were used to compare the single and dual-isotope treatment plans. RESULTS: Iodine-125 only implants were seen to be most affected by changes in tumor aggressiveness. Significant differences in organ response probabilities were seen at common dose levels. It was recognized that these differences were likely a result of suboptimal initial seed strengths. After adjusting the initial seed strength to maximize complication-free tumor control the differences between isotope combinations were minimal. This result was true even for unfavorable tumors. CONCLUSIONS: The objective of this work was to perform a radiobiologically based comparison of single and dual-isotope prostate seed implant plans. For all isotope combinations, the plans were improved by varying the initial seed strength. For the minimally-optimized treatment plans, no substantial differences in predicted treatment outcomes were seen among the different isotope combinations.

SU-E-T-517: Characterization of Relative Doses and Source Strengths for Various Plaque Sizes and Tumor Dimensions in the Treatment of Uveal Melanoma.

PURPOSE: To characterize the relative doses and source strengths for various eye plaque sizes and tumor dimensions in the treatment of uveal melanoma. MATERIALS AND METHOD: Several tumors with basal diameters ranging from 6-16mm and apical height of 5-10mm were planned using the Pinnacle3 treatment planning system following the American Brachytherapy Society's recommendations. The data is based on 5-day implant to deliver 85Gy with a dose rate of 70.9cGy/hr. Choice of plaque size is based on a 2-3 mm margin on either side of the tumor. Doses to the fixed locations: sclera, center of the eye and opposite sclera were obtained. RESULTS: From our results we see larger source strengths needed for smaller plaques due to smaller number of sources used. The exception is with 14mm and 16mm plaques containing the same number of sources where the 16mm plaque has slightly higher source strengths. For both the inner and outer sclera, the relative dose increases with increasing height. For a 12mm plaque, the relative dose of the outer sclera increases from 4-10 times that of the Rx dose at the apex of the tumor. For a 20mm eye plaque, the relative dose of the outer sclera increases from 2-5 times that of the prescription dose at the apex of the tumor. As apical height increases, the relative dose at the center of the eye increases to about 75% of the Rx dose at 10cm depth for all plaque sizes. At the inner sclera surface opposite the center of the tumor base the relative dose is 20-22% of the Rx dose for all plaque sizes. CONCLUSION: The data provided will be a helpful tool in evaluating and predicting complications in the retina and sclera based on the tumor dimensions and selected plaque size.

WE-G-BRCD-05: Evaluation of Localization Errors for CSA Delivery Using VMAT.

PURPOSE: To dosimetrically evaluate the effects of improper patient positioning in the junction area of a VMAT cranio-spinal axis irradiation technique consisting of one superior and one inferior arc. METHODS: Five (n=5) cranio-spinal axis irradiation patients were planned with 2 arcs: one superior and one inferior. The plans were then recalculated with inferior isocenter shifted, in order to mimic patient setup errors, eight times: lmm, 2mm, 5mm, and 10mm superiorly, and 1mm, 2mm, 5mm, and 10mm inferiorly. Plans were then compared to the original, non-shifted arc plan based on target metrics of conformity number and homogeneity index, as well as several normal structure mean doses. RESULTS: Percent differences were calculated in order to compare each of the eight shifted plans to the original arc plan without shifts, which would be the ideal setup of patient without error. The conformity number was on average 0.87%, 2.74%, 5.75%, and 9.10% lower for the 1mm, 2mm, 5mm, and 10mm inferiorly- shifted plans and 0.41%, 0.82%, 2.75%, and 5.99% lower for the respective superiorly-shifted plans. The homogeneity indices were, averaged among the five patients, 0.03%, 0.26%, 0.97%, and 2.84% for the inferior shifts and 0.23%, 1.17%, 6.31%, and 15.29% worse, or less homogenous for the superior shifts. Overall the mean doses to the organs at risk were less than 2% different for the 1mm, 2mm, and 5mm shifted plans. The 10mm shifted plans, however, showed percent differences from original plan of up to 5.6% on average. CONCLUSIONS: Setup errors when shifting isocenters should be minimized in order to provide the patient with the best treatment possible. Errors of 1 to 2mm can negatively affect patient treatment, most notably in the arc junction area, but are not as problematic as larger errors of 5 to 10mm.

WE-G-BRB-02: MU-EPID an EPID Based Tool for IMRT Quality Assurance.

PURPOSE: A software program (MU-EPID) has been developed to perform patient specific IMRT pre-treatment QA verification using an electronic portal imaging device. METHODS: The software converts measured images of intensity modulated beams delivered to an EPID, into fluence maps that can be imported in the treatment planning system. The dose can then be calculated in the patient anatomy and compared against the patient's treatment plan for QA purposes. We first benchmarked the software using as a patient a cylindrical phantom. An aSi-1000 EPID mounted on a Varian Novalis linear accelerator was used for the image acquisition. Finally, IMRT plans from different treatment sites were used to further validate this in- house software. QA analysis was performed by evaluation of isodose distributions, DVH comparison and 2D gamma analysis. RESULTS: The validation study with the cylindrical phantom showed that the dose to the ion chamber measurement point was in good agreement with both the original treatment plan and the MU-EPID reconstructed dose. Similar results were found for the clinical cases that we studied. A gamma analysis of the dose to the isocenter plane was performed for each plan. Using 3% and 3 mm as the evaluation criteria, resulted in an average of 97.44% of pixels passing the analysis (gamma<1). Good agreement was also observed for the DVH, isodose and profile comparisons between the clinically delivered IMRT plan and the MU-EPID derived dose calculation. CONCLUSIONS: The results of the present investigation suggest that MU-EPID can be used in a clinical environment and can be used for patient specific QA for IMRT plans. This work has been supported by the SCOA.

3D dose reconstruction of pretreatment verification plans using multiple 2D planes from the OCTAVIUS/Seven29 phantom array.

The purpose of this study is to evaluate 3D dose reconstruction of pretreatment verification plans using multiple 2D planes acquired from the OCTAVIUS phantom and the Seven29 detector array. Eight VMAT patient treatment plans of different sites were delivered onto the OCTAVIUS phantom. The plans span a variety of tumor site locations from low to high plan complexity. A patient specific quality assurance (QA) plan was created and delivered for each of the 8 patients using the OCTAVIUS phantom in which the Seven29 detector array was placed. Each plan was delivered four times by rotating the phantom in 45° increments along its longitudinal axis. The treatment plans were delivered using a Novalis Tx with the HD120 MLC. Each of the four corresponding planar doses was exported as a text file for further analysis. An in-house MATLAB code was used to process the planar dose information. A cylindrical geometry-based, linear interpolation method was utilized to generate the measured 3D dose reconstruction. The TPS calculated volumetric dose was exported and compared against the measured reconstructed volumetric dose. Dose difference, dose area histograms (DAH), isodose lines, profiles, 2D and 3D gamma were used for evaluation. The interpolation method shows good agreement (<2%) between the planned dose distributions in the high dose region but shows discrepancies in the low dose region. Horizontal profiles, dose area histograms and isodose lines show good agreement for the sagittal and coronal planes but demonstrate slight discrepancies in the transverse plane. The 3D gamma index average was 92.4% for all patients when a 5%/5 mm gamma passing rate criteria was employed but dropped to <80.1% on average when parameters were reduced to 2%/2 mm. A simple cylindrical geometry-based, linear interpolation method is able to predict good agreement in the high dose region between the reconstructed volumetric dose and the planned volumetric dose. It is important to mention that the interpolation algorithm introduces dose discrepancies in small regions within the high dose gradients due to the interpolation itself. However, the work presented serves as a good starting point to establish a benchmark for the level of manipulation necessary to obtain 3D dose delivery quality assurance using current technology.

Radiobiological and dosimetric analysis of daily megavoltage CT registration on adaptive radiotherapy with Helical Tomotherapy.

Pre-treatment patient repositioning in highly conformal image-guided radiation therapy modalities is a prerequisite for reducing setup uncertainties. In Helical Tomotherapy (HT) treatment, a megavoltage CT (MVCT) image is usually acquired to evaluate daily changes in the patient's internal anatomy and setup position. This MVCT image is subsequently compared to the kilovoltage CT (kVCT) study that was used for dosimetric planning, by applying a registration process. This study aims at investigating the expected effect of patient setup correction using the Hi-Art tomotherapy system by employing radiobiological measures such as the biologically effective uniform dose (D) and the complication-free tumor control probability (P(+)). A new module of the Tomotherapy software (TomoTherapy, Inc, Madison, WI) called Planned Adaptive is employed in this study. In this process the delivered dose can be calculated by using the sinogram for each delivered fraction and the registered MVCT image set that corresponds to the patient's position and anatomical distribution for that fraction. In this study, patients treated for lung, pancreas and prostate carcinomas are evaluated by this method. For each cancer type, a Helical Tomotherapy plan was developed. In each cancer case, two dose distributions were calculated using the MVCT image sets before and after the patient setup correction. The fractional dose distributions were added and renormalized to the total number of fractions planned. The dosimetric and radiobiological differences of the dose distributions with and without patient setup correction were calculated. By using common statistical measures of the dose distributions and the P(+) and D concepts and plotting the tissue response probabilities vs. D a more comprehensive comparison was performed based on radiobiological measures. For the lung cancer case, at the clinically prescribed dose levels of the dose distributions, with and without patient setup correction, the complication-free tumor control probabilities, P(+) are 48.5% and 48.9% for a D(ITV) of 53.3 Gy. The respective total control probabilities, P(B) are 56.3% and 56.5%, whereas the corresponding total complication probabilities, P(I) are 7.9% and 7.5%. For the pancreas cancer case, at the prescribed dose levels of the two dose distributions, the P(+) values are 53.7% and 45.7% for a D(ITV) of 54.7 Gy and 53.8 Gy, respectively. The respective P(B) values are 53.7% and 45.8%, whereas the corresponding P(I) values are ~0.0% and 0.1%. For the prostate cancer case, at the prescribed dose levels of the two dose distributions, the P(+) values are 10.9% for a D(ITV) of 75.2 Gy and 11.9% for a D(ITV) of 75.4 Gy, respectively. The respective P(B) values are 14.5% and 15.3%, whereas the corresponding P(I) values are 3.6% and 3.4%. Our analysis showed that the very good daily patient setup and dose delivery were very close to the intended ones. With the exception of the pancreas cancer case, the deviations observed between the dose distributions with and without patient setup correction were within ±2% in terms of P(+). In the radiobiologically optimized dose distributions, the role of patient setup correction using MVCT images could appear to be more important than in the cases of dosimetrically optimized treatment plans were the individual tissue radiosensitivities are not precisely considered.

A practical method to detect target failure of a helical tomotherapy unit.

PURPOSE: Helical tomotherapy has been in clinical use for several years. One of the issues with a helical tomotherapy unit is the failure of detection of the x-ray target. In this study, we are proposing a method to detect potential failure of the x-ray target. METHODS: Currently, on-board detector data from a helical tomotherapy unit are collected and sent to TomoTherapy Inc. for comparison with the so-called gold standard for the unit. However, this is sometimes time-consuming. Furthermore, the clinical medical physicists have no access to this comparison procedure. In this study, we developed a practical method to detect target failure based on one of the monthly quality assurance (QA) procedures. The commissioning cross-plane profiles were used as the comparison baseline. Larger EDR2 film (35×43 cm) were set at source-axis distance (SAD) (85 cm) and shot with 1.5 cm solid water as build-up material and 10 cm solid water as backscattering material. Cross-plane profiles obtained from the EDR2 film were compared with the commissioning profiles. RESULTS: When the cross-plane profiles from monthly QA have 1° degree difference from the commissioning profiles, it is time that the target be changed. CONCLUSION: This method enables the clinical medical physicists to easily evaluate the target status and to help improving the quality assurance of a helical tomotherapy unit.

Impact of pulse forming network and injection current parameters on output and energy variations of helical tomotherapy.

PURPOSE: In this study, an experiment was devised to establish the dependency of the impact of pulse forming network (PFN) and injection current (IC) parameters on output and energy variations of helical tomotherapy (HT) on the radiation beam output and energy. METHODS: HT has unique radiation beam characteristics due to the absence of a flattening filter. As with conventional linear accelerators, the machine output and energy should be within a + or - 2% tolerance according to published studies. However, because a dose servo is not utilized in a HT unit, these parameters may drift out of the + or - 2% tolerance due to various reasons such as high machine temperatures. With this in mind, physicists and engineers must adjust certain machine parameters to reset the output and energy to within the tolerance of the commissioned baseline. Two parameters commonly adjusted are: PFN voltage (V(PFN)) and IC voltage (V(IC)). RESULTS: Results showed that the HT unit possesses different working zones defined by the V(PFN) and V(IC) parameter settings. The working zones were classified into 5 zones: 1) low dose rate zone - radiation dose rate much lower than nominal dose rate and machine cannot run due to low dose rate fault; 2) normal dose rate zone - dose rate is within tolerance of nominal dose rate and machine can run without dose rate fault; 3) dose rate failure during treatment zone - dose rate within the tolerance of the nominal dose rate however machine interrupts during treatment due to dose rate fault; 4) high dose rate zone - dose rate is higher than nominal dose rate and machine cannot run due to high dose rate fault; and 5) inoperable dose rate zone - dose rate is much higher than the nominal dose rate and machine cannot run. CONCLUSION: The results of this study may provide a quick guide for physicists to adjust their HT unit V(PFN) and V(IC) values in order to reset the radiation beam output and energy back to within the tolerance of the commissioned baseline.

A virtual fluoroscopy system to verify seed positioning accuracy during prostate permanent seed implants.

PURPOSE: The purpose of this project was to develop a software platform to produce a virtual fluoroscopic image as an aid for permanent prostate seed implants. MATERIALS AND METHODS: Seed location information from a pre-plan was extracted and used as input to in-house developed software to produce a virtual fluoroscopic image. In order to account for differences in patient positioning on the day of treatment, the user was given the ability to make changes to the virtual image. RESULTS: The system has been shown to work as expected for all test cases. CONCLUSION: The system allows for quick (on average less than 10 sec) generation of a virtual fluoroscopic image of the planned seed pattern. The image can be used as a verification tool to aid the physician in evaluating how close the implant is to the planned distribution throughout the procedure and enable remedial action should a large deviation be observed.

Monte Carlo Modeling and Commissioning of a Dual-layer Micro Multileaf Collimator.

The purpose of this study was to commission a first-of-its-kind dual-layer micro multileaf collimator (mMLC) system by using Monte Carlo dose calculations. The mMLC is attached on a Varian 600C linac. Having a lower and an upper layer of MLC leaves, this mMLC allows for field shaping in two orthogonal directions. The commissioning of the system was performed in two steps: without and with the mMLC attached on the linac. The treatment head without and with the mMLC was modeled in the BEAMnrc Monte Carlo (MC) code. The scoring planes for the phase space files were specified below the linac's secondary collimators (jaws) and above and below the mMLC. With the mMLC attached to the linac the field size was defined by the jaws as 10 x 10 cm(2), which is also the maximum possible field size that can be shaped by the mMLC. For the commissioning of the linac, several fields of various sizes were simulated and compared against ionization chamber measurements in a water phantom. Output factors for several field sizes, as well as percent depth dose curves and dose profiles for rectangular and irregular shape fields, were calculated and compared against measurements in water. Agreement between measured and calculated data was better than 1% and less than 1.0 mm in the penumbra region for open fields. With the mMLC attached, the agreement between measurements and MC calculations is within 1.0% or 1.0 mm in the penumbra region.

On the evaluation of the relative sensitivity of commercial TLD readers using well characterized TLD chips.

PURPOSE: To characterize a relative sensitivity or performance factor between two commercial thermoluminescent dosimeter (TLD) readers that can be used in inter comparing the thermoluminescent (TL) signals from the readers. MATERIALS AND METHODS: The measurements were made with well-characterized TLD chips, TLD-100 (LiF: Mg; Ti). For illustrative purposes, we used the Harshaw TLD-5500 and the Victoreen 2800M TLD readers available in our department. A well-calibrated 6 MV beam linear accelerator was used as the source of radiation. RESULTS: A sensitivity factor between the two readers used in the illustration was measured as 3.40 +/- 0.13 with the Harshaw TLD-5500 reader producing the superior sensitivity. In terms of measurement repeatability, we observed 2.32% +/- 1.17% reproducibility with the Victoreen 2800M TLD reader and 1.86% +/- 0.95% reproducibility with the Harshaw TLD - 5500 reader. The linearity properties of the two readers were comparable. CONCLUSION: The sensitivity factor is to be interpreted as follows: when working with multiple TLD readers, in this case two, suppose the calibration of the TLD chip was performed with one of the readers, then we can use this calibration factor when measurements are made with the other reader provided we correct for differences in sensitivity with a relative sensitivity factor. This of course is true only if the TLD reader settings used at the time of measurement are similar to those used at the time of relative sensitivity characterization. Owing to a wide range of other factors that can affect the reader sensitivity, we recommend use of a relative sensitivity factor in protection level dosimetry only in situations where inaccuracies of up to 10% are acceptable.

Dosimetric evaluation of a Monte Carlo IMRT treatment planning system incorporating the MIMiC.

The high dose per fraction delivered to lung lesions in stereotactic body radiation therapy (SBRT) demands high dose calculation and delivery accuracy. The inhomogeneous density in the thoracic region along with the small fields used typically in intensity-modulated radiation therapy (IMRT) treatments poses a challenge in the accuracy of dose calculation. In this study we dosimetrically evaluated a pre-release version of a Monte Carlo planning system (PEREGRINE 1.6b, NOMOS Corp., Cranberry Township, PA), which incorporates the modeling of serial tomotherapy IMRT treatments with the binary multileaf intensity modulating collimator (MIMiC). The aim of this study is to show the validation process of PEREGRINE 1.6b since it was used as a benchmark to investigate the accuracy of doses calculated by a finite size pencil beam (FSPB) algorithm for lung lesions treated on the SBRT dose regime via serial tomotherapy in our previous study. Doses calculated by PEREGRINE were compared against measurements in homogeneous and inhomogeneous materials carried out on a Varian 600C with a 6 MV photon beam. Phantom studies simulating various sized lesions were also carried out to explain some of the large dose discrepancies seen in the dose calculations with small lesions. Doses calculated by PEREGRINE agreed to within 2% in water and up to 3% for measurements in an inhomogeneous phantom containing lung, bone and unit density tissue.

Ultra-thin TLDs for skin dose determination in high energy photon beams.

Estimation of surface dose is very important for patients undergoing radiation therapy. In this work we investigate the dose at the surface of a water phantom and at a depth of 0.007 cm, the practical reference depth for skin as recommended by ICRP and ICRU, with ultra-thin TLDs and Monte Carlo calculations. The calculations and measurements were carried out for fields ranging from 5 x 5 cm2 to 20 x 20 cm2 for 6 MV, 10 MV and 18 MV photon beams. The variation of the surface dose with angle of incidence and field size was investigated. Also, the exit dose was computed and measured for the same fields and angles of incidence. The dose at the ICRU reference depth was computed. Good agreement (+/-5%) was achieved between measurements and calculations. The surface dose at the entrance increased with the angle of incidence and/or the field size. The exit dose decreased with the angle of incidence but it increased with field size. The dose at the surface of the patient is mostly dependent on the beam energy, modality and beam obliquity rather than the field size and field separation. By correlating TLD measurements with Monte Carlo calculations, we were able to predict the dose at the skin surface with good accuracy. Knowing the dose received at the surface of the patient can lead to prediction of skin reactions helping with the design of new treatment techniques and alternative dose fractionation schemes.