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SU‐FF‐J‐56: Patient Dose From Kilo‐Voltage Cone Beam Computed Tomography (kV‐CBCT) Imaging

Purpose: To investigate patient dose from on‐board imager‐based kV‐CBCT. Method and Materials: Radiation doses from kV‐CBCT were measured using TLDs at different locations in three anthropomorphic‐phantoms (H&N, chest and pelvis) and patients retrospectively. kV‐CBCT scans were performed in standard settings (125 kV, 80 mA and 25 ms) using a Varian Trilogy linear accelerator. Both full‐fan (FOV=24 cm) and half‐fan (FOV=40 cm) modes were evaluated for H&N case while only half‐fan (FOV=45 cm) technique was studied for chest and pelvic cases. The skin dose in both phantoms and patients were measured at 4 locations: anterior, posterior, Rt‐Lat, and Lt‐Lat. Doses measured in the phantoms included different critical organs. The dosimeters used were high sensitivity TLD‐100H and only those with standard‐deviations less than 3% and sensitivity within ±3% were selected for this study. Each TLD was individually calibrated using an ion chamber under the irradiation condition. Phantom data was averaged from 3 separate measurements and patient data was averaged from 5 measurements in each category. Results: The skin dose for H&N cases were 9–10cGy for half‐fan mode in both the phantom and patients. The dose for brain and brainstem were 7.1cGy and 7.6cGy, respectively. The doses in same locations were 2–3cGy lower if the full‐fan mode is used. The skin dose for chest cases was 8–10cGy and were same for the phantom and patient measurements. Measured mean lung dose was 8.5cGy and spinal cord dose was 6.2cGy. For pelvis, measured skin dose was 2.9–4.2cGy and the prostate and rectum dose were 2.9cGy. Conclusions: For pelvic cases, kV‐CBCT dose was comparable or less than that from portal imaging. For chest and H&N cases the dose can be two times higher than that for the pelvis cases. Daily CBCT may lead to extra 400cGy to skin and 250cGy to spinal cord in 40 fractions.

SU‐FF‐P‐08: Oversight Protocols in the Management of Network of Facility Sites

Purpose: The management of quality for a network of facility sites poses challenges for an institution. Holding regular meetings, auditing treatment facilities, and requests for documentation are some mechanisms for providing clinical and quality oversights. This presentation discusses our institution protocols in recent auditing of second dosimetric checks and request for RPC‐TLD report for analysis. Methods and Materials: Our institution consists of an integrated network of 20 community (“spoke”) and academic facilities and spread about 150 miles radius from our downtown proper (“hub”). An average of 650 external beam patients are treated daily. With the diversity amongst the facilities, a common set of standards are essential to establishing quality across the facilities. This presentation examined two protocols for this Purpose: auditing second dosimetric checks and request for RPC‐TLD reports. The established protocol recommended second dosimetric checks be performed prior to the third fraction following the start of each new treatment field or field modification consistent with AAPM TG‐40 recommendation. Ten patient treatment records were randomly selected for evaluation by six auditors quarterly. The second protocol requires all RPC‐TLD reports be forwarded to the Office of Medical Physics for analysis and reporting annually. Results: Although the non‐compliance for second dosimetric checks of patient treatment records was low, there was a trend towards non‐compliance at selective facilities. RPC‐TLD analysis showed a bell‐shape curve with the median at 0.98 for the ratio of RPC measurement to institutional stated dose values. Facilities whose dose values are outside ±5% have to repeat measurements. Conclusion: Quality oversight protocols are essential to set standards of performance for institutions having a network of facilities. Establishing oversight protocols will pose challenges for any institution and two examples are given in this presentation, where data are collected by different means.

SU‐FF‐T‐424: Treatment Errors in Radiation Therapy Performed Using Modern Technology

Purpose: Conformal and intensity‐modulated radiation therapy techniques use clinical data transferred electronically from one radiation therapy equipment to another. Before treatment,patient treatment parameters are downloaded into the linear accelerator for dose delivery. This study evaluates recordable events to identify the types of treatment errors associated with these sophisticated treatment techniques. Methods and Materials:Our institution consists of 20 community and academic facilities with 30 medicallinear accelerators, and about 650 external beam patients per day are treated. All records of events are reviewed within our quality assurance program. By definition, any unintended deviation from the prescription constitutes a “recordable event.” The records of events that occurred at our institution for the years 2005 and 2006 were evaluated. Although the number of recordable events is small, it gives a trend of possible occurrence of the types of treatment errors. Results: Traditional types of treatment errors such as transcription and dosimetric errors have been minimized. New types of treatment errors classified as (a) treatment of incorrectly identified patients, (b) downloading different patient files, (c) incomplete treatment of prescribed fields, (d) failure to implement patient shift according to treatment plan, and (e) failure to invoke gating system are observed in the record of events. These constitute over half of the events. Conclusion: A failure mode and effect analysis (FMEA) can help in identifying areas where quality assurance should be emphasized. Such analysis is recommended for all new technologies implemented in busy clinics.

SU‐FF‐T‐422: The Influences of Detector Energy Dependence and Perturbation On the Determination of Small Field Output Factors

Purpose: In radiotherapy, accurate dose determination requires accurate measurements of output factors. The employment of small field size cones in radiosurgery imposes significant challenges for such accurate measurements, since the detector perturbation and its energy dependence, along with the detector size, may introduce significant errors in the output factor determination. This study investigated the impacts on the output factors introduced by the detector perturbation and energy dependence. Method and Materials: Output factors were measured on a Varian Trilogy™ linear accelerator for various collimator defined small square fields and twelve radiosurgery cones that were shipped with the machine. The measurements were performed by using four different detectors, namely, a pinpoint ion chamber, Koda EDR2 films, a stereotactic diode detector, and a pinpoint diode detector. The energy dependence was also evaluated for the ion chamber and diode detectors. Results: Both perturbation and energy dependence present non‐trivial effects in the determination of the output factors. The impact of detector perturbation was more pronounced for the small fields, while that of energy dependence was evident for all the field sizes investigated. For a cone of 5 mm diameter, the perturbation introduced by using a PinPoint ionization chamber could result in almost 30% reduction in the output factor value. The deviations caused by the energy dependence varied from 3% to 6% depending on field sizes.Conclusions: The output factor measurements of small fields must not only account for the size of detector but also its perturbation and energy dependence to ensure the measurement accuracy for stereotactic radiosurgery.

TH‐C‐T‐6E‐02: An Independent Dose Verification Method for Dynamic Intensity Modulated Radiation Therapy

Purpose:Dosimetry verification is particularly important especially for intensity modulated radiotherapy where the dosedelivery technique is complex.. The dosimetry verification is usually conducted with measurements and independent dose calculations. However, currently available independent dose calculation methods were developed for step and shoot beam delivery method, and their uses for dynamic MLCdelivery method are not clear. In this study, a dose calculation method was developed to perform independent dose verifications for dynamic MLC‐based IMRT technique. Method and Materials: This method extracts the machine delivery parameters from the dynamic MLC(dMLC) files generated by the IMRTtreatment planning system. Based on the machine delivery parameters, a monitor unit (MU) matrix, including both primary and leakage contributions, was generated. The MU matrix was used to compute the primary dose matrix and scattered dose matrix. The scattered dose was derived based on the Modified Clarkson technique. Results: The doses computed using this method were compared with both measurement (14) and treatment plans ( 25). The doses calculated using this method, on average, agreed with the measured doses to within 1% with a standard deviation of 1.9%. The computed and planned doses agreed to within 2% with a standard deviation of 1.5%. Conclusions: An independent dose calculation algorithm has been developed to perform independent dose verifications for dynamic IMRT plans. The algorithm independently computed doses that were in excellent agreement with the doses from from commercial treatment planning system. This independent dose calculation method may potentially be used for routine IMRT plan verifications.

SU-FF-T-230: Anomaly Characteristics Of Tissue Heterogeneity Curve From Three Commercially Available Tissue Characterization CT Phantoms

Purpose: Uncorrected treatment plans for tissue heterogeneities can produce errors exceeding 30% of the prescribed dose. Tissue heterogeneity correction is made by acquiring a Hounsfield Units (HU) value versus electron density relationship and applied to imaged-based treatment planning systems. This relationship is determined by scanning a tissue characterization CT phantom containing plugs of different tissues (electron densities). This study investigates the quality and provides up-to-date data on three commercially available tissue characterization CT phantoms (model RMI465, RMI467, and CIRS062) scanned using a modern helical CT-Simulator scanner. Method and Materials: These CT phantoms were scanned on the GE helical lightspeed plus CT-Simulator scanner. The phantom model RMI465 has 20 plugs while phantoms model RMI467 and CIRS062 have 17 plugs. The electron density ranges from 0.19 to 1.69. The scanning parameters for the abdomen (120 kV) with set slice thickness of 5 mm were used. Once the axial images of the CT phantom were acquired, a region of interest was drawn at the central area of each plug and the mean HU value and its standard deviation were determined. Results: The plotted HU values versus electron densities show scattered data points. One of the CT phantoms exhibiting inconsistent data may be due to chemical composition breakdown was excluded. Overall, the data points were scattered and hence direct input of measured HU values versus electron densities into the treatment planning systems for interpolation should be avoided. Instead, the data entered into the treatment planning systems should be based on fitted relationships. The data can be fitted with two linear equations. Conclusion: The HU values versus electron densities derived from commercial CT phantoms should be fitted before entering into treatment planning systems. The relationship consists of two linear equations with a point of inflection at a relative electron density of 1.0.

SU‐FF‐T‐221: Surface Dose Determination with An Interpolation‐Extrapolation Method Using EDR2 Films

Purpose: Kodak EDR2 films have been used for surface dosemeasurements in radiotherapy. However, with the conventional method, the difference of surface percent dosesmeasured with the film and with chamber could be as high as 5%. In this study, a double extrapolation method was used to correct for the overdose response due to the wrapping papers and film itself so that the surface dose can be accurately determined. Method and Materials: In the surface dosemeasurements, multiple EDR2 films were stacked together and placed on the surface of a 30 cm × 30 cm solid water phantom. Efforts were made to ensure the placement was as air tight as possible. Radiation was delivered, and the doses on the films were measured. Two curves were generated from the measureddoses. One is the percent‐depth‐dose curve for the films, the other is the percent‐depth‐dose curve for the wrapping paper, where the later curve was interpolated from the film curve. The surface percent dose was derived by extrapolating the paper curve to zero depth. The surface percent dose was also measured using a parallel plate chamber for comparison. Results: This method has been applied to the surface dosemeasurements for various open fields, oblique fields and IMRT fields. It was found that at zero degree gantry angle the surface percent dosesmeasured using this method were in agreement with the chamber measurements to within 2% for both 6 MV and 23 MV photon beams at all the field sizes for conventional and IMRT beams; the agreement was within 3% at gantry angles other than zero degree. Conclusion: An interpolation‐extrapolation method has been developed to measure the surface doses using EDR2 films. The accuracy of the method is comparable to that of a parallel plate ion chamber for both conventional and IMRT beams.

SU-FF-T-32: Imaged-Based Simulation Technique To Determine Stepping Source Dwell Position For MammoSite(r) Brachytherapy Procedures

Purpose: Incorrect dwell position for the stepping source in MammoSite® radiation therapy system would result in severe dose error to the treated volume. In many centers, CT‐simulators have replaced the fluoroscopic simulators. An alternative method must be developed for this purpose. This project evaluates the feasibility of CT‐based simulation to determine the dwell position for the stepping source of the Nucletron® High‐Dose Rate (HDR) unit. Method and Materials: A MammoSite® balloon applicator is placed in the surgical cavity intraoperatively at the time of segmented mastectomy for breast cancer. The balloon is inflated to near spherical shape with saline solution mixed with a small amount of radiographic contrast to aid in visualization. After recovery, the patient is brought to the radiation oncology facility to determine the quality of the implant and also to determine the stepping source dwell position. A dummy source train is initially inserted in the applicator and pushed to the distal end. The distance is measured using the Nucletron® measuring tool. CT scans of the breast was taken with 1 mm slice thickness. After the images have been acquired, a virtual 3‐dimensional breast is generated. Based on the virtual breast, the path of the dummy source train inside the applicator is assessed. Results: A digitally reconstructed radiography(DRR) that maximizes the projection of the pathway is created. A method is formulated to determine the center of the sphere and marks on the source pathway. The dwell position is determined by subtracting the difference of distance between the distal seed and center of the sphere from the maximum source distance as set on the HDR unit. Conclusion: For institutions where the fluoroscopic simulator has been replaced by a CT‐simulator, imaged‐based simulation allows an effective method of determining the stepping source dwell position for MammoSite® brachytherapy procedures.

SU‐FF‐T‐214: Quality Assurance Of Modern Helical CT‐Simulator Scanners: Reproducibility Of Tissue Heterogeneity Curve

Purpose: Accurate and highly reproducible tissue heterogeneity curve is essential to the proper implementation of tissue heterogeneity correction in imaged‐basedtreatment planning systems (TPS). Tissue heterogeneity curve for a CT‐Simulator scanner must be determined at the time of TPS commissioning and at time intervals not to exceed three months. This work investigates the reproducibility of tissue heterogeneity curve over a number of modern helical CT‐Simulator scanners and a period of four years. Method and Materials: The tissue heterogeneity curve is obtained by scanning a commercially available tissue characterizationCT phantom with inserts of different densities. A tissue characterizationCT phantom model CIRS062 with 17 inserts covering the relative electron density from 0.19 to 1.51 was used. Five modern GE lightspeed plus helical CT‐Simulator scanners purchased within the last four years were investigated. The scanning parameters used were 120 kV, 260 mA and 5 mm slice thickness. After the CT phantom was scanned, a region of interest was drawn in the middle of each insert on the axial image to determine the mean HU (Hounsfield Units) value and standard deviation. The mean HU values were plotted against the relative electron densities. Results: To aid in the evaluation, the tissue heterogeneity curves were fitted with two linear equations. The slopes and intercepts were obtained and compared. These curves have almost identical slopes and intercepts within the standard deviation over the stated time intervals and for different but same model CT‐Simulator scanners.Conclusion: Modern helical CT‐simulator scanners give highly reproducible tissue heterogeneity curve over time. This investigation demonstrates the improvement in performance of modern CT‐Simulator scanners compared to fourth generation CTscanners. A technique of analysis based on fitted relationship is introduced for the quality assurance of the tissue heterogeneity curve.