SU-E-T-382: An Analysis of VMAT Monthly Quality Assurance.

PURPOSE: The purpose of this study is to quantify the change in monthly VMAT quality assurance (QA) and determine the tests to maintain consistent delivery with a baseline. METHODS: VMAT monthly QA has been performed for over 14 months on two Elekta Synergy LINACs. A baseline was established at acceptance and the monthly QA results were compared to those initial values. Films were used to test the dependence on varying dose rate, gantry speed, rotational direction, and MLC speed. These parameters were tested independently and then together in a test called the synchronicity test. Ion chamber readings test a DMLC field with a varying dose rate and MLC speed. Introducing intentional errors into the tested fields allowed the detectable limits of the QA to be determined. RESULTS: The monthly QA has consistently matched the baseline within a 3% dose limit on film. Analyzing the synchronicity film with a gamma test using a 5%/0.5mm tolerance showed a monthly pass rate of over 99%. The DMLC test has been identical for the entire course of VMAT QA. Furthermore, intentional changes in the MLC speed were noticed on the synchronicity test in the form of a smaller gamma pass rate as the MLC error was increased. CONCLUSIONS: There is a monthly agreement with all films testing individual parameters, ion chamber DMLC readings, as well as agreement with the synchronicity test. This collection of data has lead to the conclusion that only the synchronicity test and DMLC readings need to be performed on a monthly basis. If those tests fail, then individual parameters need to be tested to determine the singular cause of the error. Having a single test used as a red flag increases the efficiency of the monthly QA and is being implemented with an EPID to eliminate the use of film.

SU-E-T-389: Accuracy of a VMAT Planning System for Head and Neck Radiotherapy.

PURPOSE: To determine the accuracy of VMAT treatment planning and delivery for patients undergoing radiotherapy in the head and neck. METHOD AND MATERIALS: A dosimetric study of a typical head and neck treatment plan has been carried out using Chamber, film and TLD's placed inside an anthropomorphic phantom. Lateral and AP port films were taken to verify the isocenter prior to treatment. Multiple treatments were performed to assess the reproducibility and uncertainty in the TLD measurements. Gafchromatic film was used between the phantom slices and then analyzed as an independent check on the TLD results. The two data sets, from TLDs and films, were then compared with the treatment planning system dose calculations. RESULTS: The measured dose to the primary planning tumor volume agrees with the planning system within 2%. The comparison of the measured dose to the secondary tumor volume ranges from 3-6% and the spinal cord comparison ranges from 3-10%. CONCLUSION: This new Smart Arc treatment, VMAT, has great benefits to the patient in that patient motion and internal motion of the tumor is dramatically reduced. Challenges arise in predicting the dose near heterogeneities between the treatment planning system and actual measurement.

SU-F-BRCD-03: Dose Calculation of Electron Therapy Using Improved Lateral Buildup Ratio Method.

PURPOSE: To calculate the percentage depth dose of any irregular shape electron beam using modified lateral build-up-ratio method. METHOD AND MATERIALS: Percentage depth dose (PDD) curves were measured using 6, 9, 12, and 15MeV electron beam energies for applicator cone sizes of 6×6, 10×10, 14×14, and 14×14cm2 . Circular cutouts for each cone were prepared from 2.0cm diameter to the maximum possible size for each cone. In addition, three irregular cutouts were prepared. The scanning was done using a water tank and two diodes - one for the signal and the other a stationary reference outside the tank. The water surface was determined by scanning the signal diode slowly from water to air and by noting the sharp change of the percentage depth dose curve at the water/air interface. RESULTS: The lateral build-up-ratio (LBR) for each circular cutout was calculated from the measured PDD curve using the open field of the 14×14 cm2 cone as the reference field. Using the LBR values and the radius of the circular cutouts, the corresponding lateral spread parameter (sigma) of the electron shower was calculated. Unlike the commonly accepted assumption that sigma is independent of cutout size, it is shown that the sigma value increases linearly with circular cutout size. Using this characteristic of sigma, the PDD curves of irregularly shaped cutouts were calculated. Finally, the calculated PDD curves were compared with measured PDD curves. CONCLUSIONS: In this research, it is shown that sigma increases with cutout size. For radius of circular cutout sizes up to the equilibrium range of the electron beam, the increase of sigma with the cutout size is linear. The percentage difference of the calculated PDD from the measured PDD for irregularly shaped cutouts was under 1.0%. Similar Result was obtained for four electron beam energies (6, 9, 12, and 15MeV).

SU-E-J-06: A Time Dependence Analysis of CBCT Image Quality and Mechanical Stability.

PURPOSE: To quantify the change, if any, in flexmap correction factors and image quality with the XVI system over a course of several years and from these results, assess their clinical impact. METHODS: Flexmap, a calibration procedure which corrects for imperfect gantry rotation for cone-beam CT reconstruction, and image quality tests were performed on three Elekta Synergy linacs equipped with XVI. Data was collected per month over three years. U and V values, corresponding to lateral and longitudinal shifts respectively, were acquired through the XVI software. Image quality parameters were obtained through CT imaging of the Catphan 500®. For each reconstruction, pixel values for low density polyethylene (LDPE) and polystyrene materials were recorded. RESULTS: For all three linacs, analysis of the flexmap showed a significant change in the U factor for both month-to-month comparisons and comparisons between machines. The V correction factor exhibited a small variation month to month, and showed a slight, gradual increase over time (0.2 +/-0.08 mm). Image quality analysis showed a near consistent decrease (5-10%) in LDPE and polystyrene. Despite this decrease in pixel values, the ratio of the two pixel values remained constant, thus a similar decreasing trend in contrast was not observed. CONCLUSIONS: Analysis of monthly flexmap calibration showed the general monthly change in correction shifts and their general trend over several years. For image quality, our research exhibited roughly 0.5% per month decrease in pixel values of the Catphan®. Our results imply that CBCT images obtained from XVI are not appropriate for treatment planning and despite the decrease in panel response over time, image quality with respect to contrast will remain within acceptable clinical standards. Future studies may be carried out to assess any correlation between image quality and XVI source strength.