No abstract

Purpose: The risk for radiation induced skin complications can limit the effectiveness of radiation therapy when large areas of skin are involved in the treatment. Therefore, the accurate determination of skin dose may be critical in preventing post‐therapy side effects. The purpose of this study is to quantify the skin dose received by the Volumetric Modulated Arc Therapy (VMAT) and a 7 field MLC‐based IMRT step and shoot (S&S) delivery modalities using the same dose prescription. Methods: 5 prostate, 5 head‐and‐neck, and 5 brain cancer patients were used in this study to quantify skin dose for the examined radiation modalities. 30 treatment plans were produced (15 VMAT and 15 IMRT S&S) and for each patient the skin dose was measured using ultra‐thin TLDs. Doses were measured at 11 points on the surface of a cheese phantom at 25 degree intervals using ultra‐thin lithium‐fluoride TLDs with surface dimensions of 0.3cm × 0.3cm and a thickness of 0.01cm, comparable to the thickness of the skin. For each patient, the corresponding doses were also calculated using the Pinnacle treatment planning system. Results: A 36% increase in skin dose was measured on the brain patients using VMAT vs. IMRT S&S while a 38% decrease in skin dose was measured on the H&N patients using VMAT vs. IMRT S&S. The prostate patients showed just a 12% increase in skin dose using VMAT vs. IMRT S&S. The skin doses from Pinnacle showed small differences between the two modalities. The absolute differences of the dose values between the plan and the measurements were within an order of magnitude of the measured doses. Conclusion: The skin dose differences between the VMAT vs. IMRT S&S modalities were found to be significant. However, the direction of the differences was found to be treatment site specific and its magnitude patient specific.

Purpose: To analyze the patient specific IMRT QA results at our institution over the past 3 years and evaluate the procedures and methods of our IMRT QA program. Methods: 1466 patient specific IMRT QAs were performed at our institution from March 2009 to December 2011. The passing criteria for each IMRT QA are that 90% of all evaluated points must have a gamma =1.0. The gamma is calculated using the TPS calculated planar dose as reference and the reference value is 90% of the maximum dose in the plane. A dose threshold of 10% is used in order to remove very low dose points from the calculation. The planar dose and the measurements are computed and obtained using the actual beam angles. The measurements were performed on Varian linacs equipped with Millennium 80, Millennium 120 and High‐Definition 120 MLC. Results: The IMRT QA results were analyzed with respect to the linac, treatment site, number of beams, IMRT vs. TOMO vs. VMAT, and number of control points. The overall average gamma index value was 96.85% (±1.5%). Head and Neck had the lowest gamma index (95.97%) while brain had the highest (97.85%). Conclusions: After evaluation of 700 patients, it can be determined that there are significant variations in the average measured gamma value based on treatment site, number of beams, machine type, and number of control points. This work provides a foundation for future analysis of possible underlying issues in the gamma value deviations for each comparison.

Purpose: Treatment Planning systems (TPSˈs) inaccurately quantify the dose delivered to the skin during radiation therapy, due to their inability to account for all surface dose contributing factors. The purpose of this work was to use near tissue equivalent ultra‐thin Thermo Luminescent Dosimeters to determine and compare the average skin dose for Prostate patients using the Volumetric Arc Therapy (VMAT) Delivery System with measurements taken on other delivery systems. Method and Materials: Reference beam measurements were taken at energy of 6MV for both a 10×10 open field and for an open, unmodulated arc with a 10×10 field from multiple angles. Five (n=5)_prostate patients using the VMAT delivery system were evaluated. The dose per fraction was set to 2 Gy for all patients. Two TLDs were placed at discrete locations, every 25 degrees, on the periphery of the “cheese” phantom. Results: The average skin dose for all locations of interest for the five prostate patients varied from 5.25cGy to 7.47cGy. Compared to previous work done with skin dose measurements, the VMAT delivery system delivers an overall lower skin dose than IMRT step and shoot, Helical Tomotherapy, and Serial Tomotherapy. Conclusions: The VMAT delivery method delivers lower dose to the surface than other IMRT delivery methods. The lower doses probably due to lower monitor units required for VMAT compared to static field IMRT.

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.