Purpose: To review IMRT QA measurements from several of the 50+ institutions for which we provide IMRT treatment plans and determine if institutional, anatomic site, or measurement biases exist. Method and Materials: For each patient receiving IMRT, the treatment plan is delivered to a solid water phantom and the dose measured using a small volume ion chamber and with a single EDR film placed 1 cm above the chamber plane. Of the almost 3000 IMRT treatment plans calculated and delivered in 2004, more than 1000 random, de‐identified plans were reviewed. Ratios of chamber/calculated and film‐center/calculated doses were tabulated for six anatomic sites (breast, prostate, pelvis, head & neck, brain, and other). Film dose distributions were compared to calculations using one of several commercially available QA packages. Results: The institutions with the best results had average errors of less than ±0.5% (i.e. randomly distributed about zero) with standard deviations of 1.25–1.50%. A few centers had average errors and standard deviations approaching 3%, indicating a bias in which a systematic dose measurement error was found. Agreement between chamber and film center dose was also institution specific with the best results found for those centers that had the lowest errors compared to calculation. One institution had excellent agreement between chamber and calculation (−0.2±1.7%), but 2–3% lower film dose. Although exceptions were found, little variation in the agreement between chamber measurement and calculation occurred as a function of anatomical site. Conclusion: Since all treatment plans were calculated in one central location and many centers had excellent agreement between measurement and calculation, it is likely that the higher errors were due to measurement technique rather than errors in the dose calculation. Error was not anatomic site dependent possibly due to the purposeful placement of the ion chamber in a region of relatively uniform dose.

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.

Purpose: Kodak EDR2 films have been used for surface dose measurements in radiotherapy. However, with the conventional method, the difference of surface percent doses measured 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 dose measurements, 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 measured doses. 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 dose measurements for various open fields, oblique fields and IMRT fields. It was found that at zero degree gantry angle the surface percent doses measured 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.