Methods: Five patients with 38 fields have been analyzed in this study. The plans were optimized for the following clinical sites: one liver, one lung, one brain, one prostate and one spine. The detector array used for the measurements was the PTW Seven29 array. All the plans were optimized and calculated using Eclipse v8.9. The center of the array was setup at 215 cm from the source and all the fields were measured and analyzed one by one. All the 30 measurements were performed on a NovalisTX linear accelerator equipped with a high definition multileaf collimator. The evaluation was based mainly on gamma index passing rates using 2 mm distance to agreement (DTA) and 2% dose difference.
Results: The accuracy of the Eclipse Treatment Planning System (TPS) at extended Source to Surface Distances (SSDs) using an ionization chamber was measured to be within 1.0%. All the field measurements were performed and analyzed 35 individually. The percent of the points that had a gamma index of less than 1 using 3%/3 mm was >99% for all the measurements. In order to better evaluate our process and distinguish smaller differences a new set of results was obtained by applying gamma index tolerances of 2%/2mm. In this case, the gamma index passing rates ranged from 90.8 to 100% (95.5%±3%). The profile comparison showed that the detector array measurements followed closely the calculated 40 profiles, even for fields optimized with multiple peaks and valleys.
Conclusion: The choice of the IMRT QA device has an important role in the results of the patient specific QA of the delivered dose to the patient in the case of small targets as in the treatment of spinal targets. In this study, we demonstrated that an extended SSD measurement can improve the sampling resolution of a two-dimensional (2D) detector array, in our case the PTW 45 Seven29 array. This method was shown to be accurate and efficient for measuring highly modulated small fields for pre-treatment patient specific QA.