Room: Exhibit Hall | Forum 9
Purpose: The ITV approach for SBRT lung treatment is now well established due to the availability of 4DCT. Here we investigate tumor dose coverage for each treatment fraction based on 4DCT scans obtained by CT on Rails before each treatment.
Methods: A previous retrospective study of 50 of our SBRT patients showed greater than 50% ITV variation throughout the 3-5 fraction treatment course for 16% of these patients. 5 of these patients who had more than 50% change in ITV volume over the course of treatment were chosen for this study. Using the daily 4DCT, the ITV-of-the-day was re-contoured and a treatment plan was created using the simulation-CT, 5 mm symmetric margin, RTOG 0618 criteria and a dose of 3x18Gy. MIM was used to deform the planned dose distribution onto each daily average-CT to evaluate the target coverage for the ITV-of-that-day.
Results: In general, the daily ITV variation was generally accommodated by the 5 mm symmetric PTV margin, with the 54Gy line generally covering ~98% of the ITV. However, for two cases only 89% and 84% of the ITV-of-the-day, respectively, were covered by the PTV. For these 2 cases the min dose to the ITV dropped from 57.52Gy to 53.11 Gy (7.6%) and 59.4 Gy to 44.5 Gy (25.1%) , respectively .
Conclusion: Our study demonstrates that the ITV approach is effective at covering daily variation in ITV for most patients. It also shows that for some patients non-trivial reductions in the minimum dose to the ITV may result from daily ITV variations. We are endeavoring to increase the study to include a dosimetric evaluation of more patients.