| Περίληψη: | The purpose of this study was to investigate whether it is possible to create the ITV using a reduced set of 4DCT phases.
Methods: Ten lung cancer patients were identified who had received 4DCT imaging as part of their treatment simulation, and who had a noticeable tumor motion. For each patient, a GTV was drawn on the exhale phase, based on the original physician-drawn ITV (our clinical practice is for the physician to directly draw the ITV based using the 4DCT images). This GTV was then propagated to the other phases of the 4DCT using a commercial image registration package (MIMVista, Cleveland OH). 4 different ITVs were created using N phases closest to exhale (N=1-10). For each ITV contour a RapidArc plan was created on the exhale phase CT, normalized so that the 95% isodose line covered at least 95% of the ITV. Each plan was applied to each CT phase (1-10), and the doses deformably mapped to the exhale phase. The effect of the motion was quantified in terms of the dose to the 95% of the target on the exhale phase (D95). The change in these parameters as N was reduced from 10 was calculated. Also, the difference in the 3D calculations of the original plans and the 4D calculations was noted as a function of N.
Results: Differences in 3D and 4D dose calculations varied from 3% to 14% with an average of 7% for ITV_10/10. For 9 out 10 of ten patients we can have less than 5% reduction in the D95 by using ITV_8/10. For 3 out of ten patients we can have less than 5% reduction in the D95 by using ITV_7/10 and for 1 out of 10 patients we can patients we can have less than 5% reduction in the D95 by using ITV_6/10.
Conclusions: No rule for reducing ITV works for all patients. Some reduction (8/10 phases) is possible for most of the patients but not all and also the volume reduction is small. Therefore what we are currently doing is reasonable. 4D dose calculations give different DVHs to 3D also shown by other groups. Reduction in ITV volume is possible for some patients (e.g. patient 2 17% reduction), but 4D dose calculation is necessary.
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