Session Item

The purpose of this study was the evaluation the effect of two methods of dose prescription on dose distribution for lung tumors treated by VMAT stereotactic radiotherapy in terms of plan quality and analysis of accuracy in dose delivery.

In total, 10 patients with left lung tumors previously treated with VMAT stereotaxic radiotherapy were examined. These patients were replaned using two methods of dose prescription. The first, the administered dose of 50 Gy/5fr, was prescribed on 80% isodose (80%plan), the second on 100% (100%plan) isodose covering PTV; normalization was 98% of the prescribed dose received 98% of PTV.  Both plans were three half-arcs VMAT with 6XFFF, TrueBeam (Varian, USA). Both plans (80%plan) and (100%plan) were compared in terms of dose-volume metrics by RTOG conformity index (CI) and gradient index (GI), and the constraints for the particular doses to OARs (reported by Grimm et al. (Jacmp2011)) were evaluated. The number of monitor units (MU) and were examined in terms of treatment time. Following gamma criteria in global (G) and local (L) normalization with a threshold of 10%, were (G2%,2mm), (G3%,2mm) and (L2%,2mm), (L3%,2mm) were used in the passing-rate (PR) γ-analysis. Accuracy in dose delivery was performed in EPID and phantom Octavius 4D with SRS 1600 matrix, PTW Freiburg. The accepted level for PR was ≥95%. The point dose was verified by dynamic phantom QUASAR (Modus Medical Devices, Canada) Pinpoint (PTW Freiburg).

Both 80% and 100% plans were judged clinically acceptable, but more significant differences were noticed in the plan quality parameters with the increasing volume of PTV. A significant difference in both DmaxPTV and D2%PTV favoring (80%plan) plans was observed (p<0.01). For (80%plan) and (100%plan) computed doses for OARs were not equivalent, (80%plan) achieved lower values for Dmean thoracic wall 7.07 vs. 8.65Gy, Dmean lung 3.15 vs. 3.19Gy, left lung 5.61 vs. 5.77Gy and Dmax spinal cord 10.05 vs. 16.84Gy.  Finally, mean MU for (80%plan) was 3644MU and were reduced to 3305MU for (100%plan), values were significantly smaller for a smaller volume of lesions. Acceptable results were obtained for adopted criteria for both plans (80%plan) and (100%plan) and presented in table 1.  The mean difference between measured and calculated point doses were – 0,7Gy for (80%plan) and -0.48Gy for (100%plan) in the dynamic phantom QUASAR.

Doses for lung tumors organs at risk were comparable for both methods of dose prescription 80% and 100% treated by VMAT stereotactic radiotherapy. However, in (80%plan) increased the dose for PTV while reducing doses for surrounding tissues and organs at risk. CI and other plan quality parameters were higher or slightly higher (80%plan). It was shown that the overall treatment time for patients with (80%plan) was longer. The dosimetric consistency between planned and delivered was satisfactory from (80%plan) and (100%plan) independent of dosimetric methods.