Purpose: To retrospectively analyze SBRT treatment plans.
Methods: Twenty-Five Lung SBRT treatment plans with the prescription dose of 5000 cGy delivered over 5 fractions were retrospectively analyzed. Abdominal compression was used to control the respiratory motion at 4D CT simulation and during treatments. An internal target volume (ITV) was defined based on findings of 4D CT scans and PET/CT scans. ITV was expanded isotropically by 3–5 mm to create planning target volume(PTV). All treatment plans were developed using Pinnacle treatment planning system using 1-2 VMAT arcs. Various dosimetric parameters of target volumes and normal structures were calculated and compared with RTOG 0813 protocol constraints.
Results: The average percentage PTV volume that received the prescription dose (Rx) was (96±1) %. The percentage PTV volume that received 95% of Rx ranged between 99% to 100%. The average conformity (R100%) and homogeneity (HI5%/95%) indices were (1.01 ±0.04) and (1.3±0.1) respectively. The average R50% for all plans was (5.8±1.3). The average maximum dose to any point at 2 cm from the PTV in any direction was (53±9)% of prescription dose. The average geometry index (GI=R50%/R100%) was (5.9±1.6) and the average Paddick conformation number was(0.97±0.07). Dosimetric constraints for all normal structures were met, except for rib in fifteen treatment plans.
Conclusion: The conformity index and the maximum dose 2 cm away from the PTV for all plans were well within the RTOG 0813 protocol criteria. However, the criterion of R50% was difficult to meet in some plans. This might be the result of irregular shape of some tumors and the proximity of PTV to OARs. Rib were either overlapping with PTVs or very close to them in plans that failed to meet the dosimetric constraints of this structure. In conclusion, there is potential improvement for periphery dose gradient and intermediate-dose spillage.
Dosimetry, Dosimetry Protocols
Not Applicable / None Entered.