Purpose: To investigate the dosimetric effect of temporal variation of electron density within the planning target volume in stereotactic body radiotherapy (SBRT) for lung cancer. Methods: For lung SBRT, the planning target volume (PTV) includes both gross tumor volume and surrounding lower electron density (ED) lung tissue, which accounts for the tumor motion (ITV) and setup errors. The ED of the PTV becomes temporally varied during treatment due to tumor motion. To estimate the dosimetric effect due to changes in the ED distributions during delivery, we modeled the time average effect of the tumor motion by overriding the ED of either ITV or PTV. A planning study was designed to quantify dosimetric effect by comparing plans calculated with and without ED overridden for both IMRT and 3D planning techniques. The clinically used plans were designed without ED overriding. Additional plans were created by recalculating the clinical plans after overriding the ED of either ITV or PTV with a value of 0.3, 0.6, and 0.9 g/cm3. Minimum and maximum dose, and coverage of the PTV were compared. Results: When overriding ED of ITV with different values, no significant difference was observed for both IMRT and 3D plans. However, depending on the value of the ED used to override the PTV, the difference in PTV coverage is up to 3.5%. The difference in the minimum dose of the PTV was up to 10%. Conclusions: A simplified model of temporal variation of ED within PTV showed that under dosing to the PTV may happen when a certain value of ED has been assigned to the PTV. A further comparison to the dose distribution calculated based on a 4D image set or measurement is needed to determine the planning strategy using ITV/PTV ED overriding.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging