Evaluation of OAR sparing and plan robustness of beam-specific PTV in lung cancer IMRT treatment

Abstract

Purpose: Margins are employed in radiotherapy treatment planning to mitigate the dosimetric effects of geometric uncertainties for the clinical target volume (CTV). We propose a margin concept that takes the beam direction into consideration, generating a beam-specific PTV (BSPTV) on a beam entrance view. The total merged BSPTV was considered as target using for optimization. We investigated the impact of this novel approach when applied to lung IMRT treatments, and compared the treatment plans generated using BSPTV with conventional PTV. Methods and Materials: We generated the BSPTV by expanding the CTV perpendicularly to the incident beam direction using the 2D version of van Herk’s margin concept. The BSPTV and conventional PTV margin were first analyzed in digital phantom simulation. Then, fifteen lung cancer patients were used in this planning study. First, all patient targets were performed with the CTV projection area analysis to select the suitable beam angles. Then, BSPTV were generated according to the selected beam angles. IMRT plans were optimizted with the conventional PTV and BSPTV as the target volumes, respectively. The dosimetry metrics were calculated and evaluated between these two plans. The plan robustness of both plans for setup uncertainties was evaluated with worst-case analysis. Results: Both conventional PTV and BSPTV plans satisfied the CTV coverage. The BSPTV plans improved the sparing of high doses to target-surrounding lung tissues much better than the conventional PTV plans. Both D mean of Ring PTV and Ring BSPTV were significantly lower in BSPTV plans (38.89Gy and 39.43Gy) than that in the conventional PTV plans (40.27Gy and 40.68Gy). The V20, V5 and mean lung dose of the affected lung were significant lower in BSPTV plans (16.20%, 28.75% and 8.93Gy) than that in the conventional PTV plans (16.69%, 29.22% and 9.18Gy). About 80% of target coverage in all uncertainty scenarios could be achieved for both conventional PTV and BSPTV plans. Conclusions: The results suggested that the plan robustness can be guaranteed in both the BSPTV and conventional PTV plans. However, the BSPTV plans can spare the normal tissues such as lungs significant better than the conventional PTV plans.