Purpose: In this work, as a preliminary study and proof of concept, patient specific normal tissue objective (NTO) based on PTV volumes is applied during the optimization process to improve dose gradient R50% (ratio of the 50% isodose volume/PTV volume).
Methods: Assuming PTV as a sphere, the radius can be derived from simple geometric formula. The desired 50% isodose volume can be obtained from the PTV volume times the R50% constraint from RTOG 0813 protocol, thus the radius of 50% isodose volume can be derived. The difference of the radius of these two spheres (the distance from PTV edge to 50% isodose line) is an indication of dose falloff. This distance (DNTO) is calculated according to PTV volumes and incorporated in the normal tissue objective (NTO) module of Eclipse TPS’s photon optimizer. The setting is set to achieve the distance from PTV edge to the 50% isodose line be equal to DNTO. Retrospectively 5 lung SBRT patients are re-planned utilizing the NTO module, while maintaining other OAR constraints. The R50%, CI (Conformity index) and V20 (percent lung volume receiving 20 Gy) for each plan are then compared with previously treated clinical plans where NTO objectives were not used.
Results: CI remains unchanged as expected. R50% is reduced for all plans, ranging from 1.2% to 7.7%, with an average of 4.4% when patient specific NTO are used. V20 are also reduced, ranging from 2.8-6.8%, with an average of 5.4% utilizing NTO. Further reduction is possible if more aggressive NTO setting is employed, which is under current investigation.
Conclusion: R50% can be effectively reduced using patient specific NTO settings based on PTV sizes. V20 can also be improved utilizing NTO. The correlation of R50% reduction and potential better clinical outcomes is also manifested in the improved V20 values.
Not Applicable / None Entered.