Room: AAPM ePoster Library
The new generation of rotating gamma ray systems (RGS) is designed for intra- and extra-cranial image-guided SRS/SBRT using 16 focusing 6°Co sources arranged on a 35° arc. The traditional spherical shot packing method commonly used in current intracranial radiosurgery is inefficient and sometimes time consuming for large and irregularly shaped targets. This work investigates a scanning beam method dedicated to the new RGS for efficient dose delivery with high dose conformity for intra- and extra-cranial SRS/SBRT.
An EGS4/PRESTA user code MCSIM was used for accurate dose calculation for the RGS 6°Co beams of different cone sizes. In our method, the tumor volume is divided into layers along the cranial-caudal direction, and the layer thickness and the corresponding cone size are determined based on the tumor axial dimensions. A scanning pattern of a coplanar beam set is developed with the beam edge following the tumor periphery for optimal conformity and dose fall-off. Racetrack-shaped tumors with adjustable lengths are investigated.
Tumor volumes varied from 8.88-15.80cc. Our results from manual forward planning showed that dose could be prescribed to ~50% isodose lines to achieve good target coverage (~95-95.8% at 100%PD; ~98.6-99.4% at 90%PD) with steep dose fall-off in surrounding normal tissues. Both the conformity index (Paddick CI ~0.49-0.66; RTOG CI ~1.4-1.85) and gradient index (GI ~4.1-4.68) were comparable to static gamma knife systems and dependent on the tumor size.
This study demonstrates the feasibility and great potential of the scanning beam-based dose painting delivery method for the new RGS. Future in-depth investigations will focus on the auto-optimization on beam sizes and weights and also realistic patient plans to quantify the improvement of target dose conformity and delivery efficiency.