Purpose: Dose fall off is one of the major concern of Stereotactic Radiosurgery (SRS). The objective this study is to analyze the impact of distance of dose drop-off to 50% (Δr) with planning target volume (PTV), different technique and design of multi leaf collimator (MLC).
Methods: The PTVs of known volumes ranging from 0.17cc to 43.9cc were contoured in CT image using Eclipse™ TPS and planned for 24Gy in single fraction. Plans were generated using arcs and Intensity modulated radiotherapy (IMRT) with Varian Truebeam™ STX (VTB) and only IMRT for Varian Unique™ Power (VUP). The same beam arrangements and optimization objective were used for both IMRT and arc techniques. For arc technique, the variation of △r with non-coplanar and coplanar beams using high definition MLC were analyzed. In addition, the dose drop of distance was evaluated in IMRT technique between HDMLC and millennium MLC(mMLC). The △r was measured as the difference between the radius of 50% isodose line and the radius of PTV while keeping the conformity index 1 for all plans by adjusting the reference dose isodose levels.
Results: The dose fall off from PTV edge to 50% isodose line ranges from 0.29cm-0.71cm for ARC technique with non-coplanar beams, whereas for coplanar beams it ranged from 0.34cm-1.01cm. For IMRT non-coplanar beams with HDMLC △r ranged from 0.39cm- 0.71cm whereas with mMLC it ranged from 0.34cm-0.76cm. For IMRT coplanar beams with VTB △r ranged from 0.44cm-1.01cm whereas for VUP was 0.39cm-1.01cm.
Conclusion: The study results concluded that lesser △r in ARC plans as compared to IMRT plans. It was found that a sharp dose fall in non-coplanar beams and a shallow dose fall off in coplanar beams even with HDMLC. When compared IMRT plans of two units, VTB showed a shallow fall off compared to VUP.
Dose, Stereotactic Radiosurgery, Intensity Modulation