Extend distance treatment for stereotactic body radiation therapy of lung cancer

Purpose: We investigate a delivery method in stereotactic body radiation therapy using an Extend Distance Source to Virtual Axis (EDSVA) method. This approach significantly expands the range of beam angles and improves the target confomality and the normal tissue avoidance. Methods and Materials: In EDSVA the source to the target distance varies within 100 to 130 cm. Three plans with the same prescription were sequentially calculated in Pinnacle 8.0m Treatment Planning System (TPS). The first plan utilizes ten non-coplanar beams and satisfies dosimetric criteria adopted in RTOG 0236. The plan represents suitable SBRT lung cancer treatment at SAD 100 cm. In the second plan, gantry and couch angels are kept unchanged, however, the target moved 20 cm along the central axis away from the source. In the third plan, the source to the target distance kept constant (120 cm), however, the gantry and couch angles optimized to maximize the mutual separation of the beams. All plans are calculated for custom made heterogeneous lung phantom and verified empirically. Results: We find that the dose distribution in the optimized EDSVA, the third plan, produces a more compact dose distribution. This reduction of the volume of high dose exposure of the sensitive organ (lung in our investigated case) is due to the decrease of the volume of beam overlap resulting from a more isotropic distribution of beam orientations at SAD equal to 120 cm in comparison with standard isocentric, 100 cm SAD. Conclusions: Our investigation demonstrates that EDSVA reduces the toxicity of normal tissues and is a viable approach for SBRT treatments implemented with clinical TPS and clinical linear accelerators.