TH-A-137-09: Gated Lung SRS/SBRT with Conical Collimators.
PURPOSE
To investigate the use of conical collimator equipped linacs to treat small lung tumors (< 3 cm in diameter) with gated stereotactic radio-surgery (SRS) or stereotactic body radio-therapy (SBRT) and to compare the performance of this modality with other treatment options currently being used.


METHODS
A phantom simulating chest and lung motion was used to test the effectiveness of gating a linac equipped with conical collimators. Dose to a simulated lung lesion was planned, delivered using cones, and measured with an ionization chamber and film. Plans designed using traditional modalities for treatment of a single, small lung lesion were then compared with plans using conical collimators to determine if any advantages exist when using conical collimators.


RESULTS
Conical collimator equipped linacs may be used with respiratory gating during SRS/SBRT. For Varian Trilogy linacs, using beam arcs with cones rather than static beams would require software modifications that prevent minor-interlocks from reinitiating the beam after gantry motion is stopped due to gating. A significant advantage was found when using static beams with cylindrical collimators to treat small lung lesions where the healthy tissue at risk is limited by mean dose to the tissue. It is possible to deliver 30% higher doses to the target lesion under these circumstances. When the healthy tissue at risk is limited by maximum dose to surrounding critical structures, such as the spinal cord, using the cone collimators does not provide a significant advantage over other modalities.


CONCLUSION
The results show that, depending on the type of sensitivity exhibited by the tissue at risk, use of conical collimators with gated static beam SRS/SBRT on small lung tumors would allow delivery of higher target doses than conventional modalities, while still sparing surrounding normal tissue from damaging dose levels.