Room: Exhibit Hall | Forum 7
Purpose: To investigate and quantify the sensitivity of intracranial, dynamically collimated proton therapy (DCPT) treatments using a Dynamic Collimation System (DCS).
Methods: A set of intracranial treatments were planned with an in-house planning system capable of modeling asymmetric proton beamlets resulting from the DCS. These nominal plans were each modified to reflect the estimated uncertainties in beam spot placement, collimation rod positioning accuracy during translation, and the alignment of the DCS during mounting. Each offset was represented using a normally distributed random variable with a standard deviation of 0.25 mm. The resulting coverage was recalculated to quantify the changes in the PTV homogeneity, conformity, and the surrounding OAR sparing.
Results: Shifts in the DCS alignment resulted larger PTV coverage changes than random offsets in beamspot and collimation rod position accuracy. The magnitude of these dosimetric changes can be minimized by prohibiting collimator positions near a beamlet's central axis. This results from the fact that proton fluence and the resulting dose distribution are sensitive to small positional changes in the collimator when near the beamlet central axis. This sensitivity decreases as a function of collimator distance from the central axis. The combination of potential offsets was found to change the PTV D95% of a 50 Gy chordoma treatment plan by as much as 3.16 Gy if the minimum allowable off-axis collimator distance was restricted to 0.5 mm. Increasing this distance to 1.5 mm reduced potential changes in the PTV D95% to within 0.45 Gy.
Conclusion: Increasing the minimum allowable off-axis collimator distance improves the robustness of a DCPT plan. However, this comes at the cost of reducing the degree of healthy tissue sparing achievable. An acceptable balance between these parameters is still under investigation to mitigate the effects of systematic and random errors while maintaining a high dose conformity.