Purpose: In proton therapy, conforming the dose distribution to an organ at risk (OAR) using the sharp dose fall-off at the distal beam edge may improve OAR sparing. Currently, the lateral beam edge is often used instead to reduce the effects of uncertainties in the in vivo proton range. However, range uncertainty reductions may allow for greater use of the distal beam edge. We quantified the benefits of such novel beam arrangements in a set of nine cases with targets abutting the brainstem.
Methods: For each case, six treatment plans robust to 2 mm set-up as well as 0-5% range uncertainty were created for the traditional clinical beam arrangement and a novel beam arrangement making greater use of the distal beam edge to conform the dose distribution to the brainstem. Metrics including the brainstem normal tissue complication probability (NTCP) with the endpoint of necrosis were determined for all plans and set-up and range uncertainty scenarios.
Results: For the traditional beam arrangement, reducing the range uncertainty from the current level of approximately 4% to a potentially achievable level of 1% reduced the brainstem NTCP by up to 0.6 percentage points when averaging over all scenarios and by up to 0.9 percentage points in the worst-case scenario. A switch to the novel beam arrangement at 1% range uncertainty increased these values by a factor of about 2, to 2.0 percentage points and 2.9 percentage points. The novel beam arrangement achieved a lower brainstem NTCP in all cases at a range uncertainty of 2% when averaging over scenarios and at a level of 1% in the worst-case scenario.
Conclusion: The benefits of novel beam arrangements may be of the same magnitude or even exceed the direct benefits of range uncertainty reductions. The benefits of range uncertainty reductions may therefore previously have been underestimated.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Federal Share of program income earned by Massachusetts General Hospital on C06-CA059267, Proton Therapy Research and Treatment Center, and by the National Cancer Institute grant R01-CA229178, Fast Individualized Delivery Adaptation in Proton Therapy. S.T. was supported by a scholarship awarded by the Monika Kutzner Foundation.