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The Impact of Range Uncertainties On Organ at Risk Doses in Proton Therapy

S Tattenberg*, B Gorissen, T Madden, T Bortfeld, J Verburg, Massachusetts General Hospital and Harvard Medical School, Boston, MA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Proton therapy’s main advantage is the steep dose fall-off at the beam’s distal edge. Due to range uncertainties, this advantage is usually not fully utilized. To prevent unintended effects of range errors on the dose to organs at risk (OARs), plans are made robust and instead use the lateral edge of the pencil-beams, which has a shallower dose fall-off, to conform to OARs. We created plans of varying robustness to analyze the benefit that a reduction of the proton range uncertainty would have on the dose to the brainstem and other healthy tissues.


Methods: 11 head and neck patients with clival tumors were selected, and their clinical treatment plan was re-optimized to yield a total of six plans with different levels of range error robustness. Of the six different treatment plans created for every patient, one plan was not robust, and the five others were robust to range uncertainties ranging from 1%-5%, in increments of 1%. The target coverage was kept consistent between all plans. The resulting dose distributions were analyzed, and various metrics such as mean OAR doses were calculated.


Results: The mean dose to the brainstem increased by an average of 0.54 Gy for a 1% increase in the range error robustness, with a maximum increase of 2.13 Gy. The maximum brainstem dose increased by an average of 0.30 Gy between robustness levels, and increases of up to 100 cc and 70 cc in the volume of healthy tissue receiving 10 Gy and 30 Gy were also observed.


Conclusion: We quantified the increase of OAR dose as a function of range error robustness, which is significant to determine which patients will benefit from various prompt gamma-based, PET-based and other methods currently under investigation that aim to reduce proton range uncertainty.

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