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The Influence of Patient Positioning Uncertainties in Proton Radiotherapy On Proton Range and Dose Distributions

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J Liebl

J Liebl1,2,3*, H Paganetti2 , B Winey2 , (1) EBG MedAustron GmbH, Wiener Neustadt, Austria, (2) Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, (3) Medical University of Graz, Graz, Austria


SU-E-T-324 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Proton radiotherapy allows radiation treatment delivery with high dose gradients. The nature of such dose distributions increases the influence of patient positioning uncertainties on their fidelity when compared to photon radiotherapy. The present work quantitatively analyzes the influence of setup uncertainties on proton range and dose distributions.

38 clinical passive scattering treatment fields for small lesions in the head were studied. Dose distributions for shifted and rotated patient positions were Monte Carlo-simulated. Proton range uncertainties at the 50% and 90%-dose falloff position were calculated considering 18 arbitrary combinations of maximal patient position shifts and rotations for two patient positioning methods. Normal tissue complication probabilities (NTCPs), equivalent uniform doses (EUDs) and tumor control probabilities (TCPs) were studied for organs at risk (OARs) and target volumes of eight patients.

We identified a median 1σ proton range uncertainty at the 50%-dose falloff of 2.8 mm for anatomy-based patient positioning and 1.6 mm for fiducial-based patient positioning as well as 7.2 mm and 5.8 mm for the 90%-dose falloff position respectively. These range uncertainties were correlated to heterogeneity indices (HIs) calculated for each treatment field (38% < R² < 50%). A NTCP increase of more than 10% (absolute) was observed for less than 2.9% (anatomy-based positioning) and 1.2% (fiducial-based positioning) of the studied OARs and patient shifts. TCP decreases larger than 10% (absolute) were seen for less than 2.2% of the target volumes or non-existent. EUD changes were up to 178% for OARs and 35% for target volumes.

The influence of patient positioning uncertainties on proton range in therapy of small lesions in the human brain and target and OAR dosimetry were studied. Observed range uncertainties were correlated with HIs. The clinical practice of using multiple compensator-smeared treatment beams selected to avoid distal OAR sparing is considered to be safe.

Funding Support, Disclosures, and Conflict of Interest: J. L. was supported by a scholarship of the University of Vienna

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