Abstract
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. Thirty-eight 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. The authors 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 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. For target volumes TCP decreases by more than 10% (absolute) occurred in less than 2.2% of the considered treatment scenarios for anatomy-based patient positioning and were nonexistent for fiducial-based patient positioning. EUD changes for target volumes were up to 35% (anatomy-based positioning) and 16% (fiducial-based positioning). The influence of patient positioning uncertainties on proton range in therapy of small lesions in the human brain as well as target and OAR dosimetry were studied. Observed range uncertainties were correlated with HIs. The clinical practice of using multiple fields with smeared compensators while avoiding distal OAR sparing is considered to be safe.
Highlights
Deviations of patient positions in radiotherapy occur between the patient position at the acquisition of a computed tomography (CT) image acquired for radiotherapy planning and patient setup position(s) for delivery of the treatment
The thereby created planning target volume (PTV) is used for treatment planning and shall increase the probability of appropriate dose delivery to the clinical target volume (CTV) taking into account patient setup uncertainties
The current study aims to quantify the effects of patient setup errors on proton range as well as organ at risk (OAR) dose and target dose variations, in cranial and head and neck locations due to smaller treatment fields and close proximity of organs at risk (OARs) to targets
Summary
Deviations of patient positions in radiotherapy occur between the patient position at the acquisition of a computed tomography (CT) image acquired for radiotherapy planning and patient setup position(s) for delivery of the (fractionated) treatment. Uncertainties in reproducing the patient position for delivery of irradiation are generally accounted for by the addition of a margin to any clinical target volume (CTV).. The thereby created planning target volume (PTV) is used for treatment planning and shall increase the probability of appropriate dose delivery to the CTV taking into account patient setup uncertainties.. It is essential to reduce healthy tissue irradiation for a patient treatment, keeping the margins added to a CTV as small as possible while achieving a favorable actual dose delivery to the CTV during treatment. To the creation of a PTV for treatment planning, in proton therapy a range overshoot is applied to account for proton range uncertainties (i.e., errors in dose delivery and/or dose calculation). Burr proton therapy center of the Massachusetts General Hospital (MGH) is 1 mm and 3.5% of the most distal proton range. Additional margins are applied for specific geometrical scenarios [e.g., lung or complex geometries (interfaces) in the head]
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