Dosimetric impact of range uncertainty in passive scattering proton therapy

Abstract

Purpose: The objective of this study was to investigate the dosimetric impact of range uncertainty in a large cohort of patients receiving passive scatter proton therapy.

Methods: A cohort of 120 patients were reviewed in this study retrospectively, of which 61 were brain, 39 lung, and 20 prostate patients. Range uncertainties of ±3.5% (overshooting and undershooting by 3.5%, respectively) were added and recalculated on the original plans, which had been planned according to our clinical planning protocol while keeping beamlines, apertures, compensators, and dose grids intact. Changes in the coverage on CTV and DVH for critical organs were compared and analyzed. Correlation between dose change and minimal distance between CTV and critical organs were also investigated.

Results: Although CTV coverages and maximum dose to critical organs were largely maintained for most brain patients, large variations over 5% were still observed sporadically. Critical organs, such as brainstem and chiasm, could still be affected by range uncertainty at 4 cm away from CTV. Coverage and OARs in lung and prostate patients were less likely to be affected by range uncertainty with very few exceptions.

Conclusion: The margin recipe in modern TPS leads to clinically acceptable OAR doses in the presence of range uncertainties. However, range uncertainties still pose a noticeable challenge for small but critical serial organs near tumors, and occasionally for large parallel organs that are located distal to incident proton beams.

Keywords: dose calculation; passive scatter proton therapy; proton range uncertainty; retrospective study.

Fig. 1

Distribution of deviations from planned values for maximum doses to OARs for brain tumor cases.

Fig. 2

Boxplots of distribution of deviations from planned values for maximum doses to OARs, minimum dose to CTV, and D95 of CTV for brain tumor cases. The indices 1 and 2 represent overshooting plan and undershooting scenarios, respectively. The tops and bottoms of each box are the 25th and 75th percentiles of the samples, respectively. The distances between the tops and bottoms are the interquartile ranges. The line in the middle of each box is the sample median. The whiskers are lines extending above and below each box. Whiskers are drawn from the ends of the interquartile ranges to the furthest observations within the whisker length (the adjacent values). Observations beyond the whisker length are marked as outliers. The outlier is a value that is more than 1.5 times the interquartile range away from the top or bottom of the box. Outliers beyond the box were plotted in red markers.

Fig. 3

Maximum dose vs. minimum distance between OAR for brain tumor cases where the red and black triangles denote overshooting and undershooting scenarios, respectively.

Fig. 4

Dose distribution of lung and esophagus for lung tumor cases.

Fig. 5

Boxplots of dose distribution of lung, heart, and esophagus for lung tumor cases. In the boxplots, id 1 and 2 represent overshooting scenario and undershooting scenario, respectively. The tops and bottoms of each box are the 25th and 75th percentiles of the samples, respectively. The distances between the tops and bottoms are the interquartile ranges. The line in the middle of each box is the sample median. The whiskers are lines extending above and below each box. Whiskers are drawn from the ends of the interquartile ranges to the furthest observations within the whisker length (the adjacent values). Observations beyond the whisker length are marked as outliers. The outlier is a value that is more than 1.5 times the interquartile range away from the top or bottom of the box. Outliers beyond the box were plotted in red markers.

Fig. 6

Dose volume vs. minimum distance between OAR and CTV for lung tumor cases. The red triangle indicates the dose by overshooting scenario, and the black triangle indicates the dose by undershooting scenario.

Fig. 7

Dose distribution of bladder and rectum of prostate tumor cases.

Fig. 8

Dose distribution of bladder and rectum of prostate tumor cases. In the boxplots, id 1 and 2 represent overshooting scenario and undershooting scenario, respectively.

Fig. 9

Dose volume vs minimum distance between OAR and CTV for prostate tumor cases.