Purpose: The clinical efficacy of intensity modulated proton therapy (IMPT) requires accurate delivery of the spot pattern. As a result, patient specific quality assurance (QA) is required for each new plan. However, sensitivity of commonly used patient QA systems to spot positional errors, and the conditions where position delivery errors can be detected in patient QA have not previously been examined. This work evaluates the sensitivity of current patient-specific IMPT QA procedures to minor spot positioning errors.
Methods: Dosimetric effects of delivery uncertainties in proton spot position was evaluated for ten head-and-neck patients. Each spot position was perturbed randomly assuming a uniform distribution with interval-width of 0.75, 1, 1.5, 2.0, 2.5, and 3.0 mm at isocenter. Dose distributions and dose-volume histograms of original and perturbed plans were recalculated and compared using CTV coverage metrics. Additionally, absolute dose and 2D-planar dose measurements using 2D array ion chamber (MatriXX-PT) were performed. The impact of spot position uncertainties on the patient QA results was examined using a gamma criteria of ±3%/3mm and the gamma passing rate greater than 90% considered as criteria for validation of a clinical plan.
Results: Comparison of TPS calculated dose between original and perturbed plans suggest the effect of random spot position uncertainties on target dose parameters such as D2%,, D95%, D98% was not significant (less than 2%) until 1.5mm internal-width. Additionally, the patient-specific IMPT QA all perturbed plans up to 1.5mm tolerance was acceptable within clinical criteria (±3%/3mm).
Conclusion: Our results show that the target coverage was not sensitive to random spot position uncertainty up to 1.5 mm. Additionally, this uncertainty did not affect patient IMPT QA results.