Optimizing QACT Frequency and Setup Uncertainty in Cranial Proton Therapy for Normal Tissue Sparing.

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Proton therapy offers precise cranial dose distributions but is sensitive to setup variations.
• Quality assurance computed tomography (QACT) scans improve accuracy but increase patient dose and workflow burden.
• Optimal setup uncertainty margins and QACT frequencies require further investigation for robust cranial proton therapy.

Purpose of the Study:

• To evaluate the dosimetric robustness of cranial proton therapy with reduced setup uncertainty.
• To assess the impact of setup uncertainty on target coverage and organ at risk (OAR) sparing.
• To inform adaptive planning and QACT frequency requirements based on beam trajectory and target location.

Main Methods:

• Retrospective review of 122 adult patients treated with cranial proton therapy (2019-2023).
• Analysis of adaptive planning rates based on beam paths through the neck or nasal cavity.
• Re-planning of 20 selected patients using 2 mm setup uncertainty versus the clinical 3 mm standard, with synthetic CT generation for daily verification.

Main Results:

• Only 6 patients required adaptive plans, with 2 cases attributed to anatomical changes in patients with beams traversing the nasal cavity.
• Clinical target volume (CTV) coverage goals were met in 95.7% (2 mm) and 99.6% (3 mm) of plans.
• Overall clinical goals were met in 99.3% for both 2 mm and 3 mm uncertainty plans, with minimal dose deviations.

Conclusions:

• A 2 mm setup uncertainty margin provides robust target coverage and OAR sparing in cranial proton therapy.
• Tailored QACT schedules, considering beam trajectory and target location, can reduce imaging burden without compromising accuracy.
• Proton therapy protocols can be refined by implementing reduced setup margins and selective QACT based on patient-specific factors.