Objectives:

To explore the pros and cons of single-field optimization (SFO) versus multi-field optimization (MFO) in pencil beam scanning (PBS) proton lattice radiotherapy (LRT).

Methods:

Both SFO and MFO LRT treatment plans were created retrospectively for 12 representative patients with bulky head-and-neck, thoracic, and abdominal tumors. Mean gross tumor volume (GTV) was 710.7 cc (323.28-3463.14 cc). The plans were developed in the RayStation treatment planning system (version 2023B), adhering to established consensus guidelines for the prescription dose and planning technique. For each plan, 6-8 vertices with an average diameter of 1.4 cm (1.0-2.2 cm) were positioned approximately 3.5 cm apart. The prescribed dose was 18 Gy to each vertex and 3 Gy to the GTV. The dosimetric parameters evaluated included GTV Dmean, D95%, generalized equivalent uniform dose (gEUD a=-10), vertex D90%, peak-to-valley dose ratio (PVDR), and skin D1% for plan quality assessment. Plan robustness was also assessed by comparing dose metrics between the nominal and second-worst-case scenarios of the robust analysis (N=12 scenarios).

Results:

For all 12 patients, both SFO and MFO plans achieved a PVDR close to 4 across the three treatment sites. No significant differences in primary dose metrics were observed between SFO and MFO plans, except for skin D1%, which was reduced by an average of 25% in the MFO plans (p< 0.05). Robustness evaluation indicated significant increases in PVDR, GTV Dmean, and skin D1% between nominal and second worst-case scenarios for MFO plans compared to SFO (p< 0.05).

Conclusion(s):

Both SFO and MFO techniques demonstrate viability with current proton beam quality standards and advanced treatment planning algorithms. SFO offers improved plan robustness in maintaining the originally optimized metrics under uncertainties, whereas MFO enhances the PVDR and the capability to spare critical organs. These results will serve as the basis for our decision in selecting SFO/MFO for clinical proton lattice treatment, ensuring optimal treatment precision and effectiveness.