Ultrafast proton delivery with pin ridge filters (pRFs): repurposing single-energy delivery for motion management in proton therapy.

OBJECTIVE

Highly conformal proton stereotactic body radiation therapy (SBRT) remains vulnerable to respiratory interplay, and standard mitigation techniques are either patient-dependent or enlarge the irradiated volume. Long delivery times dominated by energy-layer switching (ELS) limit the use of breath-hold (BH). We propose using pin-ridge filters (pRF) to eliminate ELS, shorten beam-on time, and reduce margins.

APPROACH: A monoenergetic pRF plan was constructed, with the highest clinical energy (Emax) obtained from the reference internal target volume (ITV)-based intensity modulated proton therapy (IMPT) plan. To avoid per-patient commissioning, we used a bank of pre-commissioned monoenergetic beams spanning 110-200 MeV. Each beam model enables the development of a gross tumor volume (GTV)-based downstream IMPT (IMPT-DS) plan (reduced margins in the BH setting). Next, a nested pencil-beam-direction (PBD) spot-reduction process iteratively removed low-weighted spots from each PBD and, finally, generated pRFs with coarser resolution using a single-energy. This method was validated on five liver and one lung SBRT cases (10 Gy ×5) by comparing the relative total volume receiving the prescription dose (conformity index CI(GTV+5mm) =VRx/VGTV+5mm) and the mean lung-GTV and liver-GTV doses between the conventional IMPT and pRF plans.

MAIN

RESULTS

Across five liver SBRT cases, the average CI(GTV+5mm) improves from 2.64 (IMPT) to 1.16 (IMPT-pRF). The reduced dose cloud results in ~26% reduction in Dmean to liver-GTV. The mean beam-on time per field dropped from 43.02 s (IMPT) to 7.14 s (IMPT-pRF), i.e., an average saving of 35.88 s per beam (83.4% shorter). Lung SBRT follows a similar trend for CI(GTV+5mm) (3.22 to 1.58), lung-GTV Dmean (10.3 to 6.9 Gy), and average beam-on time (decreased from 47.2 to 9.5s per beam). These dosimetric improvements are primarily due to change in the planning approach (free-breathing ITV to BH GTV), whereas the key contribution of the pRF is shortening of beam-on time, which makes BH delivery feasible and thereby allows this margin reduction.

SIGNIFICANCE: This proposed method significantly reduces delivery time and organ-at-risk dose. Future work should focus on planning quality assurance (QA) and clinical implementation in patients with limited breath-hold capacity.