New energy layer reduction method for rapid intensity-modulated proton therapy (IMPT) using a ridge filter.

BACKGROUND

Efficient delivery of intensity-modulated proton therapy (IMPT) plays a pivotal role in improving the effectiveness of proton therapy. Accelerating delivery not only increases patient throughput but also improves the ability to manage dynamic target motion and reduces uncertainties, thereby enhancing treatment accuracy. Additionally, rapid delivery enables the effective use of breath-hold techniques, allowing for smaller target margins and providing better protection for healthy tissues. Since energy layer switching time constitute the majority of delivery time, reducing the number of energy layers can be highly beneficial for improving delivery efficiency.

PURPOSE

This work proposes a novel energy layer reduction method for rapid IMPT using a ridge filter.

METHODS

The width of the Bragg peak (BP) is extended using a ridge filter, referred to as the extended Bragg peak (EBP). The machine file of the generated EBP is subsequently incorporated in our in-house treatment planning system for inverse planning. The extended peak width reduces the number of energy layers required for target coverage. However, relying solely on EBP can compromise the accuracy of energy modulation. To address this limitation, pristine BP is utilized to fill small gaps left by EBP and ensure rapid dose fall-off at the distal and proximal edges of the target. To optimize the selection of BP and EBP energies, the problem is formulated as a mathematical model incorporating a group sparsity constraint. The model is then solved by a variant of block orthogonal matching pursuit algorithm, which selects BP and EBP energies in a greedy manner.

RESULTS

The combined use of BP and EBP, referred to as the BP-EBP, considerably reduced the number of energy layers required compared to conventional IMPT. For example, in an abdomen case, the BP-EBP method decreased the energy layers from 79 to 28 (11 for BP and 17 for EBP) while maintaining comparable plan quality. Similarly, in a lung case, it reduced the energy layers from 70 to 30 (9 for BP and 21 for EBP), achieving slightly better plan quality than the conventional approach. Moreover, the BP-EBP reduced delivery time by more than 50% in both cases compared to conventional IMPT. Furthermore, the feasibility of BP-EBP was experimentally demonstrated on the ProteusONE proton therapy system.

CONCLUSIONS

A novel energy reduction method is proposed and experimentally demonstrated that leverages BP and EBP generated by a ridge filter that significantly decreases the number of energy levels required in IMPT, thereby enhancing delivery efficiency.