The following is a summary of “Reducing the lateral dose penumbra in IMPT by incorporating transmission pencil beams,” published in the June 2024 issue of Oncology by Kong et al.

In intensity-modulated proton therapy (IMPT), the characteristic Bragg peaks produce a sharp distal dose fall-off, while the lateral dose fall-off is generally less pronounced than in photon therapy. High-energy, pristine transmission (‘shoot through’) pencil beams, which lack a Bragg peak within the patient, demonstrate a sharp lateral penumbra at the target level. This study investigates the potential benefits of integrating Bragg peaks with transmission pencil beams (TPBs) in IMPT, aiming to enhance head-and-neck cancer treatment plans by leveraging TPBs’ steep lateral dose fall-off.

To achieve this, researchers extended the automated multi-criteria IMPT plan optimization system to accommodate the combined optimization of Bragg peaks (BPs) and TPBs. The system generates a Pareto-optimal plan for each patient based on a generic ‘wish list’ containing prioritized planning objectives and complex constraints. The study group evaluated this approach in a cohort comprising eight patients with nasopharynx cancer (NPC) and eight patients with oropharynx cancer (OPC). For each patient, the IMPT&TPB plan was compared against a conventional IMPT plan, which utilized only Bragg peaks and was generated using the same optimization framework but excluded transmission pencil beams.

The results demonstrated that all clinical organ-at-risk (OAR) and target constraints were met across all plans; notably, including transmission pencil beams in the optimization process led to significant improvements in many OAR plan parameters: 14 out of 25 for NPC and 9 out of 17 for OPC. Only one parameter for OPC exhibited a minor yet statistically significant deterioration. For NPC, specific improvements included a reduction in the mean dose to the cochlea by up to 17.5 Gy and a decrease in the D2% dose to the optic nerve by up to 11.1 Gy. The remaining parameters showed non-significant differences.

In conclusion, TPBs in IMPT (IMPT&TPB) provide comparable target coverage to conventional IMPT while offering superior OAR sparing. This enhancement is primarily attributed to the steeper lateral dose fall-off achieved with TPBs, which reduces the dose to adjacent OARs. The findings suggest that the IMPT&TPB approach holds promise for improving the therapeutic ratio in head-and-neck cancer treatments.

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