Proton pencil beam scanning for mediastinal lymphoma: treatment planning and robustness assessment

Abstract

Background: Modern radiotherapy (RT) for lymphoma is highly personalized. While advanced imaging is largely employed to define limited treatment volumes, the use of proton pencil beam scanning (PBS) for highly conformal lymphoma RT is still in its infancy. Here, we assess the dosimetric benefits and feasibility of PBS for mediastinal lymphoma (ML). Materials and methods: Ten patients were planned using PBS for involved-site RT. The initial plans were calculated on the average four-dimensional computed tomography (4D-CT). PBS plans were compared with 3D conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and proton double scattering (DS). In order to evaluate the feasibility of PBS and the plan robustness against interand intra-fractional uncertainties, the 4D dose was calculated on initial and verification CTs. The deviation of planned dose from delivered dose was measured. The same proton beamline was used for all patients, while another beamline with larger spots was employed for patients with large motion perpendicular to the beam. Results: PBS provided the lowest mean lung dose (MLD) and mean heart dose (MHD) for all patients in comparison with 3D-CRT, IMRT, and DS. For eight patients, internal target volume (ITV) D 98% was degraded by <3%; and the MLD and MHD deviated by <10% of prescription over the course of treatment when the PBS field was painted twice in each session. For one patient with target motion perpendicular to the beam (>5 mm), the degradation of ITV D 98% was 9%, which was effectively mitigated by employing large spots. One patient exhibited large dose degradation due to pericardial effusion, which required replanning across all modalities. Conclusions: This study demonstrates that PBS plans significantly reduce MLD and MHD relative to 3D-CRT, IMRT, and DS and identifies requirements for robust free-breathing ML PBS treatments, showing that PBS plan robustness can be maintained with repainting and/or large spots.

Proton therapy for adults with mediastinal lymphomas: the International Lymphoma Radiation Oncology Group guidelines

“…Several dosimetric studies comparing proton with photon plans [8][9][10][11][12][13][14][15][16][17][18][19][20][21] have been conducted in an effort to establish the benefit and indications for the use of proton therapy in lymphoma, with the vast majority focusing on mediastinal involvement. They showed that there is an individualized and potential benefit in patients with dose reduction to the heart, lungs, and breasts with proton therapy compared with 3D conformal radiotherapy or IMRT.…”

Evidence-based Review on the Use of Proton Therapy in Lymphoma From the Particle Therapy Cooperative Group (PTCOG) Lymphoma Subcommittee

“…For treatments delivered by proton pencil beam, the interplay between the scanning beam and a moving target creates dose inhomogeneities within the target, degrading the dose distribution. Larger spots and dose repainting are used to maintain robustness of mediastinal lymphoma proton pencil-beam treatments (61). With the small cohorts in dosimetric studies comparing photon and proton lymphoma plans, it is difficult to discern if all plans are of deliverable quality, especially because some use the approach of simple geometric expansions as motion management without motion evaluation.…”

“…Of the 9 published studies evaluating the dosimetric benefits of proton therapy on cardiac dose, 7 demonstrated improvements compared with photon techniques (53,(55)(56)(57)(58)(59)(61)(62)(63) (Table E1; available online at www.redjournal.org). Seven of these studies evaluated passive-scanning proton therapy, and 2 evaluated an active scanning technique.…”

“…The 3 studies with the largest difference in mean dose to the lungs with proton therapy compared with 3D RT reported actual treatment with proton therapy (53,61,63). These studies may have seen larger differences in mean dose to the lungs owing to selection bias in that patients expected to benefit more from proton therapy, and because of large bulky mediastinal disease, might preferentially be referred for proton therapy.…”

Comparing Breath Hold and Free Breathing during Intensity-Modulated Radiation Therapy and Proton Therapy in Patients with Mediastinal Hodgkin Lymphoma

“…In the treatment of mediastinal lymphoma, there has been increased interest over the last 5 years in utilizing proton therapy techniques as well as breath-hold techniques with 3dimensional conformal radiation therapy (RT) or intensity-modulated RT in an effort to reduce the radiation dose to the organs at risk [1][2][3][4][5][6]. Combining both breath-hold techniques and proton therapy has been less common owing to concerns about the reproducibility of breath hold, the impact of poor reproducibility when using proton therapy, and the lack of volumetric image-guided RT and real-time diaphragm monitoring of the breath hold.…”