Treatment Planning of Bulky Tumors Using Pencil Beam Scanning Proton GRID Therapy

Aditya Halthore; Zachary Fellows; Anh Tran; Curtiland Deville Jr; Jean L Wright; Jeffrey Meyer; Heng Li; Khadija Sheikh

doi:10.14338/IJPT-22-00028

Treatment Planning of Bulky Tumors Using Pencil Beam Scanning Proton GRID Therapy

Int J Part Ther. 2022 Dec 22;9(3):40-49. doi: 10.14338/IJPT-22-00028. eCollection 2023 Winter.

Authors

Aditya Halthore^{1

2}, Zachary Fellows², Anh Tran², Curtiland Deville Jr^{1

2}, Jean L Wright^{1

2}, Jeffrey Meyer¹, Heng Li^{1

2}, Khadija Sheikh^{1

2}

Affiliations

¹ Department of Radiation Oncology and Molecular Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
² Department of Radiation Oncology, The Johns Hopkins Proton Center, Washington, DC, USA.

Abstract

Purpose: To compare spatially fractionated radiation therapy (GRID) treatment planning techniques using proton pencil-beam-scanning (PBS) and photon therapy.

Materials and methods: PBS and volumetric modulated arc therapy (VMAT) GRID plans were retrospectively generated for 5 patients with bulky tumors. GRID targets were arranged along the long axis of the gross tumor, spaced 2 and 3 cm apart, and treated with a prescription of 18 Gy. PBS plans used 2- to 3-beam multiple-field optimization with robustness evaluation. Dosimetric parameters including peak-to-edge ratio (PEDR), ratio of dose to 90% of the valley to dose to 10% of the peak VPDR(D90/D10), and volume of normal tissue receiving at least 5 Gy (V5) and 10 Gy (V10) were calculated. The peak-to-valley dose ratio (PVDR), VPDR(D90/D10), and organ-at-risk doses were prospectively assessed in 2 patients undergoing PBS-GRID with pretreatment quality assurance computed tomography (QACT) scans.

Results: PBS and VMAT GRID plans were generated for 5 patients with bulky tumors. Gross tumor volume values ranged from 826 to 1468 cm³. Peak-to-edge ratio for PBS was higher than for VMAT for both spacing scenarios (2-cm spacing, P = .02; 3-cm spacing, P = .01). VPDR(D90/D10) for PBS was higher than for VMAT (2-cm spacing, P = .004; 3-cm spacing, P = .002). Normal tissue V5 was lower for PBS than for VMAT (2-cm spacing, P = .03; 3-cm spacing, P = .02). Normal tissue mean dose was lower with PBS than with VMAT (2-cm spacing, P = .03; 3-cm spacing, P = .02). Two patients treated using PBS GRID and assessed with pretreatment QACT scans demonstrated robust PVDR, VPDR(D90/D10), and organs-at-risk doses.

Conclusions: The PEDR was significantly higher for PBS than VMAT plans, indicating lower target edge dose. Normal tissue mean dose was significantly lower with PBS than VMAT. PBS GRID may result in lower normal tissue dose compared with VMAT plans, allowing for further dose escalation in patients with bulky disease.

Keywords: GRID; bulky tumors; proton; spatial fractionation.

Grants and funding

Funding: The authors have no funding to disclose.