The Organ Sparing Potential of Different Biological Optimization Strategies in Proton Therapy

Helge Henjum; Tordis J Dahle; Lars Fredrik Fjæra; Eivind Rørvik; Sara Pilskog; Camilla H Stokkevåg; Andrea Mairani; Kristian S Ytre-Hauge

doi:10.1016/j.adro.2021.100776

The Organ Sparing Potential of Different Biological Optimization Strategies in Proton Therapy

Adv Radiat Oncol. 2021 Aug 17;6(6):100776. doi: 10.1016/j.adro.2021.100776. eCollection 2021 Nov-Dec.

Authors

Helge Henjum¹, Tordis J Dahle^{1

2}, Lars Fredrik Fjæra¹, Eivind Rørvik¹, Sara Pilskog^{1

2}, Camilla H Stokkevåg^{1

2}, Andrea Mairani^{3

4}, Kristian S Ytre-Hauge¹

Affiliations

¹ Department of Physics and Technology, University of Bergen, Bergen, Norway.
² Department of Oncology and Medical Physics, Haukeland University Hospital, Bergen, Norway.
³ Centro Nazionale di Adroterapia Oncologica (CNAO Foundation), Pavia, Italy.
⁴ Heidelberg Ion Beam Therapy Center, Heidelberg, Germany.

Abstract

Purpose: Variable relative biological effectiveness (RBE) models allow for differences in linear energy transfer (LET), physical dose, and tissue type to be accounted for when quantifying and optimizing the biological damage of protons. These models are complex and fraught with uncertainties, and therefore, simpler RBE optimization strategies have also been suggested. Our aim was to compare several biological optimization strategies for proton therapy by evaluating their performance in different clinical cases.

Methods and materials: Two different optimization strategies were compared: full variable RBE optimization and differential RBE optimization, which involve applying fixed RBE for the planning target volume (PTV) and variable RBE in organs at risk (OARs). The optimization strategies were coupled to 2 variable RBE models and 1 LET-weighted dose model, with performance demonstrated on 3 different clinical cases: brain, head and neck, and prostate tumors.

Results: In cases with low ${(α / β)}_{x}$ in the tumor, the full RBE optimization strategies had a large effect, with up to 10% reduction in RBE-weighted dose to the PTV and OARs compared with the reference plan, whereas smaller variations (<5%) were obtained with differential optimization. For tumors with high ${(α / β)}_{x},$ the differential RBE optimization strategy showed a greater reduction in RBE-weighted dose to the OARs compared with the reference plan and the full RBE optimization strategy.

Conclusions: Differences between the optimization strategies varied across the studied cases, influenced by both biological and physical parameters. Whereas full RBE optimization showed greater OAR sparing, awareness of underdosage to the target must be carefully considered.