Novel methodology for re-irradiation treatment plan optimisation
OC-0126
Abstract
Novel methodology for re-irradiation treatment plan optimisation
Authors: Louise Murray1, Christopher Thompson2, Christopher Pagett2, John Lilley2, Bashar Al-Qaiseh2, Stina Svensson3, Kjell Eriksson3, Michael Nix2, Michael Aldred2, Lynn Aspin2, Stephen Gregory2, Ane Appelt4
1Leeds Institute of Medical Research at St James’s and Department of Clinical Oncology, University of Leeds and Leeds Cancer Centre, Leeds, United Kingdom; 2Department of Medical Physics, Leeds Cancer Centre, Leeds, United Kingdom; 3Research Group, RaySearch Laboratories AB, Stockholm, Sweden; 4Leeds Institute of Medical Research at St James’s and Department of Medical Physics, University of Leeds and Leeds Cancer Centre, Leeds, United Kingdom
Show Affiliations
Hide Affiliations
Purpose or Objective
The STRIDeR (Support Tool for Re-Irradiation Decisions guided by Radiobiology) project aims to create a clinically viable re-irradiation (reRT) planning pathway within a commercial treatment planning system (TPS) that takes account of the full previous dose distribution, fraction-size-effects, tissue recovery and anatomical changes between two radiotherapy courses.
Material and Methods
The STRIDeR pathway allows an original dose distribution to be used as background dose to guide optimisation of a reRT plan. Organ at risk (OAR) planning objectives in equivalent dose in 2Gy fractions (EQD2) can be applied cumulatively across both treatment courses, with optimisation of the reRT plan performed on a voxel-by-voxel level in EQD2; this is a unique feature within a commercial TPS. Normal tissue recovery can be incorporated and different a/b values applied per OAR. Different approaches to image registration can be employed to account for anatomical change.
Validation of the EQD2 optimisation process was performed and data from 21 patients who received 5-fraction pelvic Stereotactic Ablative Radiotherapy (SABR) reRT were used to evaluate the pathway. A selective approach for dose mapping was used: 1) the original planning CT was registered to the reRT planning CT with deformable image registration (DIR), and the original dose mapped to the reRT planning CT based on the deformation vector field; 2) for each OAR where the DIR was considered reliable, the original dose was used as background dose for reRT optimisation and cumulative planning objectives in EQD2 applied across both dose distributions (Fig 1); 3) for any OAR where the DIR was considered unreliable, the previous maximum dose to that OAR within 2cm of the reRT Planning Target Volume (PTV) was used to calculate the ‘dose remaining’ for reRT and then determine an appropriate planning objective, applied to the reRT dose distribution only. The PTV coverage was optimised in physical dose (applied to reRT dose distribution).
STRIDeR plans were compared to those produced using a standard manual method based on the same registration strategy.
Work was performed in RayStation Research version 9B in collaboration with RaySearch Laboratories AB (Stockholm, Sweden).
Results
The STRIDeR pathway was successfully used in all 21 cases, with clinically acceptable plans created in 20/21 (optimal clinical goals achieved in 12, relaxation of optimal clinical goals or incorporation of recovery required in 7 and reduced prescription dose required in 1; Fig 2). Compared to plans produced using the more laborious manual method, less clinical goal relaxation was required or a higher reRT dose could be prescribed in 3/21 cases.
Conclusion
The STRIDeR pathway uses background dose for scientifically meaningful reRT treatment planning in a commercial TPS. This provides a standardised approach that offers more informed reRT, improved cumulative OAR dose evaluation, and the potential for enhanced understanding of OAR reRT tolerance.