Copenhagen, Denmark
Onsite/Online

ESTRO 2022

Session Item

Sunday
May 08
16:55 - 17:55
Mini-Oral Theatre 1
15: Treatment plan optimisation & adaptation
Edmond Sterpin, Belgium;
Lena Nenoff, USA
2570
Mini-Oral
Physics
Novel optimization functions designed for re-irradiation treatment planning
Jakob Ödén, Sweden
MO-0638

Abstract

Novel optimization functions designed for re-irradiation treatment planning
Authors:

Jakob Ödén1, Kjell Eriksson1, Stina Svensson1, Erik Setterquist2, John Lilley3, Christopher Thompson3, Christopher Pagett3, Ane Appelt4, Louise Murray5, Rasmus Bokrantz1

1RaySearch Laboratories AB, Research, Stockholm, Sweden; 2RaySearch Laboratories AB, Development, Stockholm, Sweden; 3Leeds Cancer Centre, St. James' University Hospital, Medical Physics and Engineering, Leeds, United Kingdom; 4Leeds Cancer Centre, St. James' University Hospital, Medical Physics and Engineering, Leeds Institute of Medical Research at St James's, University of Leeds, Leeds, United Kingdom; 5Leeds Cancer Centre, St. James' University Hospital, Clinical Oncology, Leeds Institute of Medical Research at St James's, University of Leeds, Leeds, United Kingdom

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Purpose or Objective

Re-irradiation (reRT) for locoregional recurrence or second cancers after previous radiotherapy (RT) is a promising treatment technique, though anatomical changes between courses and radiobiologically-appropriate dose summation of multiple RT courses are recognised challenges. Optimally, reRT treatment planning should take the previously delivered dose into account. However, if traditional normal tissue threshold dose levels are already exceeded in previous treatments, regular optimization functions might cause suboptimal reRT plans. We propose a set of novel optimization functions designed for reRT treatment planning.

Material and Methods

The standard physical optimization functions in RayStation that quadratically penalize voxel doses above a threshold dose level were adjusted to quadratically penalize the total equivalent dose in 2 Gy fractions (EQD2) above a threshold EQD2 level (EQD2level). The EQD2level was further adjusted in a voxelwise fashion to account for the accumulated delivered EQD2 and a selected minimum allowed reRT EQD2 (EQD2delivered+reRTmin) as,


EQD2*level[x] = max(EQD2level[x], EQD2delivered+reRTmin[x]),


where EQD2*level[x] is the adapted EQD2level for voxel x in a ROI. An α/β=3 Gy was used for all voxels in this study. To account for partial tissue recovery between the RT courses, a set of reRT scale factors ([0,1]) that scale the EQD2 for each previous RT course could be selected. These adjustments were implemented in a research version of RayStation 11A for the maximum EQD2/EUD/DVH functions and the EQD2 fall-off function (Fig. 1). To demonstrate proof of concept, reRT VMAT plans of 35 Gy in 5 fractions (EQD2=70 Gy) were optimized to fulfil D95%≥95% and D2%≤105% using standard and novel optimization functions (EQD2reRTmin=1–2 Gy) for a spherical relapse (20 cc) in three positions (scenarios A, B and C) following a fictive pelvic treatment of 60 Gy in 20 fractions (EQD2=72 Gy) on a human phantom (CIRS 801-P) (Fig. 2). The homogeneity index (HI=EQD295%/EQD25%), EQD2 conformity index (CI=Vtarget, 95%/V95%) and EQD2 gradient index (GI=[V20%-V80%]/Vtarget) were evaluated.



Results

All reRT plans fulfilled D95%≥95 % and D2%≤105%. The EQD2 distributions in Fig. 2a and 2b resulted in 0.93, 0.91 and 8.54 (HI, CI, GI) using the standard functions compared with 0.95, 0.92 and 5.50 using the novel functions for scenario A. The corresponding values for scenario B were 0.90, 0.90 and 9.03 for the standard functions vs. 0.92, 0.96 and 6.81 for the novel functions, whereas for scenario C the standard functions resulted in 0.90, 0.70, 14.81 vs. 0.91, 0.98, 9.86 for the novel functions.


Conclusion

The novel optimization functions were well-suited for reRT with increased HI and CI as well as reduced GI compared to standard functions for all scenarios in this proof-of-concept study. These novel functions are being tested in the Support Tool for Re-Irradiation Decisions guided by Radiobiology (STRIDeR) project and have the potential to be instrumental in the future of reRT planning.