ESTRO 2024 - Abstract Book

S4716

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

For each patient, sCT was generated from T2-weighted MR images using MRI planner (Spectronics, Sweden), a commercial neural network-based software. The sCT was co-registered to the pCT using both automatic and manual registration in RayStation. Delineated volumes were copied from the pCT to the sCT, and the photon and proton plan if available were recalculated on the sCT. Dose volume histograms (DVH) from the original plans based on pCT were compared to the ones recalculated on sCT. Additionally, a global gamma index analysis with a gamma criterion of 2%/2mm was done, comparing the recalculated dose to the original dose for all plan pairs.

Results:

While DVH parameters for the photon plans based on pCT are similar to the parameters for the plans recalculated on sCT, larger differences can be seen for the proton plans for all structures, except the brainstem (Figure 1). For the proton plans, the difference of D 2 and D 99.5 between the plan based on pCT and sCT is larger than for D mean , but this trend is not visible for the photon plans in Figure 1. As an example, Figure 2 shows that the dose calculated on pCT compared to the dose calculated on sCT differed more for protons than photons, resulting in larger regions with gamma index above 1. The mean passing rate of the global gamma index with criterion 2%/2mm was 93 ± 8 % and 79 ± 7 % for photons and protons, respectively. Therefore, the gamma index analysis also confirms that sCT dose calculations were worse for proton compared to photon plans.

Inaccurate patient positioning and image registration of sCT to pCT can explain some of the discrepancies between the clinical and recalculated dose plans.

Figure 1: Absolute dose difference between dose volume histogram (DVH) parameters for plans calculated on planning CT (pCT) and synthetic CT (sCT). The analysis was done for both photon (left) and proton (right) plans. Abbreviation: clinical target primary volume prescribed 68 Gy (CTV p, 68 ).