S1873
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Conclusion: Systematic delineation of cardiac substructures from AI-based contouring has reshaped planning paradigms. Without substructure cardiac constraints, IMRT was more favorable for left-sided treatments, but with explicit LAD objectives, VMAT becomes more appropriate to reduce cardiac exposure. For right- sided targets, unaffected by LAD, IMRT is the optimal approach. These results partly diverge from Fogliata et al. [3], whose flowchart mainly relies on nodal involvement; our data identify laterality as the primary determinant guiding the IMRT-VMAT selection. References: [1] Koivumaki T et al. Physica Medica 136 (2025).[2] De Rose F et al. Radiother Oncol 202 (2025).[3] Fogliata A et al. Adv Radiat Oncol 9 (2024). Keywords: Breast radiotherapy; IMRT; VMAT; Halcyon SPECT-guided radiotherapy on the MR-linac for functional liver avoidance SBRT for liver tumors: A planning study Albert J. Everard 1 , Thomas Willigenburg 1 , Jonna K. Vulpen 1 , Marnix G.E.H. Lam 2 , Bas W. Raaymakers 1 , Martijn P.W. Intven 1 , Casper Beijst 1 1 Radiotherapy, UMC Utrecht, Utrecht, Netherlands. 2 nuclear medicine, UMC Utrecht, Utrecht, Netherlands Purpose/Objective: Liver tumors are traditionally treated with surgery, chemotherapy and/or (radiofrequency/microwave- )ablation. However, due to advances in radiotherapy, SBRT has become a validated treatment option, particularly for patients for whom surgery and ablation are not feasible due to prior treatments or tumor location. For these patients liver function, which Digital Poster 2077 is known to be heterogeneously distributed, is of importance. Functional liver avoidance radiotherapy (FLART) guided by HIDA-SPECT imaging, which reveals spatial distribution of liver function, enables treatment planning that accounts for regional liver function. Combining FLART with MRI-guided SBRT delivery allows for online adaptation and motion mitigation, which may further enhance the effectiveness of FLART by enabling optimal sparing of highly functional liver tissue. In this planning study, we demonstrate the feasibility of SPECT-guided radiotherapy on the MR- linac for functional liver-avoidance SBRT. Material/Methods: HIDA-SPECT scans were acquired in five patients prior to MRI-guided SBRT for liver tumors. The HIDA scans were registered to the planning T2-weighted MRI, on which target and organ delineations were performed by the radiation oncologist. Functional liver volumes (FLVs) were defined by thresholding at 50% of the
maximum of the activity distribution. excluding the bile ducts. Based on these delineations, 5 × 12 Gy IMRT plans were generated with and without functional liver avoidance. FLART plans were achieved by selecting beam orientations that avoided passage through the FLV and by applying an IMRT constraint to minimize dose to the FLV while maintaining gross tumor volume (GTV) and planning target volume (PTV) coverage. Mean dose and V18Gy to the FLV were calculated for each plan. Results:
For each patient, both standard and FLART IMRT plans were successfully created with adequate GTV and PTV coverage and compliance with dose constraints. An example of a standard and FLART plan is presented in Figure 1. For all patients, the FLART plans resulted in reduced mean dose and V18Gy to the FLV compared to the standard IMRT plans (Figure 2), with significant mean reductions of 213.6 cGy (SD=139.0, P<0.05) in mean dose and 34.9 cc (SD=20.9, P<0.05) in V18Gy. The magnitude of improvement varied among patients and was influenced by tumor proximity to the FLV.
Conclusion: This study demonstrates the feasibility of SPECT- guided radiotherapy on the MR-linac by incorporating functional liver information into MR-linac radiotherapy planning to reduce dose to highly functioning liver tissue. The benefit of FLART planning is patient-specific and depends on the spatial relationship between the tumor and functional liver regions. Keywords: MR-linac, SPECT-guided, Functional liver volume
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