ESTRO 2020 Abstract book

S166 ESTRO 2020

The combined model with ADC+FMISO+K trans resulted in a peak of PAC-values around 78-99%, which was both more distinct and shifted towards higher probabilities. An average PAC peak value of 98% for the combined model was obtained at 6 patients, while 3 patients had not a distinct peak in the combined model as well as for single parameters. Single parameter models from either FMISO, ADC, K trans had lower probabilities in the range 18-97%; 79- 96%; 17-96%, respectively, and the PAC histogram distributions varied substantially. In the six patients with a distinct peak of PAC for the combined parameter model, all single parameters contributed to that result, especially ADC with a peak in the PAC range of 85-93%. The spatial distribution of PAC with a threshold of 90%, resulted for the combined model in a regular and connected distribution in 3 patients and an irregular pattern in 6 patients. Patients with a regular spatial distribution of PAC all had peaks for every single parameters in the range of PAC ≥76% (Figure).

Material and Methods 65 PTVs of 54 patients treated with gantry-based (GS) SBRT (with breath-hold-gating and ultrasound surveillance) or robotic-based (RS) SBRT (with real-time fiducial tracking) were available for long-term follow up. Post-SBRT MMA were contoured manually on each T1-weighted contrast- enhanced MRI for follow-up schedules of 3 months (n=64), 6 months (n=41), 9 months (n=37), 12 months (n=32), 24 months (n=13) and 24-36 months (n=4). Volumetric data, imaging characteristics of the MMA and liver volume were documented for each FU. Results After 3-9 months, 95% of lesions have shown an MMA with a median volume of 119cc characteristically correlated to the delivered dose as published previously (1). In the course of FU, MMAs dissolved completely for 20 PTVs (30%) between 6 months and 2 years post-SBRT. 41 MMAs were still present at the last follow-up (6 months to 3 years and beyond). MMAs, which did not dissolve completely, were however successively reduced in volume with longer FU. Equating the first FU MMA volume to 100%, it was reduced to a median of 28% after 9 months, to 27% after 12 months, to 14% after 24 months and to 6% after 2-3 years (Table 1, Figure 1). Liver volumes were also measured for all FUs; however, no compensatory liver hypertrophy or relevant liver volume changes were observed.

Figure 1. Patient treated with GS-SBRT 3, 6, 9 and 12 months post-treatment (top row, from left to right). MMA contoured in red, liver in blue. Patient treated with RS- SBRT 3, 6, 12 and 36 months post-treatment (bottom row, from left to right)

Conclusion The inter-patient heterogeneity for different imaging parameters and their relative contribution to the combined PAC varied substantially. By combining imaging parameters the certainty and regularity of PAC increased, whereas for single parameters the PAC regularity was low. PD-0298 Long-term characterization of MRI- morphologic alterations after active motion- compensated liver SBRT G. Miebach 1 , C. Dreher 1 , P. Wojtal 1 , H. Oppitz 1 , A. Jahnke 1 , G. Sarria 1 , F.A. Giordano 1 , O. Blanck 2 , J. Boda- Heggemann 1 1 Universitätsmedizin Mannheim- Medical Faculty Mannheim- Heidelberg University, Department of Radiation Oncology, Mannheim, Germany ; 2 University Medical Center Schleswig-Holstein, Department of Radiation Oncology, Kiel, Germany Purpose or Objective Post-radiotherapy dose-correlated MRI-morphologic alterations (MMA) of the healthy peritumoral liver tissue have been described in the early post-SBRT phase (1); however, data on long-term devolution and clinical impact of these MMA are unknown. We characterized the post- SBRT MRI-morphologic alterations in long-term survivors with MRI-based follow-up (FU) in a pooled patient cohort treated by actively motion managed gantry-based and robotic liver SBRT.

Table 1. Absolute volume (cc) and relative volume to first FU volume = 100% of MMA during long-term follow-up

Conclusion Post-radiogenic MRI-morphologic alterations in the peritumoral healthy liver tissue dissolve completely between 6-24 months post treatment in 30% of the cases. In the other 70% the MMAs resolve at least partially which might be interpreted as a post-radiogenic scar. Reasons for different behavior, correlation to liver function and prognostic value for toxicity and recurrence prediction will be further elucidated. References 1. Boda-Heggemann, J., Jahnke, A., Chan, M.K.H., Ernst, F., Ghaderi, A.L., Attenberger, U., Hunold, P., Schafer, J.P., Wurster, S., Rades, D. , et al. (2019). In-vivo treatment accuracy analysis of active motion-compensated liver SBRT through registration of plan dose to post- therapeutic MRI-morphologic alterations. Radiother Oncol 134 , 158-165.