ESTRO 2021 Abstract Book

S1662

ESTRO 2021

Stockholm, Sweden). For GTV-BTVs agreements 95% of the Hausdorff distance (HD95%) from GTV to the BTVs were calculated, additionally BTV portions outside of the GTV and coverage by the 95% isodose contours were also determined. In case of recurrence, the latest MR images were co-registered to planning CT to evaluate its location relative to BTVs and 95% isodose contours. Results Average (range) GTV, BTV1.7 and BTV2.0 were 46.58 (6-182.5), 68.68 (9.6-204.1), 42.89 (3.8-147.6) cm 3 respectively. HD 95% from GTV were 15.5 mm (7.9-30.7 mm) and 10.5 mm (4.3-21.4 mm) for BTV1.7 and BTV2.0 respectively. Based on volumetric assessment 58.8% (28-100%) of BTV1.7 and 45.7% of BTV2.0 (14-100%) were outside of the standard GTV. (Figure 1), still all BTVs were encompassed by the 95% dose. All recurrences were confirmed by follow-up imaging, all occurred within PTV, with an additional outfield recurrence in a single case, which was not DOPA positive at the beginning of treatment . The average time span between the two PET acquisitions was 56 min (35-83 min). Good correlation was found between the mean and median values of PET/CT and PET/MRI segmented volumes relative to corresponding brainaccumulated enhancement (r=0.75; r=0.72).

Conclusion 18F-FDOPA PET resulted in substantial larger tumor volumes compared to MRI, however its added value is unclear as vast majority of recurrences occurred within the prescribed dose level. Use of PET/CT signals proved to be feasible in the absence of direct segmentation possibilities of PET/MR in TPS. The added value of 18F-FDOPA may be better exploited in the context of integrated dose escalation.

PO-1951 Comparison between the use of PET and MRI 3.0 T in meningiomas L. Anemoni 1 , M.S. Cadeo 1 , A. Mancin 1 , I.E. Mascayano 1 , L. Preda 2 , S. Tampellini 1 , E. Orlandi 1 1 Fondazione CNAO, Radiotherapy, Pavia, Italy; 2 IRCCS Policlinico San Matteo, Radiology, Pavia, Italy

Purpose or Objective Positron emission tomography (PET-CT) PET-CT 68-gallium (PET-CT 68Ga-DOTATOC), in combination with traditional imaging are used in hadrontherapy both in the planning phase and follow-up. The primary endpoint of this retrospective study is the assessment of the correlation between the treatment volumes defined with PET-CT 68Ga-DOTATOC and the potential clinical impact of any discrepancies. The secondary objective is to assess the sensitivity, specificity and positive and negative predictive value of the multi-methodological approach of high-field Magnetic Resonance Imaging (MRI) in the assessment of early response to particle therapy, also estimating the degree of concordance with PET-CT method and with only morphological MRI. For this study both PET-CT and MRI images of 34 patients suffering meningioma treated at CNAO with adrotherapy were analysed. Based on histology, patients underwent PET-CT 68Ga-DOTATOC a Gross Tumor Volume (GTV) of reference PET (GTVrifPET) and another one evaluated on MRI images (GTV 3T ) by using a multi-methodological approach of high-field MRI (3.0 T) have been defined. The variables of the study were expressed as a volume (cm3). The clinical evaluation confirm that the effective response to the treatment will be performed by routine follow-up examinations after the end of radiotherapy treatment Materials and Methods

A GTVrifPET and a GTV were defined, evaluated on MRI

(GTV 3T ) images using a multi-methodological approach of high-field MRI (3.0 T).