1197OSELECTIVE MEDIASTINAL NODE IRRADIATION IN NSCLC IN THE IMRT/VMAT ERA: ADDED VALUE OF EBUS-TBNA-MAPPING TO PET-CT.

ABSTRACT Aim: FDG-PET-CT based selective lymph node (LN) irradiation is the standard when using 3D-conformal techniques (3D-CRT) for locally advanced NSCLC. With 3D-CRT, adjacent LN not included in the target volume still receive a substantial radiation dose. However, with current new techniques (IMRT/VMAT), radiation dose to non-involved LNs decreases, which raises the question whether selective nodal irradiation based on FDG-PET-CT is still safe. We therefore evaluated the impact of adding EBUS-TBNA to FDG-PET-CT in selective nodal irradiation. Methods: Literature data that included sensitivity and specificity of EBUS-TBNA in FDG-PET-CT staged NSCLC were identified. As false negative (FN) LNs have the largest consequence to avoid geographical miss, FN rates for different constellations of CT, PET and EBUS-TBNA were calculated. This algorithm was tested on 325 mediastinal LN in 25 consecutive NSCLC-patients with N2/N3 disease based on PET-CT who underwent full mediastinal EBUS-TBNA mapping. Results are expressed as mean +/- SD and range. Results: A practical algorithm when to include LNs in the GTV was made based on data from 5 large meta-analyses. The expected cancer prevalence, taking into account the FN rate of EBUS of 20%, were calculated. The resulting algorithm shows that for PET-positive and EBUS-negative LNs, FN rates are 16% for enlarged LNs, and 14% for normal-sized LNs. Furthermore, adding EBUS to FDG-PET-CT decreases the FN-rate of FDG-PET: from 13% to 3% for enlarged LNs, and from 6% to 1% for normal-sized LNs. In our population, 325 mediastinal LNs were visually analyzed on PET-CT scan. Sixty-seven were enlarged (≥10mm), of which 63 were PET-positive. Twelve normal-sized LNs were PET-positive. All accessible LNs were reviewed with EBUS. Fifty suspicious LNs were biopsied with EBUS-TBNA (mean: 2/patient +/-0.96; 1-5): 28 were malignant, 22 normal. EBUS-TBNA detected 1 cancer-containing normal-sized LN without FDG-uptake, thus 1/25 geographical miss (4%). Conclusions: When the incidental nodal irradiation is low, such as in IMRT or VMAT, EBUS-TBNA should be added to FDG-PET-CT for mediastinal staging. PET-positive and EBUS-negative LNs should be included in the GTV as FN-rates remain high. Adding EBUS to FDG-PET-CT decreases the risk of geographical miss. Disclosure: All authors have declared no conflicts of interest.