ESTRO 37 Abstract book

ESTRO 37

S539

characteristics showed significant associations with PET- and CT- slopes for FEV1 (p=0.017, p=0.056), COPD status (p=0.007, p=0.083), and age (p=0.073, p=0.074). Further associations for PET-slope were tumor stage (p <0.001), N stage (p=0.004), and chemotherapy regimen(p=0.041).

PO-0975 Relationship of dose, FDG PET, CT lung response imaging, and radiation pneumonitis in NSCLC patients M. La Fontaine 1 , G. Defraene 2 , J. Van Diessen 1 , S. Van Kranen 1 , B. Reymen 3 , D. De Ruysscher 3 , J. Belderbos 1 , J.J. Sonke 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands 3 MAASTRO, Radiation Oncology, Maastricht, The Netherlands Purpose or Objective Radiation pneumonitis (RP) is an important dose limiting toxicity in NSCLC patients undergoing (chemo- )radiotherapy. Imaging modalities such as PET and CT have shown high inter-patient variability in lung tissue response to dose, yet its association is still unclear with clinically scored RP. This study evaluated imaging metrics in predicting RP, and their effectiveness in comparison to dose metrics. In addition, pre-treatment patient characteristics were investigated to find associations to inter-patient dose sensitivity derived from imaging. Material and Methods 65 patients with inoperable, stage II-III NSCLC were treated with (chemo)radiotherapy as part of the (NCT01024829) PET-boost trial. All patients received an FDG PET/CT scan at 3 months post-treatment, which was deformably registered to the planning CT. For lungs minus GTV, a linear region was found between planned dose and 1) post-treatment FDG PET scan, and 2) pre- to post-treatment CT HU change (delta CT). Slopes of the linear regions for post-treatment FDG PET (PET-slope) and delta CT (CT-slope) were investigated in individual patients for RP prediction (Grade ≥2 according to CTCAE). Due to a low number of RP events in this cohort (6 patients), the F1 score, a harmonic mean of sensitivity and precision, was used. The F1 score (ranging from 0 to 1) was compared between PET- and CT- slopes, as well as for dose (EQD2: α/β =3 Gy) metrics of lung V20, lung V5, mean lung dose (MLD), and mean heart dose (MHD). Univariate analysis was applied (alpha = 0.10) to search for pre-treatment patient characteristics associated with patient sensitivity for dose in CT-slope and PET-slope. Results Post-treatment FDG PET SUV displayed higher linearity (median r: 0.87 [IQR:0.64-0.94], p=0.002) to dose than delta CT (0.67 [IQR:0.28-0.84]) for individual patients. In addition, PET-slope had a higher F1 score than CT-slope with values of 0.59 and 0.40, respectively. In comparison, dose metrics displayed F1 scores of 0.50 (lung V20), 0.42 (lung V5), 0.36 (MLD), and 0.12 (MHD) (Figure 1a). The best identifier for RP was a combination of MLD and PET- slope with a F1 score of 0.83 (sensitivity: 0.83, precision: of 0.83) (Figure 1 b-c). Univariate analysis of patient

Conclusion While both CT and PET metrics demonstrated high linearity with dose, PET-slope was not only a better indicator of RP in comparison to CT-slope, but also in relation to dose metrics. However, the PET-slope metric was further improved upon by adding the MLD to predict RP (F1 score=0.83). In addition, univariate analysis indicated several patient characteristics, which may be useful for future multivariate studies in identifying patients with higher dose sensitivity in both PET and CT. PO-0976 Rectal cancer radiochemotherapy: pathological response predicted by modeling early tumor regression C. Fiorino 1 , C. Gumina 2 , P. Passoni 2 , A. Palmisano 3 , S. Broggi 1 , G.M. Cattaneo 1 , A. Di Chiara 3 , M. Mori 1 , R. Raso 1 , N. Slim 2 , F. De Cobelli 3 , R. Calandrino 1 , N. Di Muzio 2 1 San Raffaele Scientific Institute, Medical Physics, Milano, Italy 2 San Raffaele Scientific Institute, Radiotherapy, Milano, Italy 3 San Raffaele Scientific Institute, Radiology, Milano, Italy Purpose or Objective Predictive models based on the individual response during neo-adjuvant radio-chemotherapy (RCT) of rectal cancer (Rca) have huge potential in treatment individualization; up to now, models based on the response during RCT were not reported. The aim of this study was to individually assess a radiobiological parameter (TCP early ) based on early tumor regression; then, the ability of TCP early in predicting the tumor pathological response was investigated. Material and Methods Seventy-four consecutive patients (pts) treated according to our protocol (41.4 Gy in 18 fractions, 2.3 Gy/fr) delivering an adaptive (ART) concomitant boost on the residual tumor (GTV) in the last 6 fractions (3 Gy/fr, mean dose to GTV: 45.6 Gy) were considered. Chemotherapy consisted of oxaliplatin 100 mg/m 2 on days -14, 0 (start of RT), and +14, and 5-fluorouracil 200 mg/m 2 /d from day -14 to the end of RT. High resolution T2-weighted MRI were taken before RT (for the initial