ESTRO 37 Abstract book

ESTRO 37

S540

plan, MRI pre ) and at half therapy (fr 9, for the ART plan, ). GTVs were contoured by a single clinician on ). Based on the Poisson-like tumor control probability formula, assuming the volume regression as proportional to the fraction of killed cells and neglecting the inter- patient variability of the kinetic of their removal, the parameter TCP early = -ln[(1 – (V half / V pre )) Vpre ] was suggested as a robust surrogate of early response. The discriminative power of TCP early in predicting the tumor pathological response was quantified by ROC curves (AUC, sensitivity, specificity, positive and negative predictive values (PPV and NPV)); three end-points were considered: pathological complete response (pCR); pCR or clinical complete response without surgery (cCR); limited response (residual vital cells (RVC) in the surgical specimen >10%). Results Complete data were available for 65/74 pts: pCR, cCR and RVC>10% were 20, 2 and 19. The discriminative power of TCP early was moderately high with AUC values equal to 0.79 (95% CI:0.67-0.89), 0.81 (0.69-0.89) and 0.75 (0.62-0.84) for pCR, pCR+cCR and RVC>10% respectively (p<0.0005). TCP early was highly sensitive for all considered end-points (85%, 85%, 90% respectively) with very high NPV (90-94%). In figure 1, the rates of pCR+cCR and of RVC>10% grouped by TCP early quartiles are plotted, showing the ability of TCP early in discriminating the response. In Figure 2, the logistic regression between TCP early and the rate of pCR+cCR (p<0.0001, OR: 0.90 (95%CI: 0.84-0.96), H&L test:0.71) is plotted together with the true rates, grouped by quartiles. MRI half both MRIs and GTV volumes were assessed (V pre , V half

formula has relevant potentials in guiding strategies for ART and treatment individualization.

PO-0977 Assessment of motion and SUV recovery in 3D and 4D PET/CT: a multicentre phantom study K.J. Ortega Marin 1 , M. Lambrecht 1 , A.J. Arends 2 , S. Adebahr 3 , U. Nestle 4 , W. Vogel 5 , M. Verheij 6 , J.J. Sonke 6 , C.W. Hurkmans 1 1 Catharina Hospital, Radiation Oncology, Eindhoven, The Netherlands 2 Catharina Hospital, Medical Physics, Eindhoven, The Netherlands 3 University Medical Center Freiburg, Radiation Oncology, Freiburg, Germany 4 University Medical Center Freiburg / Kliniken Maria Hilf, Radiation Oncology, Freiburg / Mönchengladbach, Germany 5 The Netherlands Cancer Institute, Nuclear Medicine, Amsterdam, The Netherlands 6 The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective In PET/CT studies, tumour motion results in the apparent decrease of SUV and apparent increase of its volume in t he PET image with respect to static conditions. The goal of this multicentre dynamic phantom study is to quantify the variation in SUV and motion recovery in 3D and 4D PE T/CT images. Material and Methods A customized CIRS- 008A phantom was scanned at 13 institutions participatin g in the EORTC LungTech trial on SBRT for centrally locat ed lung tumours. The phantom body contains a moving cy linder of 63.5 mm diameter with a sphere of either 15 m m or 25 mm diameter placed inside it. The body, cylinder and sphere were filled with homogeneous 18F- FDG solutions representative of activity concentrations in mediastinum, lung and tumour, respectively. 3D and 4D PET/CT scans were acquired for static and dynamic tumo urs. Motion was simulated with cos 6 respiratory patterns o f various amplitudes and periods: 15mm/6s, 15mm/3s an d 25mm/4s. Institutional scan protocols were used. SUVm ax and SUVpeak (average SUV of the volume encompassed by a 70% SUVmax threshold) for all scans acquired were a ssessed centrally using MATLAB. To recover the range (am plitude) of motion (ROM), the centre of mass of the tumo ur was estimated from a volume segmented for SUVs larg er than 40% of the SUVmax in each bin (respiratory phase) . SUV metrics were normalized to their corresponding sta tic values. Wilcoxon rank tests were used for assessment of SUV variations. Results Variations in SUVmax due to tumour motion with 3D PET/ CT were the largest for the 25mm/4s pattern (median=- 36.4%, range=[- 44.7%; 268.4%]) and significantly different from the 15m m/6s pattern (-27.6% [-36.3%; - 9.3%], p=0.037) and from the 15mm/3s pattern (-23.6% [- 35.3%; - 10.7%], p=0.009) (Figure 1a). These variations decreased significantly with 4D PET/CT (p<0.001): -10.4% [- 35.2%; 241.1%] for the 25mm/4s pattern, -5.1% [- 42.2%; 81.3%] for the 15mm/6s pattern and -5.1% [- 32.3%; 41.4%] for the 15mm/3s pattern (Figure 1b). There were no significant differences in the variations between patterns with 4D PET/CT (p>0.15). Nevertheless, a larger spread in variations was observed in 4D PET/CT for the 1 5mm/6s and 15mm/3s patterns. Moreover, an overestima

Conclusion TCP early

, individually assessing the early response from tumor regression was able to accurately predict the tumor pathological response in a relatively large group of pts treated within an ART protocol with RCT for Rca. The