ESTRO 36 Abstract Book

S918 ESTRO 36 2017 _______________________________________________________________________________________________

generating would enhance/visualize the signal changes; (ii) correlation of MRiBC with the dose distribution, for the purpose of identifying a cumulative dose threshold. Material and Methods Data of three patients treated at PSI where target was involving or immediately next the spinal cord resulting in an evident bone change (fatty replacement) after the treatment was selected. All patients received a pre- and a post-treatment MRI and a subset of the acquired sequences (T2 space transversal,T1 vibe dixon transversal with contrast media) were included into this analysis. Rigid registration on the bony structure has been performed in the region of interest of the bone change between the planning CT and the MRIs (T1 and T2 sequences), both pre and post treatment. To generate a differential map enhancing only the fatty replacement, it is important that same tissues have the same signal across the different time points. Therefore, we evaluated the stability of MRI signal pre- and post- treatment on vertebral bodies outside the treatment area. Finally, the post treatment MR Intensity was correlated with dose distribution. Results Variations in signal intensity of the same ROI and in different acquisition days show a mean value of the mean signal intensity difference of 50 and a max-min difference of 130 (grey intensity scale). Therefore, generating a reliable subtraction map is extremely difficult with conventional sequences. For the considered 3 cases, the correlation between the MRiBC and dose distribution looks good for all 3 patients (Fig. 1). In this case, the bone change lies between the 20%-30% isodose line. differential maps which

Results Once the dUVH was created in terms of uptake values (Fig.1.bottom left) the histogram was normalized to the uptake value of the very first peak assuming to correspond to the normal rectal tissue glucose uptake. Subsequently, the maximum S/B ratios were sampled for all patients (Fig.1.bottom right). Table 1 represents results of two- sample paired t-test indicating that patients with complete response to radiation (pT0) have higher and significantly different S/Bmax when compared to non- responders. While there are some contradictory data in the literature in regards to SUVmax and clinical outcomes, our results are in agreement with notion that more aggressive tumors (that spend longer periods of time in M phase, known to be more radiosensitive) have better response to radiation.

Conclusion While the dUVH method was initially developed to extract different biological sub-volumes (glucose phenotypes) within tumors, results presented here suggest that the same method can help in defining the background uptake on PET images. The method described here shows an alternative to sampling the background (normal) uptake within contralateral (healthy) tissue in the case of paired organs (lung, brain, etc). Furthermore, reconstructed S/Bmax values may prove to represent a prognostic factor of tumor response to radiation and hence allow for tailoring of more patient specific treatment strategies. EP-1702 Evaluation of radiation induced MRI intensity change in vertebral bodies after proton beam scanning L. Placidi 1 , R. Poel 1 , A.J. Lomax 1 , D.C. Weber 1 , M. Peroni 1 1 Paul Scherrer Institute PSI, Centre for Proton Therapy, Villigen PSI, Switzerland Purpose or Objective This retrospective study aims at evaluating the magnetic resonance intensity bone change (MRiBC) [Gensheimer et al. 2009] induced by pencil beam scanning (PBS) proton treatment. Two fundamental aspects are tackled: (i) the MRI signal reproducibility, especially if the imaging is performed at different time points, with the goal of

Conclusion MRI signals are not quantitative enough to use and it is difficult to manipulate MRI settings to make it more quantitative. Image processing, as normalisation, smoothing, etc., can lead to a substantial variation of the original data set and shows relevant signal histogram differences. A possible common solution could be a quantitative MRI sequences (as a T2 relaxation time map), that is already under investigation and could reduce signal variation of an order of magnitude. As the dataset at our disposal is consisting of pre- and post-treatment MRIs only, it is difficult to say whether bone marrow change is more like a threshold or if there is in fact a useable gradient. In order to monitor the onset of the fatty replacement, track it during the entire process and better correlate it to the delivered dose, periodic MRI acquisition during treatment is necessary. Nevertheless, for these patients, it is clear that there is a correlation between MRiBC and dose.

Electronic Poster: Physics track: Images and analyses