Abstract Book

S1150

ESTRO 37

1 Kumamoto University Hospital, Radiation Oncology, Kumamoto, Japan 2 Kumamoto University Hospital, Diagnostic Radiology, Kumamoto, Japan 3 Kumamoto University Hospital, Radiological Technology, Kumamoto, Japan 4 Kumamoto University, Medical Imaging, Kumamoto, Japan Purpose or Objective We evaluated the influence of previous treatments on the parametric discrepancies between dose–volume histograms (DVHs) and dose–function histograms (DFHs) generated based on 99 m Tc-labeled diethylene triamine pentaacetate-galactosyl human serum albumin ( 99 m Tc- GSA) single photon emission computed tomography (SPECT) images of hepatocellular carcinoma (HCC) patients treated with stereotactic body radiation therapy (SBRT). Material and Methods This retrospective study based on prospectively acquired data was approved by our institutional review board, and each patient provided written informed consent. Twelve consecutive HCC patients underwent SBRT at 30–40 Gy in five fractions. Surgery had been performed in three (25%) patients before treatment. Nine (75%) patients had undergone radiofrequency ablation, transarterial chemoembolization, and/or percutaneous ethanol injection therapy for HCC in the liver or remnant liver, and no patient received SBRT before treatment. The mean number of previous treatments for lesions other than SBRT-treated lesions was 1.8 (range, 0–4). All patients underwent 99 m Tc-GSA SPECT/ computed tomography (CT) imaging within 1 month before SBRT planning. Attenuation-corrected SPECT and CT images and planning CT images, including delineated structures and dose distributions, were transferred to Velocity AI (version 3.0.2; Varian Medical Systems, Palo Alto, CA, USA). After registration between SPECT and CT images by hardware arrangement, we registered SPECT/CT images onto the planning CT images: a rigid image registration followed by a non-rigid deformable registration. Structures of the irradiated volumes of the liver parenchyma were generated at 5-Gy dose increments, based on the dose distribution information. DFH parameters were calculated as follows: Fx = (sum of the counts within the liver volume receiving a dose of more than x Gy/sum of the counts within the whole liver volume) × 100. The discrepancy between Fx and Vx (Dx = Fx − Vx), where Vx = [(normal liver volume receiving a dose of more than x Gy/whole normal liver volume) × 100], was also calculated. The effect of the number of previous treatments on the absolute value of Dx was evaluated using the Mann–Whitney U test.

(p = 0.106), 0.96 and 0.75 at D 30

(p = 0.343),

0.71 at D 25

and 0.74 and 0.72 at D 35 (p = 1.000) for ≥2 and <2 previous treatments, respectively, of lesions other than SBRT-treated lesions.

Conclusion Previous treatment significantly influences the parame- tric discrepancy between DFH and DVH. EP-2094 Ultra high-field MRI for identifying complete response after neoadjuvant therapy for rectal cancer S. Hoendervangers 1 , C.S. Arteaga de Castro 1 , A.M. Couwenberg 1 , M.M. Lacle 2 , W.M.U. Van Grevenstein 3 , M.P.W. Intven 1 , M.E.P. Philippens 1 , H.M. Verkooijen 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands 2 UMC Utrecht, Pathology, Utrecht, The Netherlands 3 UMC Utrecht, Surgery, Utrecht, The Netherlands Purpose or Objective In patients with complete response following neoadjuvant therapy for rectal cancer, the question arises whether extensive surgery is necessary or whether a wait-and-see approach is justified. However, selection of patients for such an organ sparing approach is challenging, as small residual tumor might be missed on conventional imaging. High-field ( 7 Tesla) MRI, allowing increased resolution and functional imaging, may be valuable in improving identification of complete response after neoadjuvant therapy for rectal cancer. Here we present the first results of protocol optimization of 7 Tesla MRI for rectal cancer. Material and Methods 5 patients with rectal cancer and 5 resected specimens of patients with rectal cancer were scanned on a 7 Tesla MRI (Philips). Possibilities for improving resolution on anatomical MR sequences and the use of functional imaging were explored. We were particularly interested in chemical exchange saturation transfer (CEST), a new endogenous contrast reflecting acidity, which was acquired using 2D gradient echo with 45 offsets (10 ppm Although higher resolutions could be achieved at 7 Tesla, no clear benefit was found when compared to 3 Tesla in terms of tumor contrast. Nevertheless, with 7 Tesla MRI we could investigate endogenous contrasts, which differed between tumorous and non-tumorous tissue. For example, ATP metabolism was measured, showing an increased CEST effect in some areas, possibly caused by active tumor cells. range). Results

Results Dx %

ranged from −3.4% to 6.8%. Dx was positive for all parameters for five (42%) patients and ranged from negative to positive for seven (58%) patients. The mean absolute values of Dx were 2.66 and 1.69 at D 5 (p = 0.755), 3.22 and 1.67 at D 10 (p = 0.048), 2.86 and 1.11 at D 15 (p = 0.018), 2.00 and 0.77 at D 20 (p = 0.048), 1.32 and