ESTRO 38 Abstract book

S413 ESTRO 38

chemoembolization. A recent study revealed that a dose– function histogram (DFH) using 99m Tc-GSA SPECT provides dosimetric information of the liver function of HCC patients who underwent SBRT. Introduction of functional information of 99m Tc-GSA SPECT for use in inverse planning for IMRT might be a reasonable approach to spare liver function. We evaluated the impact of 99m Tc-GSA SPECT image-guided inverse planning on the DFH parameters of SBRT planning in HCC patients. Material and Methods Eleven patients with HCC were enrolled in this study. We used a SPECT/CT system (Symbia T16; Siemens Healthcare, Erlangen, Germany) for the SPECT/CT imaging. Attenuation-corrected SPECT and CT images and planning CT images were transferred to a Velocity AI (version 3.0.2; Varian Medical Systems, Palo Alto, CA, USA). We registered the SPECT/CT images onto the planning CT images: a rigid image registration followed by a non-rigid deformable registration. The functional liver structure (FLS) as an avoidance structure for optimization was derived from SPECT thresholds of 60%–80% of the maximum pixel value. Two treatment plans optimized without FLS (plan C) and with FLS (plan F) were designed for 50 Gy in 5 fractions to the planning target volume (PTV) by using a 2-arc RapidArc (Clinac iX; Varian Medical Systems, Palo Alto, CA, USA). DFH parameters were calculated as follows: Fx = (sum of the counts within the liver volume receiving a dose >x Gy/sum of the counts within the whole liver volume) × 100. Other parameters for the PTV included the absorbed dose received by 95% of the PTV (D 95 ), mean dose, conformity index (CI), and homogeneity index (HI). Results In comparison with plan C, plan F significantly reduced the DFH parameters of F 5 to F 40 (p < 0.05), and plan F did not significantly increase F 45 and F 50 . There were no significant differences in the DVH parameters of D 95 , mean dose, CI, and HI for the PTV between plans C and F. There were no significant differences in the parameters of the OARs of the stomach, duodenum, spinal cord, and kidneys between plans C and F. There was no significant difference in the MUs between plans C and F.

Conclusion DFH analyses revealed that 99m Tc-GSA SPECT image-guided inverse planning provided dosimetric benefits related to sparing of liver function while maintaining coverage of the PTV and may reduce hepatic toxicities. PO-0798 Response assessment to neoadjuvant chemoradiotherapy for esophageal cancer using PET/CT and DW-MRI A. Borggreve 1 , L. Goense 1 , P.S.N. Van Rossum 2 , S.E. Heethuis 2 , R. Van Hillegersberg 3 , J.J.W. Lagendijk 2 , A.L.H.M.W. Van Lier 2 , S. Mook 2 , J.P. Ruurda 3 , M. Van Vulpen 4 , F.E. Voncken 5 , B.M.P. Aleman 5 , A. Bartels- Rutten 6 , J. Ma 7 , P. Fang 8 , B.C. Musall 7 , S.H. Lin 8 , G.J. Meijer 2 1 UMC Utrecht, Radiation Oncology and Surgical Oncology, Utrecht, The Netherlands ; 2 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands ; 3 UMC Utrecht, Surgical Oncology, Utrecht, The Netherlands ; 4 Holland PTC, Holland Particle Center, Delft, The Netherlands ; 5 The Netherlands Cancer Institute - Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands ; 6 The Netherlands Cancer Institute - Antoni van Leeuwenhoek Hospital, Radiology, Amsterdam, The Netherlands ; 7 The University of Texas MD Anderson Cancer Center, Imaging Physics, Houston, USA ; 8 The University of Texas MD Anderson Cancer Center, Radiation Oncology, Houston, USA Purpose or Objective Around one third of the patients have a pathologic complete response (pCR) after neoadjuvant chemoradiotherapy (nCRT) for esophageal cancer. Accurate preoperative identification of this group could omit esophagectomy in these patients. The purpose of this prospective multicenter study was to evaluate the distinct and combined value of 18 F-FDG PET/CT and DW-MRI during and after nCRT to predict pathologic response in esophageal cancer patients. Material and Methods In this prospective multicenter study, patients scheduled to receive nCRT followed by esophagectomy for resectable esophageal cancer underwent 18 F-FDG PET/CT and DW-MRI scanning prior to start of nCRT, during nCRT and 0-2 weeks before esophagectomy. Response to nCRT was measured using the tumor regression grading system based on histopathological evaluation of the resection specimen (TRG1-4). Relative changes in 18 F-FDG PET/CT (ΔSUV and ΔTLG) and DW-MRI (ΔADC) parameters were compared between patients with a pCR (TRG1) and non-pCR (TRG2- 4) groups (Figure 1). Multivariable logistic regression analysis with bootstrapped 95% confidence intervals and