Abstract Book

S810

ESTRO 37

s/mm 2 ). Two observers, one radiologist and one radiation oncologist, both with experience in rectal cancer diagnosis and treatment, delineated GTV on T2 (T2 GTV ) and DWI (DWI GTV ) sequences, blinded and independently from each other. GTV volume (cm 3 ) was measured for T2 and b1000 DWI images for each observer, and compared using Mann-Whitney test. Observer agreement was assessed using DICE index, Bland-Altman analysis (mean difference, 95% limits of agreement) and intraclass correlation coefficient (ICC). Results Median T2 GTV and DWI GTV volumes were 13.24 cc (7.32- 17.54 cc) and 8.14 cc (6.06-14.07 cc) for radiologist, and 12.70 cc (7.22-19.75 cc ) and 8.77 cc (4.41-16.27 cc) for radiation oncologist, respectively. T2 showed slightly largest volumes, near to a significant difference, compared to DWI (p<0.057 and p<0.131 for both observes). Mean DICE index for T2 GTV and DWI GTV were 0.63 and 0.59, respectively. The mean difference between the two readers was -0.61 cm 3 (95%CI: -7.44 to 6.22) and -1.65 cm 3 (95%CI:-9.93 to 6.64) for T2 and DWI volumes (Figure 1). The ICC for T2 MRI volumes was 0.970 (95%CI: 0.941-0.984) (p<0.001) and 0.960 (95%CI: 0.922- 0.979) (p<0.001) for DWI volumes, respectively.

Conclusion In our study, neoadjuvant chemoradiotherapy was associated with good results in terms of sphincter function, late toxicities and QoL indexes. A routine use of assessment scales could contribute to a better selection of patients with increased risk of developing functional disorders, who could benefit from neoadjuvant therapy. EP-1493 T2 and Diffusion weighted MRI for GTV delineation in rectal cancer: agreement between volumes. C. ROSA 1 , L. Caravatta 1 , M. Di Tommaso 1 , R. Cianci 2 , A. Delli Pizzi 2 , A. Vinciguerra 1 , I.A. Zecca 3 , M. Di Nicola 3 , D. Genovesi 1 1 SS. Annunziata Hospital- “G. D’Annunzio” University, Department of Radiation Oncology, Chieti, Italy 2 “G. D’Annunzio” University, Department of Neuroscience- Imaging and Clinical Sciencies, Chieti, Italy 3 “G. D’Annunzio” University, Laboratory of Biostatistics- Department of Medical- Oral and Biotechnological Sciences, Chieti, Italy Purpose or Objective To achieve high pathological response rates with low toxicity in rectal cancer radiotherapy, an accurate delineation of boost volume is required. T2 weighted MRI imaging is currently considered the gold standard for rectal cancer staging and is often used in addition to CT scan for rectal cancer radiotherapy planning. Functional Diffusion-weighted MRI (DWI) images are able to improve lesion detection and could represent a valid instrument for an accurate delineation of Gross Tumor Volume (GTV) in rectal cancer, as reported in several studies. Our study aimed to evaluate the difference in GTV delineation based on T2 weighted and DWI MRI images, by evaluating volumes and inter-observer agreement between radiologist and radiation oncologist, in locally advanced rectal cancer. Material and Methods Thirthy-six patients affected by non-mucinous rectal cancer underwent 3T MRI images. Imaging study including axial T2 weighted MRI and echo planar DWI images, using six different b-values (0, 400, 500, 600, 800, 1000

Conclusion DWI resulted in smaller volumes compared to T2 weighted MRI in GTV delineation, with a good agreement between radiologist and radiation oncologist. Then, DWI images could be used to improve quality in radiation planning. In addition, given the investigational ability of DWI, performed prior and during treatment, in detection of responder patients, a DWI guided planning could represent a valid instrument for accurate boost volumes delineation when a dose-intensification could be adopted, as in no-responders. A study to evaluate GTV delineation on DWI images during radiation treatment is planned in our institution.

EP-1494 Patterns of failure in rectal cancer with positive circumferential resection margin Y. Kim 1 , D.Y. Kim 1 , T.H. Kim 1 , S.Y. Kim 2 , J.Y. Baek 3 , M.J. Kim 3 , H.J. Chang 3 , M.J. Kim 3 , S.C. Park 3 , J.H. Oh 3