Abstract Book

ESTRO 37

S597

resonance (MRI) imaging is the gold standard for rectal cancer staging and the diffusion-weighted sequence (DWI) allows for the best definition of the tumor. We compared the rectal gross tumor volume (GTV) delineated on computed-tomography (CT), T2w MRI and DWI MRI images to evaluate inter- and intra-observer agreement. Material and Methods LARC patients (pts) underwent CT without contrast medium for radiotherapy treatment planning and subsequently T2w and DWI (b = 1500 s / mmq) MRI axial sequences. CT and MRI were acquired in the prone position and pts had the same bladder preparation for both procedures. Rigid CT-MRI co-registration of image series was obtained. Four independent observers (ob) with different skill levels and experience delineated the GTV on CT, T2w and DWI MRI images. Conformity index (CI) was calculated between each ob-pair per patient per technique and between each technique-pair per patient per ob. Reliability between techniques and between ob was assessed using the intraclass correlation coefficient (ICC). Comparisons within and between groups were analyzed by mixed repeated-measures ANOVA, with Geisser-Greenhouse adjustments for non-sphericity with Bonferroni’s post hoc correction. Results Twenty consecutive pts were included. CT, T2w MRI and DWI MRI mean GTV in cm 3 were 43.01, 34.98 and 36.10 respectively for ob one; 42.88, 34.39 and 41.89 for ob two; 47.20, 36.39 and 36.70 for ob three; 40.41, 33.67 and 34.15 for ob four. ICC among ob for CT, T2W MRI and DWI MRI volumes were 0.76, 0.81 and 0.94 respectively and this reliability were confirmed by the comparison between ob-pairs. Volumes are significantly related to imaging technique (p=0.0003) with CT volumes larger than T2w and DWI and they do not depend on the observer. Median CIs among all ob are 0.45, 0.54 and 0.74 for CT, T2w and DWI, respectively. The average of the median values for pairs of ob are 0.62, 0.53 and 0.73 respectively for the three modalities. There is a statistically significant difference between the CIs (p=0.007) and also the relationship between CIs and the modalities tends to significance (p=0.062). Mean difference of urinary bladder volume between CT and MRI was 25 cm 3 but this did not affect CIs. Conclusion MRI, mostly DWI series, showed high agreement among different ob and therefore it could be useful for the definition of GTV in LARC. Studies with a larger number of pts are needed to confirm whether such agreement is significant. The best imaging technique for the definition of the true tumor volume is still to be determined. PO-1066 Delineation uncertainty and parotid gland doses and estimated NTCPs in head and neck proton therapy G. Engeseth 1 , M. Brydøy 1 , J. Dale 1 , C. Boer 1 , H. Gripsgård 1 , J. Moi 1 , L. Muren 2 , C. Stokkevåg 1 1 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway 2 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark Purpose or Objective Proton therapy may be beneficial for head and neck cancer patients, mainly due to the potential reduction in doses to organs at risk (OAR). Late side effects like xerostomia have been associated with parotid gland (mean) dose. Treatment plans are typically optimised to spare the parotid gland and therefore depends on

Results The CIs for T1, T2, T2W-SPAIR, DWI, DCE, sdyn-eTHRIVE and CT were 0.646, 0.648, 0.680, 0.676, 0.718, 0.716, respectively, implying that the DCE and sdyn-eTHRIVE could specifically show the preoperative details of the mass, closely followed by DWI. The DCs between the volumes defined by CT and the two MRI sequences (DCE and sdyn-eTHRIVE) were 0.67± 0.20, 0.66 ± 0.22 for LCs and 0.87 ± 0.06, 0.86 ± 0.07 for PTVs, respectively. The GMI and NTI based on T1 and T2 were more obvious than the other MRI sequences with CT. Conclusion The pre-MRI does improve the target volume delineation, especially for that large, pendulous, breast size. The target volumes of LC and PTV generated according to a MRI sequence are smaller than those according to CT mostly. The DCE and sdyn-eTHRIVE both might be the better sequence for precise definition of LC. The PTV-MRI volumes, defined by the DCE and sdyn-eTHRIVE, were comparable with those of PTV-CT for the majority of cases studied. PO-1065 Role of CT-MRI co-registration in tumor delineation for preoperative radiotherapy of rectal cancer E. Palazzari 1 , M. Lupattelli 2 , C. Galuppo 3 , M. Napoletano 4 , A. Podlesko 5 , V. Bini 6 , M. Iacco 7 , C. Fulcheri 7 , G. Verzini 2 , A. De Paoli 1 , C. Aristei 8 1 Centro di Riferimento Oncologico IRCCS Aviano, Radiation Oncology Department, Aviano, Italy 2 Santa Maria della Misericordia Hospital, Radiation Oncology Department, Perugia, Italy 3 Santa Maria della Misericordia Hospital, Radiology Department, Perugia, Italy 4 Perugia University, Radiology Department, Perugia, Italy 5 Perugia University, Radiation Oncology Department, Perugia, Italy 6 Perugia University, Internal Medicine- Endocrine and Metabolic Science Section, Perugia, Italy 7 Santa Maria della Misericordia Hospital, Medical Physics Department, Perugia, Italy 8 Santa Maria della Misericordia Hospital and Perugia University, Radiation Oncology Department, Perugia, Italy Purpose or Objective Intensification of preoperative radio-chemotherapy in locally advanced rectal cancer (LARC) is gaining progressive interest with a view to increasing response rates and to developing new organ preservation approaches. Dose escalation programs are providing promising results but they require optimization of boost volume definition. T2 weighted (T2w) magnetic