I.M. Jürgenliemk-Schulz

First patients treated with a 1.5 T MRI-Linac : Clinical proof of concept of a high-precision, high-field MRI guided radiotherapy treatment

The integration of 1.5 T MRI functionality with a radiotherapy linear accelerator (linac) has been pursued since 1999 by the UMC Utrecht in close collaboration with Elekta and Philips. The idea behind this integrated device is to offer unrivalled, online and real-time, soft-tissue visualization of the tumour and the surroundings for more precise radiation delivery. The proof of concept of this device was given in 2009 by demonstrating simultaneous irradiation and MR imaging on phantoms, since then the device has been further developed and commercialized by Elekta. The aim of this work is to demonstrate the clinical feasibility of online, high-precision, high-field MRI guidance of radiotherapy using the first clinical prototype MRI-Linac. Four patients with lumbar spine bone metastases were treated with a 3 or 5 beam step-and-shoot IMRT plan. The IMRT plan was created while the patient was on the treatment table and based on the online 1.5 T MR images; pre-treatment CT was deformably registered to the online MRI to obtain Hounsfield values. Bone metastases were chosen as the first site as these tumors can be clearly visualized on MRI and the surrounding spine bone can be detected on the integrated portal imager. This way the portal images served as an independent verification of the MRI based guidance to quantify the geometric precision of radiation delivery. Dosimetric accuracy was assessed post-treatment from phantom measurements with an ionization chamber and film. Absolute doses were found to be highly accurate, with deviations ranging from 0.0% to 1.7% in the isocenter. The geometrical, MRI based targeting as confirmed using portal images was better than 0.5 mm, ranging from 0.2 mm to 0.4 mm. In conclusion, high precision, high-field, 1.5 T MRI guided radiotherapy is clinically feasible.

A volumetric analysis of GTVD and CTVHR as defined by the GEC ESTRO recommendations in FIGO stage IIB and IIIB cervical cancer patients treated with IGABT in a prospective multicentric trial (EMBRACE)

PURPOSE To quantify the gross tumor volume at diagnosis (GTVD) and high-risk clinical target volume (CTVHR) at brachytherapy (BT) and describe subgroups of patients with different patterns of response to chemoradiotherapy (CRT) in patients with FIGO stage IIB and IIIB cervical cancer treated with image-guided adaptive brachytherapy (IGABT). Additionally, to evaluate the feasibility of IGABT achieving adequate target coverage in these groups. MATERIALS AND METHODS Patients with FIGO stage IIB and IIIB cervical cancer enrolled in the EMBRACE study were analyzed. T2-weighted MRI scans were obtained at diagnosis and at BT. GTVD and CTVHR were defined as per the GEC ESTRO recommendations. Patients were classified taking into account that initial tumor volume and response to CRT represented by the volume of residual disease (CTVHR) and extent of residual parametrial disease are all major factors determining local dose delivery by BT, local control, and overall disease outcome. These factors were quantified applying the following criteria: (1) volume of the GTVD relative to the median volume of the GTVD; (2) the ratio (R) of CTVHR to GTVD for each patient; (3) the extent of residual parametrial disease at the time of BT. Accordingly, patients were classified into six groups (G1-G6): stage IB1-like tumors (G1), tumors with good response and any size (G2), small tumors with moderate response (G3), large tumors with moderate response (G4), tumors with poor response (G5) and those with progressive disease (G6). Tumor and treatment characteristics were then compared among the first five groups (only 3 patients were allocated to G6). RESULTS A total of 481 patients were evaluated. The number of patients in the 6 groups were 55, 78, 123, 147, 75 and 3, respectively. The mean (SD) GTVD was 43.6 (32.8)cm3 and the mean (SD) CTVHR was 31.6 (16.1)cm3. The mean GTVD and CTVHR were 12.6cm3 and 23.7cm3 in G1 (R>1.1), 47.5cm3 and 25.3cm3 in G2 (R<0.9), 23.9 cm3 and 29.9cm3 in G3 (R 0.9-1.1), 73.4cm3 and 38.5cm3 in G4 (R 0.9-1.1), 79.4cm3 and 59.5cm3 in G5 (R>1.1), respectively. Parametrial disease extent at BT was as follows: no involvement in G1 and G2, proximal at most in G3 and G4, distal or to the pelvic wall in G5, progressive in G6. The use of interstitial needles was progressively higher among the groups (mean 0, 0, 2, 3, 6 in G1-5, P<0.001). The mean GTVBT D100 in G1-5 was 103.1Gy, 91.8Gy, 93.5Gy, 88.3Gy and 87.1Gy. The mean CTVHR D90 in G1-5 was 95.1Gy, 92.1Gy, 92.6Gy, 87.6Gy and 88.4Gy. CONCLUSIONS In patients with FIGO stage IIB and IIIB disease, intra-FIGO stage heterogeneity and overlap between the two stages exist with respect to tumor volume, treatment response and extent of parametrial disease at BT. Taking into account GTVD, parametrial disease at BT and the ratio of CTVHR/GTVD, five major groups exist. These enable prediction of GTVBT and CTVHR dose coverage through BT. IGABT, as performed in EMBRACE, accommodates to a considerable degree for the different variants of tumor regression in these groups through adaptation of the treatment technique including the use of needles. However, major variations remain at present with regard to dose to GTVBT and to CTVHR, which are most pronounced in G4 and G5. This new classification will be validated in future in regard to clinical outcome in EMBRACE.

PD-0468: The practical use of our 1,5 Tesla MRI/HDR treatment room for patients with cervical cancer

fractions) Adaptive Radiotherapy (ART) approach delivering a concomitant boost to the MRI-based residual tumour during the last 6 fractions. Materials and Methods: T3/T4N0 or N+ rectal adenocarcinoma patients (pts) were enrolled in an observational trial. Concomitant chemotherapy consisted of Oxaliplatin 100mg/m on days -14, 0, +14, and 5-FU 200mg/m/day from day -14 to the end of radiotherapy (day 0 is the start of radiotherapy). Radiotherapy consisted in the delivery of 41.4Gy in 18 fractions (fr) (2.3 Gy/fr) with Tomotherapy to the tumor and regional lymph-nodes (PTV) defined on CT/MRI imaging. After 9 fr, CT and MRI were repeated for the planning of the adaptive phase: PTVadapt was generated by adding a 5mm margin to the residual tumour visible on MRI images. In the last 6 fr, a boost of 3.0 Gy/fr (total dose: 45.6 Gy in 18 fr) was delivered to PTVadapt while concomitantly delivering 2.3 Gy/fr to PTV outside PTVadapt. Data regarding acute toxicity and outcome were analyzed. Results: From September 2009 to April 2014, 50 pts completed the preoperative treatment and were evaluable. No G4 toxicity occurred: the G3 toxicity was gastrointestinal only: diarrhoea in 9/50 pts (18%), and proctitis in 2/50 (4%). Diarrhoea started before the adaptive phase in all cases and all affected patients were women. Two pts achieved complete response (cCR) and refused surgery, 1 pt was lost, 1 pt had early distant progression. Forty-six pts underwent surgery (43 R0, 3 R1): thirteen pts (28 %) had pathological complete response (pCR); 22/46 (47%) showed TRG3 response: 13/46 (28%) and 6/46 (13%) had ≤5%, and 6-10% residual viable cells, respectively. Regarding the two patients with cCR who refused surgery, one is still cCR after 54 months while the other had local relapse and underwent transanal resection 1 year after treatment. Concerning treatment feasibility, two pts interrupted radiotherapy after 7 and 13 fr respectively, the remaining pts (48/50=96%) completed the treatment, and the median duration of RT was 25 days (22-36 days). 43/50 pts (86%) and 40/50 pts (80%) received the full dose of oxaliplatin and FU, respectively: 14% of pts received moderately reduced doses (60%-90%), and only two pts (4%) received less than 60% of the planned dose. Conclusions: This study confirms that adaptive boost strategy is feasible with an acceptable G3 toxicity rate and a very encouraging tumour response rate. The results suggest that there should still be room for further dose escalation with the aim of increasing pCR and/or cCR rates.

OC-0486: Nodal failure after (chemo-)radiation and MRI guided adaptive BT in cervical cancer: a sub-analysis within EMBRACE

Nodal failure after (chemo-)radiation and MRI guided adaptive BT in cervical cancer: a sub-analysis within EMBRACE C.N. Nomden, A.A.C. De Leeuw, E.M. Monninkhof, M. Schippers, K. Tanderup, P.S. Kroon, A. Ramlov, J.C. Lindegaard, R. Pötter, C. Haie-Meder, P. Hoskin, B. Segedin, I.M. Jürgenliemk-Schulz UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands UMC Utrecht, Julius Center for Health Sciences and Primary Care, Utrecht, The Netherlands Aarhus University Hospital, Oncology, Aarhus, Denmark Comprehensive Cancer Center Medical University of Vienna, Oncology, Vienna, Austria Gustave Roussy, Radiation Oncology, Villejuif, France Mount Vernon Hospital, Cancer centre, Northwood, United Kingdom Institute of Oncology, Radiotherapy, Ljubljana, Slovenia

PO-0846: MR-based adaptive radiotherapy for cervical cancer: A concept of fast, automated IMRT planning

PO-0845 Evaluation of interplay effects of target and MLC during VMAT by using 3D gel measurements S. Ceberg, C. Ceberg, M. Falk, P. Munck af Rosenschöld, S. Bäck Skåne University Hospital, Medical Radiation Physics, Lund, Sweden Lund University, Medical Radiation Physics Department of Clinical Sciences Lund, Lund, Sweden Radiation Medicine Research Center, Radiation Oncology Rigshospitalet, Copenhagen, Denmark

147 oral MULTICENTRE STUDY OF MRI-GUIDED BRACHYTHERAPY TREATMENT PLANNING: COMPARISON AMONG TANDEM OVOID APPLICATOR USERS

therapy. Such a culture, open and non-punitive, should be actively encouraged through regular multidisciplinary meetings to promote inter-professional communication while respecting and understanding the different roles and responsibilities of individuals. Communication among staff members is essential for all aspects of the radiotherapy process, since mistakes may be made because of lack of adequate communication, incorrect information, or poor understanding of correct information. As a concluding remark it should be acknowledged that education and training is not the only contributing factor to improving safety in radiotherapy. Nevertheless, the different approaches to acquisition of knowledge, skills and competencies are certainly a fundamental pillar supporting patient safety. Proffered paper