ESTRO 37 Abstract book

but in all cases there was a considerable reduction of V25 for VMAT with respect to 3D-CRT. In the case of the lung, great reductions were observed in the V20 values obtained with VMAT, accomplishing the V20<20% constraint in all cases (even in those where it wasn’t achieved with 3D-CRT). For low doses (V10 and V5) we observe larger irradiated volumes when VMAT was used, especially for V5, which was worse for all patients. Conclusion Based on the results obtained, we can say that a better and more homogeneous PTV coverage is obtained with VMAT, while a significant reduction in heart doses and lung V20 is observed. This fact makes VMAT a better technique for treatment planning of this pathology. However, an increment in the volume of lung irradiated with low doses is also produced, especially in those cases with a very large PTV, so an individualized analysis for each case is recommended. EP-2369 Dosimetric impact of imaging modality (CT versus MRI) for cervical cancer radiotherapy S. Burke 1 , V. Batumalai 1,2,3 , K. Lim 1,2 , D. Roach 2 , G. Dinsdale 1 , L. Mahoney 4 , M. Jameson 1,2,3 , C. Ochoa 1 , J. Veera 5 , L. Holloway 1,2,3 , S. Vinod 1,2 1 Department of Radiation Oncology, South Western Sydney Local Health District, New South Wales, Australia 2 South Western Clinical School, University of New South Wales, New South Wales, Australia 3 Ingham Institute of Applied Medical Research, Medical Physics, New South Wales, Australia 4 Faculty of Health, University of Newcastle, New South Wales, Australia 5 Department of Radiation Oncology, Peter MacCallum Cancer Centre, Victoria, Australia Purpose or Objective The use of magnetic resonance imaging (MRI) is becoming more prevalent in cervical cancer external beam radiotherapy (RT). To our knowledge, no planning study has been conducted to evaluate the dosimetric implications of imaging modality for cervical cancer RT. The aim of this study was to evaluate the dosimetric impact of computed tomography (CT) versus MRI based contouring variability for cervical cancer RT. Material and Methods 5 definitive cervical cancer patients had clinical target volumes (CTV) independently contoured by three radiation oncologists on CT and MRI scans as part of a previous study). Planning target volumes (PTV) were generated by applying 1cm margin around the CTV. Gold standard PTVs for each image modality were generated using the Simultaneous Truth and Performance Level Estimation (STAPLE) algorithm. All organs at risks (OAR) were contoured by one of two radiation therapy technologists (RTT) on CT scan and checked by a radiation oncologist. Two volumetric modulated arc therapy plans were generated for all patients; one using CT-STAPLE volume, and another using MRI-STAPLE volume. A prescription of 50Gy in 25 fractions was used for all cases. All plans were generated by one RTT and checked by another senior RTT, both experienced in cervical cancer radiotherapy planning. Dose metrics for planning target volumes (PTV) and OARs were compared to analyse the difference based on imaging modality (CT vs MRI). Paired t-tests were used to analyse the differences between the two modalities. Results Table 1 shows the average target and OARs doses comparing CT and MRI. Paired t-tests showed no statistically significant differences between the two modalities for all dose metrics assessed. Structure Dose metric CT based plans MRI based plans PTV D95 (Gy) 49.1 49.1

PTV PTV PTV

D50 (Gy) 51 D98 (Gy) 48.9

51.1 48.2 52.5 42.2 76.7

D2 (Gy)

52.2

Small bowel

V40Gy (%) 40.8 V40Gy (%) 77.5 V45Gy (%) 57.2

Rectum Bladder

54

Left head

femoral

V30Gy (%) 18.1

15.6

Right femoral head 14.3 Table 1: Dose metrics averages (Dx - dose to x% of the PTV; Vx - the percentage of the volume receiving more than x% of the prescribed dose) Conclusion This preliminary study showed no dosimetric differences between CT and MRI based target volumes. Further work will include planning on a larger sample size and analysing contouring similarity metrics to correlate with dosimetric outcomes for cervical cancer radiotherapy. V30Gy (%) 13.9 EP-2370 Cone Beam Computed Tomography for radical Head and Neck Radiotherapy D. Thornberry 1 , R. McLauchlam 1 , D.M. Gujral 1 1 Imperial College NHS Trust, Radiotherapy, London, United Kingdom Purpose or Objective The aim of this study was to compare the use of kV planar imaging with combined kV and CBCT imaging in patients receiving radical Intensity Modulated Radiotherapy (IMRT) for head and neck cancer Material and Methods The verification images of 30 patients receiving radical IMRT for head and neck cancer were retrospectively reviewed by a single, experienced observer. Standard imaging protocol was kV planar imaging on days 2, 3 and then weekly with a CBCT acquired on day 1 and week 4. Additional kV images were taken in addition to the weekly schedule if corrections or gross displacements in shifts occurred for either kV or CBCT. Displacements were recorded in all 3 planes with the addition of the Pitch, Roll and Rotation (PRR) from the CBCT images. PRR were evaluated in the matching but could not be corrected as we do not have a 6 degrees of freedom treatment couch in our department. PRR values <3 degrees were accepted, if greater than this the patient was re-setup. Online shifts for both kV and CBCT images were performed for displacements of >0.3 cm. The displacement values in the 3 planes were used to produce population random and systematic errors using the method as outlined in On Target(1) Results The addition of 3-dimensional information of patient position provided by CBCT gave a more precise match to bone. Population random and systematic errors for kV (219 AP, 238 Lateral images) and CBCT (72 images) are shown below (Figure 1). Errors were similar for kV and CBCT matching except systematic error in the Superior- Inferior which was larger for CBCT. Figure 1. Population random and systematic errors Margin recipes (Van Herk2 and Stroom3) were calculated to determine the clinical target volume (CTV) to planning target volume (PTV) and are shown in Table 1. Uncorrected data uses raw displacement measurements assuming patient position was not corrected and gives Electronic Poster: RTT track: Image guided radiotherapy and verification protocols