ESTRO 38 Abstract book

S1167 ESTRO 38

Conclusion The implementation of every technical advance in radiation oncology requires a comprehensive formation and experience to achieve the best results. In our experience, the dosimetric values in lung cancer have been clearly improved with the greater knowledge and use of VMAT technique. EP-2110 Developing a QA programme for the Elekta Unity MR-linac J. Chick 1 , I. Hanson 1 , S. Nill 1 , U. Oelfke 1 1 Royal Marsden Hospital NHS Trust and the Institute of Cancer Research, Joint Department of Physics, Sutton, United Kingdom Purpose or Objective In all modalities of treatment and diagnosis a well- structured Quality Assurance (QA) programme is necessary to ensure a high quality service and to minimise the risks to patients and staff. The Unity MR-linac (Elekta AB, Stockholm) has recently been brought into clinical service at the Royal Marsden Hospital (RMH), and requires a QA programme to include dosimetry, image quality and safety aspects. Here the RMH QA programme is outlined and initial results reported for the first month of clinical use. Material and Methods The QA programme is currently completed in daily 60-90 minute sessions before patient treatment, including daily mandatory tests (45minutes), and weekly and monthly tests on a rotating cycle. A summary of the programme is described in Table 1, detailing frequency and equipment used. Many of the tests use a QA platform, which is a jig that fits onto the couch top, with set up marks to enable equipment to be set up reproducibly. The Syzygy phantom consists of spheres aligned along a plumb line and water level, to provide an absolute frame of reference (Hanson et al. ESTRO 37, QC-0079). The alignment of the MR and MV co-ordinate systems is measured daily using an Elekta Phantom which has 7 Zr0 2 spheres which are visible on MV, with corresponding signal voids on MR. Further MR tests will be done quarterly, to include B0 homogeneity, B1 flip angle accuracy, dynamic stability, ghosting and spurious noise (Tijssen et al, ESTRO 36, QC- 0257). Annual tests will include dose linearity, gantry angle attenuation, and MLC transmission/leakage.

1 Capio-Fundación Jimenez Díaz, Radiation Oncology Department, Madrid, Spain Purpose or Objective RTOG 0617 trial established among other conclusions the advantages of IMRT/VMAT in locally advanced non small cell lung cancer (LA-NSCLC) mainly due to better dosimetric values comparing to 3DRT. Our objective is to determine if the better knowledge and experience in the development of VMAT in lung cancer is related with an improvement in dosimetric parameters. Material and Methods We started to employ VMAT for lung cancer in January 2014. Since then, 125 locally advanced non small cell lung cancer and limited disease small cell lung cancer have been treated with this technique. All patients were treated following our protocol treatment. The simulation consisted in a 4D CT simulation (3mm slices) with an Elekta frame based immobilization system and free breathing. We delimited the planning target volume and organs at risk according to RTOG guidelines. For constraints, Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) was used. The plan was designed using Volumetric Modulated Arc Therapy (VMAT) with one or two arcs and 6-10 MV photons. It was calculated by the Monaco treatment planning system (version 5.10). The plan was delivered using Elekta Agility Linac with Precise treatment system (precise table) and Elekta XVI CBCT system for quality assurance. We have analyzed and compared dosimetric values (lung V5-10-13-20 and mean lung dose; mean heart dose) between the first 20 patients treated with VMAT (group 1) and the last 20 patients treated with VMAT (group 2). To achieve an interpretable results a volume ratio have been employed to ensure homogeneus groups: V PTV / V TOTAL LUNG. Where V PTV : Plannig target volume (cc) and V TOTAL lung: Total lung volume (cc) Results Both groups were homogeneus with the same volume ratio: 0.16. Significative differences were registered between groups in all the dosimetric values analyzed, all of them in favour of group 2, mainly in mean heart dose, with a great difference of 28,6% for the group 2. Every lung dosimetric values were clearly better in group 2, especially Mean lung dose and V5 (see figure 1) FIGURE 1 (DOSIMETRIC VALUES VMAT LUNG CANCER) V PTV /V TOT AL LUN G MEA N LUN G DOS E LUNG V5 L UNG V10 LUN G V13 LUN G V20 MEAN HEART DOSE

GRO UP 1 (FIRS T 20 PATI ENTS ) GRO UP 2 (LAS T 20PA TIEN TS) DIFF EREN CE (%)

16,

15,

0,

74,67

55,29

45,

28,

24 Gy

09 Gy

%

17%

16

%

91%

15,

10,

0,

67,95

52,46

43,

27,

05 Gy

78 Gy

%

87%

16

%

65%

4

_

9,0

5,1

28,6

7,3

2,9

,3