Rapid Arc Treatment Verification: post evaluation on Delta-4 and proposal of a new verification protocol G. Pittomvils 1,,L. Paelinck 1, T. Boterberg 1, G. De Meerleer 1 and C. De Wagter 1 (1)Division of Radiotherapy, Ghent University Hospital, Gent, Belgium 50 prostate and 61 rectal cancer patient plans were evaluated in order to find an alternative for the Delta-4 patient specific dosimetry. Using class solution criteria, an excellent linear correlation was observed between the beam aperture based modulation factor extracted from the DICOM RTPLAN file and the total number of MU of the arc divided by the isocenter dose (on delta-4). This reduces the patient specific dosimetric work load on the Delta-4 phantom from 111 to 23 patients. Introduction Discussion and Conclusion Results Material and Methods  In our department, patient specific dosimetry for Rapid Arc treatments is done using a Delta-4 phantom. It compromises a cylindrical phantom with two orthogonal 2D diode detector arrays in a 5 mm grid in a central 6x6cm² plane and 10 mm at the borders. 50 randomly selected prostate cancer patients (38x Gy depending on the rectum tolerance) and 61 rectal cancer patients (25x1.8 Gy) were evaluated. In order to reduce the workflow some class solution specific criteria were set; multiple arc plans were excluded; the average PTV volume should be within one standard deviation of the average value; the isocentre should be in a high-dose region. The other patients were maintained in the Delta-4 measuring protocol.  Six beamlets with leave positions; -10/-7 cm; -10/-4cm; -10/-1cm; 1/10cm; 4/10cm and 7/10 cm at a SSD of 90 cm and jaws settings of 16x20cm² at 10 cm depth for 60MU(18MV)/70 MU(6MV) are measured using radiochromic films and 0,6 cc ionization chambers and compared with the treatment planning calculations.  Small and Narrow Fields on axis and off axis  Small and Narrow Fields on axis and 5cm/5cm off axis are investigated to test the modelling in Pinnacle. Field widths varying from 0.8 cm to 3 cm and field lengths of 2 and 6 cm are evaluated using a diamond detector in order to compare measurements with calculated output factors at a SSD of 90 cm and at 10 cm depth.  Radiochromic Films  Radiochromic film can be used as a relative and absolute dosimeter with an accuracy of 2-3 %. Using several radiochromic films in a clamped PMMA slab phantom in order to avoid air cavities near the films, 3D information of the treatment planning is obtained.  Fine tuning using abutting fields  The stability of the leaf calibration and the agreement between treatment planning and treatment delivery was consequently evaluated during a period of several months using 14 matching fields (width cm) over a range of 24 cm along the leaf trajectory.  An independent check of 101 beamlets (width 1 cm), each shifted over 0.5 mm from Y2 towards Y1 with fixed jaw positions X1=X2=3cm and Y1=Y2=20 cm was used to compare the point dose calculation with the measured dose reading of a 0,6 cc reference ionization chamber.  Output factors for regular fields – After automatic modelling, after small modifications in density of the wedge factor and after increasing the off axis softening factor of 18 MV modelling good agreement was obtained between measurements and calculations: for central open beams (6 MV: 0.71% ± 0.31%; 18 MV: -0.06% ± 0.29%; for off-axis beams (6 MV: -0.02% ± 0.37%; 18 MV: -0,48% ± 1,14 %). – For a 6x6cm² field an agreement of 2% or better was achieved along the leave setting for of axis distances from -10 cm to + 10 cm.  Inverse Pyramid – The calculated profiles for 6MV (70 MU) and 18MV (60 MU) at 10 cm depth are compared with several EBT-film measurements and with chamber recordings at the plateaus of the profile to benchmark the EBT-films as a absolute dosimeter and to compare the calculated point dose contribution of each individual segment with the measured data. – The results for 18 MV are depicted in figure 1 and each individual segment remained within tolerance (in beam 6MV: -0.55% ± 0.76 %; scattered dose 17% ± 16%) and after adjustment of leaf transmission for 18MV (in beam 18MV: -0.27% ± 0.84%; scattered dose -5% ± 12%).  Small and narrow fields on axis and off axis - Shielding of the source influences stability of the output factors of small fields. For fields without shielding effects NCS 15 allow 3% tolerance for in axis fields and 4 % tolerance for the off axis fields. - All on axis fields are within the NCS tolerance except the 8mm wide 6cm long field and the average differences are for the 6 cm long fields -0.40% ± 1.73% for 6MV and -0,83% ± 1.22 % for 18 MV; for the 2 cm long fields -1,62 % ± 0.58% for 6 MV and 0,80% ± 1.43% for 18 MV. - All off axis fields for both 2cm and 6 cm long fields are within the 4 % tolerance except the smallest fields (width <1.2cm). Leaf position variations (2  =1mm) are indeed introducing extra uncertainties in the output factors of the smallest field sizes.  Overal treatment verification - Figure 2 illustrates the agreement for the at the Ghent University Hospital used dose painting protocol, where the dose level is correlated to the intensity of the PET signal emitted by the tumour cells; 97,5 % of the data points have Gamma values below 1. - Figure 3 illustrates the agreement for a IMAT treatment of a cervix tumor. The results of the gamma evaluation are depicted in table 1.  Fine tuning for the abutting field setup of IMAT treatment delivery – The results of IMAT treatment planning are less accurate then those of IMRT and dose painting planning. Leaf tip positioning was one of the items of investigation. A tolerance of 0.5mm (instead of 1 mm) between treatment planning and the treatment machines was introduced. – The increase in agreement after the leaf tip shift in the treatment planning of 0.6mm is tabulated in table 2. – For the 101 beamlet prescription, the error between calculation and measured data decreased from 15% to 6%. An agreement of 2% was achieved when comparing dose delivered in the 20-80% region and the calculated dose. Figure 3 : EBT film data of an IMAT treatment for a cervix tumor; 25 fractions of 208 cGy are prescribed (left); Dose prediction of Pinnacle 8.0 m of the treatment plan calculated on a 1 cm slab PMMA phantom (CARPET Phantom) (center); 3 %, 3 mm Gamma evaluation data between film data and dose prediction of Pinnacle (right). Figure 1 : The inverse pyramid for 6/18 MV (70/60 MU per segment; SSD=90 cm, depth 10 cm) field size 16x20 cm², MLC beamlets (-10/-7cm; -10/-4cm; -10/-1cm; 1/10cm; 4/10cm; 7/10cm) The inverse pyramid allows the evaluation of open beam data, scatter beam data and leaf transmission. Where film only allows evaluation scatter data and leaf transmission on the central axis, chamber measurements allows off axis leaf transmission and scatter characteristics for several distances to the open field segment. The results showed excellent agreement. Above mentioned tests resulted in excellent agreement for Dose Painting treatment plans, but a decrease in tolerance to 0.5 mm between treatment planning and treatment delivery was necessary in order to obtain 90% of the data point within the 3%/3mm gamma criterion. This is due to the large amount of small long abutting segments in the optimisation process. This difference in leaf tip modelling was already reported in by Olteanu et al. (1) The approximation in the calculation of the IMAT treatment delivery (static segments every 8°), absorption of dose in the Sinmed table (2) top are partially responsible for the remaining differences between treatment planning and treatment delivery. References [1] NCS report 15 (Netherlands Commission on Radiation Dosimetry) [2] L. Olteanu et al. IFMBE proceedings 25/I, (2009) [3] S. Gillis et al. Radioth. Oncol. 75, (2005) [4] van Herk M, Seminars in Radiation Oncology 14 (2004), Figure 2 : EBT film data of the Dose Painting treatment; a dose escalation from 216 cGy to 300 cGy on the PET positive lesion is administered correlated to the intensity of the PET signal (left); Dose prediction of Pinnacle 8.0 m of the treatment plan calculated on a 2 mm slab PMMA phantom (center); 3 %, 3 mm Gamma evaluation data between film data and dose prediction of Pinnacle (right). Gamma value <1<1.2<1.4<1.6<1.8>2 % of the data points 79.9 % 92.12%95.24%99.49%99,53%0,10% Table 1 : Gamma evaluation data of the IMAT treatment of a cervix tumor Gamma value <1<1.2<1.4<1.6<1.8>2 Slice 1 before fine tuning % 87.62%93.44%96.63%98,23%0,84% Slice 1 after fine tuning % 93.18%96.14%97.70%98,52%0,90% Slice 2 before fine tuning % 92.31%95.01%96.48%97,66%1,34% Slice 2 after fine tuning % 94.08%95.83%97.09%98,19%1,71% Table 2 : Gamma evaluation data of the IMAT treatment of before and after the leaf tip optimalisation