EP-1393 SMALL-FIELD DOSIMETRY IN A 6 MV PHOTON BEAM USING ALANINE AND LIQUID IONISATION CHAMBER

S528

The deviation of D20/D10 of the composed field to D20/D10 of the single field was 0.1%. The result of the measurements with modulated beam on Tomotherapy machine was Q = D20/D10 = 0.627. According to the DIN 6800-2 (2008) kQ is 0.999 which is very close to kQ = 1 proposed by Accuray Inc.. Conclusions: A beam modulation to produce a flattened field which is equivalent to a 10x10 square field is valid to determine kQ. EP-1391 RESULTS FROM TESTING THE OCTAVIUS PRE-TREATMENT QA SYSTEM EQUIPPED WITH THE INCLINOMETER E. Vanetti de Palma1, A. Fogliata1, G. Nicolini1, A. Clivio1, L. Cozzi1 1 Oncologic Institute of Southern Switzerland, Department of Medical Physics, Bellinzona, Switzerland Purpose/Objective: The Octavius (PTW) pre-treatment QA system is now equipped with an inclinometer device, able to provide information on the gantry position during an arc treatment delivery. In the present study the capabilities of this system for RapidArc (RA) pre-treatment QA have been investigated. Materials and Methods: Five RA plans made up of one complete arc have been selected, presenting different modulation levels. Pretreatment QA measurements were done with the 729 ionization chambers detector array (PTW) inserted into the Octavius phantom. A 6MV beam from a UNIQUE (Varian) linac equipped with MLC-120 Millennium was used. Each measurement was repeated with the octagonal phantom placed in its 8 possible positions (one per each supporting side). In this way the detector and the Octavius hollow layed in all possible positions. For each image acquired with the inclinometer, the contribution from a definable gantry angular interval is determined by the associated PTW Verisoft software. Pretreatment QA was evaluated in the present study for the original full arc and for given gantry angular intervals, distinguishing the sectors irradiating the detector array laterally, the phantom from its anterior position and trhough the Octavius hollow. Acquired contribution was compared with the corresponding calculated dose image. Calculations were done in the Varian Eclipse planning system using Acuros XB algorithm. Comparisons between acquired and calculated dose images have been perfomed through the gamma analysis (DTA=2mm and DD=1%, 2% and 3% on a global basis) using the Verisoft software, in the defined sectors. Results: Gamma analysis results of complete arcs cases were always satisfactory at least for the conditions with DD=2% and 3%: the percentage of points fulfilling the gamma criteria (GAI) was greater then 95%. Results concerning gantry intervals evidentiated differences: very good agreement was shown for sectors orthogonal and anteriorly to the detector, where the GAI was greater than 95% also for DD=1%; for sectors irradiating the detector laterally the GAI was lower than 95% also for DD=3%. Conclusions: The use of the inclinometer together with the Octavius system allowed investigating the behaviour of the 729 detector respect to different sectors of RA arcs. In general the system behaves adeguately for the most irradiation conditions investigated. Only for small sectors the results becomes suboptimal. With the different positions of the Octavius phantom and the inclinometer, an accurate pre-treatment QA is possible for all sectors. EP-1392 IMRT PRE-TREATMENT VERIFICATION USING EBT3 FILM AND FILMQA PRO SOFTWARE E.M.A. Kok1, B.P.H.M. Geelen1 1 Reinier de Graaf Groep, Radiotherapy, Delft, The Netherlands Purpose/Objective: The purpose of this study was to investigate the accuracy of the new Gafchromic EBT3 film[1] in combination with FilmQA Pro software[1]. Gafchromic EBT3 film has only been commercially available since the end of 2011, and is the successor of EBT1 and EBT2. This study was carried out with a view to implementing this technique in our clinic for pre-treatment verification of prostate IMRT treatments. Materials and Methods: All measurements were performed using a Varian Clinac 2100c/d, 1 cm leaves, 15 MV and dynamic IMRT. The maximum isocenter dose was 250 cGy per fraction. Calculations were performed using Brainlab Iplan 4.1.2 software.The calculated IMRT plan was projected onto a RW3 slab phantom using Iplan phantom mapping. Ionisation chamber point dose measurements were performed using a calibrated type 31016 3D pinpoint ionisation

ESTRO 31

chamber (PTW-Freiburg). For the film dosimetry measurements Gafchromic EBT3 film was used. Both film and ionisation chamber were irradiated at isocenter depth. Films were scanned using an Epson 10000XL flatbed scanner, ±15 hours after exposure. All films were both scanned in portrait and landscape mode. The EBT3 film was calibrated using calibration patches (Mosaiq) over a range from 0-300 cGy. The calculated isocenter dose was compared to both ionisation chamber and EBT3 film measurements, both for the total IMRT plan, and for each individual beam. In the case of the EBT3 measurements a quantitative analysis of the dose distribution was carried out using FilmQA Pro software, and the ‘Triple Channel dosimetry’ [2] method. Gamma [3% 2 mm], differential dose [3%] (Differential Delta) and distance to dose agreement [2 mm] between Iplan and the EBT3 film were compared, using a threshold of 10%. Results: Pointdose measurements show good agreement (all within 3 %) with the calculated isocenter dose. De oriëntation of the EBT3 film on the flatbed scanner turned out to be of influence regarding the measured dose. Best results were obtained using portrait mode. A gamma (3%, 2 mm) > 99 % was achieved. Conclusions: Preliminary results show excellent agreement for EBT3 film in combination with triple channel optimalisation of the Film QA pro software, providing the film is scanned in ‘portrait’ orientation. [1] ISP Advanced Materials Group [2] Micke, Lewis, and Yu: Multichannel film dosimetry with nonuniformity correction, Med. Phys. 38 (5), May 2011 Table 1: Results dose measurement for 2 pre-treatment IMRT prostate verifications Measurement Point dosimetry * Scan [% dev. in ISO as Orientation compared to TPS] IMRT prostate 0.2% 1 IMRT prostate -1.2% 2

Gamma [3%, 2 mm]

Differential Dose [3%]

Portrait

99.8%

95.0%

Landscape Portrait

99.6% 99.2%

89.0% 87.2%

Landscape

96.7%

80.3%

* corrected for dose deviation in linac output EP-1393 SMALL-FIELD DOSIMETRY IN A 6 MV PHOTON BEAM USING ALANINE AND LIQUID IONISATION CHAMBER S. Zimmermann1, H.L. Riis1, M. Hjelm-Hansen1, R. Ottosson2, C.E. Andersen2, J. Helt-Hansen2 1 Odense University Hospital, Radiofysisk Laboratorium, DK-5000 Odense C, Denmark 2 Technical University of Denmark, Center for Nuclear Technologies, DK-4000 Roskilde, Denmark Purpose/Objective: Dosimetry of small field sizes in MV photon beams is an increasingly important subject, and a generally accepted guideline for clinical measurements is still lacking. The present comparative study was carried out to further investigate the use of alanine and the PTW microLion ionisation chamber for small-field dosimetry in liquid water. Materials and Methods: The measurements were carried out on a Siemens Primus 58 leaves MLC. The alanine dosimeters were cylindric Ø4.9 mm × 3.0 mm and density of 1.2 g/cm3. The alanine dosimeters were placed on the top of a solid water stick of Ø4.9 mm× 88 mm. The stick was held vertically in a PTW MP3 water tank. A latex sleeve of Ø5 mm was used to protect the dosimeter from the water. Measurements were carried of at four depths, 0.2, 15, 100 and 200 mm. The distance from the target to the water surface was 100 cm in all experiments. In addition, the measurements were carried out at seven symmetric field sizes with the length of the field edges at the water surface: 0.8, 1.0, 1.4, 2.0, 3.0, 4.0 and 10.0 cm. As a minimum three dosimeters were irradiated to 2500 MU in each configuration. The dosimeters were placed in the dose maximum of each field and depth. This dose maximum was measured for each field using a ScanditronixWellhöfer photon field diode. The same measurements were carried out using a liquid ion-chamber, PTW microLion, irradiated by 500 MU. The output of the accelerator was controlled by a PTW semiflex ionchamber placed in the radiation head of the accelerator. The calibration of the accelerator output provides 1.0 Gy for 100 MU at dose maximum. The alanine dosimeters were measured with an EMXmicro EPR spectrometer. Measurement parameters were: modulation amplitude 10G, conversion time 41 ms, time constant 82

ESTRO 31

ms, 2 times 4 sweeps with a 90° turn in between, total measurement time was 3 minutes per dosimeter. The alanine dosimetry system was calibrated using a Co-60 gamma cell ensuring traceability to a national standard. Results: The results are summarized in Table 1. At depths below 2 mm, we noted a systematic deviation between the tw wo systems with field size. At 10x10cm, the deviations were insiggnificant (mean 1.01+/-0.01), but at 0.8 x 0.8 cm2, the deviation was w as large 10% (mean 0.90+/-0.02). At 2 mm depth, the data seemed to be subject to more variability than can be expected from the readou ut procedures.

S529

values obtained with the PTW 60008 have a range r between 5-6% while the all three values of PTW 60012 are lower than t 3%.

ers indicate that diode Conclusions: The OCR and St paramete detectors have superior performances tha an ionization chambers. Furthermore, since PTW 60012 diode has be etter St values than PTW 60008, PTW 60012 is preferable for smalll field measurements in Cyberknife SRS Unit. EP-1395 FILM BASED DELIVERY QUALITY ASSU URANCE FOR ROBOTIC RADIOSURGERY 1 1 1 O. Blanck , J. Dunst , D. Rades , G. Hildebran ndt2 1 University Clinic Luebeck, Radiation Oncologgy, Luebeck, Germany 2 University Clinic Rostock, Radiation Oncologgy, Rostock, Germany Conclusions: The study confirms the difficulty related with small-field dosimetry and the importance of detector choice (ma aterial and size) and positioning procedure. No corrections for volume e averaging and spatial sensitivity of the EPR spectrometer over the e volume of the alanine dosimeter were applied, and this may explain part of the measured deviations. A practical difference between the two systems was that the alanine measurements were much more time consuming than the liquid ionization chamber measurements. EP-1394 WHICH DETECTOR TYPE SHOULD BE SELECTED FO OR SMALL-FIELD MEASUREMENTS IN CYBERKNIFE SRS UNIT? ION CHAMBER R OR DIODE T. Yolcu1, M. Cengiz1, M. Yeginer1, Y. Akdeniz1, M. Gurkaynak1 1 Saglik Bilimleri, Radiation Oncology, Ankara, Turkey Purpose/Objective: Small field measurements can n imply serious uncertainties because of lateral electronic disequilibrium and volume effect of the dosimeter. In this study our aim is to fin nd out the most appropriate detector for the small field measuremen nts of off-center ratios (OCRs) and collimator factors (St) in CyberKnife.. Materials and Methods: The measurements for 5, 7.5 7 and 10 mm collimators of CyberKnife SRS unit were performed using u a PTW M3 water phantom. Ionization chambers and solid state e semiconductor detectors with different effective volume were used d to collect the data. Ionization chambers include PTW 30013, PTW 31010 and PTW PinPoint with effective volumes of 0.6cc, 0.125ccc and 0.015cc, respectively. PTW 60008 and PTW 60012 diodes were used as semiconductor detectors with the effective volum me of 0.56 mm diameter and 2.5 µm thickness. The reference meassurements were acquired by PTW Diamond detector. OCR measurementts were taken at the depths of 50mm and 200 mm with 800mm SSD. The T 60 mm cone was used as reference for St measurements. Results: Penumbra broadening and full width at half maximum (FWHM) differences based on the reference values were w calculated. Table 1 shows the values provided by each detector. The PTW 30013 has inferior performance than other detectors. Among A the ion chambers, although the best results are obtained wiith the PinPoint chamber, its values for 5mm and 7,5mm collim mator are also inacceptable. PTW 60008 and PTW 60012 have the best agreement with the diamond detector such that their penumbra broadening and FWHM difference are lower than 0.5mm and 1mm m, respectively. The St values for PTW 30013 were found to be higher h than 50% whereas PTW 31010 values are 43.8%, 22% and 11.1% for f 5mm, 7.5mm and 10mm cones, respectively. PinPoint has again best St values among the other ionization chambers. Its results are e 20.3%, 7.96%, 3.57% for the 5mm, 7.5 mm and 10 mm cones, re espectively. The

Purpose/Objective: For complicated treatme ents such as IMRT or IMAT guidelines and tool exist for delivery qualiity assurance (DQA). Yet there is currently no consent on DQA for the t CyberKnife® Robotic Radiosurgery System. Our objective was to in nvestigate gafchromic film based DQA for radiosurgery using the CyberK Knife™ (Accuray, USA) and the FilmQA™ software (3Cognition, USA). Materials and Methods: For our study Ash hland/ISP (USA) provided newly released gafchromic EBT2 films for th he CyberKnife BallCube II phantom. For this study we used DQA planss with a maximum scaled prescribed dose of 4-12Gy, for which the film f was calibrated. The plans were overlaid on the BallCube II with the head phantom for intracranial cases and the hemisphere on the motion platform for extracranial moving cases. We evaluated the e tests with FilmQA™ and used acceptance criteria of > 95% for 2% Dose D / 2mm Distance-toagreement (DTA) and > 98% for 3% Dose / 3mm DTA pixels passing Gamma. As further evaluation we corrected for translational and registration errors and evaluated pixels passin ng Gamma with 1% Dose / 1mm DTA. Results: The FilmQA™ analysis for 10 intracra anial test cases showed on average 99.04% (3% Dose / 3mm DTA) and 95.84% (2% Dose / 2mm DTA) pixels passing Gamma when comparin ng film to the predicted dose distribution. Only the case with a maxim mum scaled dose of 12Gy failed the acceptance criteria for 2% Dose / 2mm DTA, but passed 3% Dose / 3mm DTA. For 10 extracranial movin ng test cases the analysis showed on average 99.39% (3% Dose / 3mm DTA) D and 96.28% (2% Dose / 2mm DTA) pixels passing Gamma. For the corrected evaluation we found for all 20 cases 93.63% passing Gamma with 1% Dose / 1mm DTA and a mean correction of 1.06mm. The avera age time for the DQA was 32min and 10min for FilmQA™ evaluation.