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Polarity Correction Factor and Out of Field Dose Measurement for FFF and FF Beam Using Two Microchambers

T Chiu*, B Yang , K Tang , C Cheung , W Lam , K Cheung , S Yu , Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To quantify the change of polarity factor for ExradinA16 Ion Chamber and PTW PinPoint3D under different photon beam field sizes and energies. To assess the performance of these detectors in out-of-field dose measurement in Cyberknife and VersaHD.

Methods: Polarity measurement was performed by using PTW UNIDOSwebline and Standard Imaging Blue Water Phantom. Measurements were performed at 5cm depth. In VersaHD, 100cmSSD was set and in Cyberknife, 800cmSAD was set. Polarity factors for A16 and PinPoint3D were determined by using AAPMTG51. Two dosimetric parameters, field size and beam energy, were investigated. 6MV FF, FFF beam and 10MV beam from VersaHD(Elekta) and 6 MV FFF beam from Cyberknife M6(Accuray) were used. Field sizes ranged from 2cm to 30cm for linac and 0.5cm diameter to 6cm diameter cone for Cyberknife were tested. kpol at 10cm field size from VersaHD was set as reference value. In out-of-field dose measurements, 6cm cone were set in Cyberknife and both microchambers and one FC-65G chamber were used.

Results: kpol determined for both microchambers exhibited field size dependence. In linac measurements, kpol for A16 increased with field size and that of PinPoint3D had an opposite trend. Difference between values at 2cm field size and reference value were -0.2% and +0.1% for A16 and PinPoint3D respectively under 6MV. In Cyberknife measurements, field size dependence existed and the largest deviation from reference value was -0.5% and -0.3% for A16 and PinPoint3D respectively. In out-of-field measurement, both types of chambers showed a lower reading than the commissioned data. The deficiency was around 0.2%.

Conclusion: Our study showed a significant dependence of kpol on field size. Besides, the investigated chambers gave a lower reading in out-of-field measurements. This effect could affect point-dose verification of Cyberknife treatment as out-of-field dose makes a significant contribution to target dose in this delivery technique.


Not Applicable / None Entered.


TH- Radiation dose measurement devices: ion chamber

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