Room: AAPM ePoster Library
Purpose: test the code of practice (IAEA-AAPM) of small field dosimetry for Tomotherapy.
Methods: dosimetry of small fields for 6MV flattening filter free (FFF) beam from Tomotherapy was carried out to determine beam quality, corrected field output factors and to estimate uncertainty.
Measurements were performed at in water and solid water for field sizes ranging from the largest possible 5 cmx 40 cm to the smallest 1 cm x 0.625 cm. Two ionization chambers [IBA CC01, PTW Pin-Point] and one unshielded electron field diode (EFD,IBA) were used. Beam quality at the reference field size of 10 cm x 10 cm, [TPR 20,10] was determined by analytical expression by Palmans. Relative dosimetry was performed for clinical field sizes (fclin) of 1cm X 0.625cm, 1cm X 1.25cm, 2.5cm X 2.5cm, 5cm X 5cm and 5cm X 10cm (MSR field size) at 10 cm depth. Measurements were carried out in water as well as solid water slabs. Combined uncertainty was estimated for all the detectors.
Results: variation of TPR20,10(S) at MSR field size and smallest field size among three detectors was 0.34% and 1.41% respectively. The validation of Palman/s equation was successfully done for 5 cm x 10 cm and 5 cm x 5 cm. Corrected field output factors for all three detectors were in good agreement except for the smallest field size. PTW Pinpoint ion chamber has the highest, whereas IBA EFD diode has the least positional uncertainty at the smallest field size.
Conclusion: ion chambers CC01 and Pinpoint can be used for output factors measurement at field sizes larger than 2.5cm X 2.5cm. Unshielded EFD diode is preferred for Output factor measurement at smaller field sizes due to its low uncertainty compared to ion chambers and availability of output correction factors.