ESTRO 38 Abstract book

S472 ESTRO 38

arguments: (i) the center of the radiation field can be determined from lateral beam profiles using the IC in a parallel orientation as recommended in TRS-483, (ii) OCFs were lower in parallel compared to perpendicular orientation for the smallest fields in all studied ICs. PO-0893 Absolute dosimetry with polymer gels – A TLD reference system A. Schwahofer 1 , P. Mann 1 , C. Karger 1 1 German Cancer Research Center DKFZ, Department of Medical Physics in radiation therapy, Heidelberg, Germany Purpose or Objective Absolute dosimetry in 3D with polymer gels (PG) is generally complicated and usually requires a second independent measurement with conventional detectors. This is why, PG are often used only for relative dosimetry. To overcome this drawback, we combine PG with a 1D thermoluminescence (TL) detector within the same measurement. The TL detector information is then used as additional information for calibration of the gel. Material and Methods The PAGAT dosimetry gel was used in combination with TLD600 (LiF:Mg,Ti). TL detectors were attached on the surface of the PG container placed inside a cylindrical Phantom (figure 1). To test the usability of this setup, two irradiation geometries were carried out: (a) homogeneous target coverage and (b) small-field irradiation. PG was evaluated with magnetic resonance imaging (MRI) and the TL detectors with a Harshaw 5500 hot gas reader.

Material and Methods The measurements were made on Elekta Versa HD linac for 6 and 10 MV beams with and without flattening filter. FOFs were acquired for nine small square fields ranging from 0.5 to 10.0 cm (nominal sizes) shaped with MLC and jaws, at SSD = 90 cm and 10 cm depth in a 3D water phantom. Nominal field sizes were converted to equivalent square field sizes (S clin ) as S clin = (AxB) 1/2 , where A and B correspond to the measured field width and length defined at the FWHM. Discrete values of FOFs were determined with two reference detectors, Exradin W1 plastic scintillator and EBT3 radiochromic films, fitted with the analytical function FOF(S clin ) proposed by Sauer and Wilbert - Eq. (1). EBT3 films were irradiated in an RW3 Solid Water phantom under the same set-up as described above and analyzed with Radiochromic.com software. Four types of ICs - IBA Razor, IBA CC04, PTW 31022 3D PinPoint and PTW 31023 PinPoint - were used in parallel and perpendicular orientations to measure signal ratios M(S clin )/M(S ref ) between clinical and reference (10 x 10 cm 2 ) fields. The orientation was kept unchanged (parallel or perpendicular) for the scanning procedure and the point measurements. Discrete values of OCFs determined with Eq. (2) were fitted by the function shown in Eq. (3).

Results For the smallest fields, OCFs were always lower (closer to unity) in parallel than in perpendicular orientation for all ICs, regardless of the beam energy used (Fig. 1).

Results PG dosimetry alone showed deviations of up to 4% as compared to calculations. Including additionally the dose information of the TL detectors for PG calibration, this deviation was decreased to less than 1% for both irradiation geometries (figure 2). This is also reflected by the very high -passing rates of >96% (3%/3mm) and >93 % (2%/2mm), respectively.

Conclusion For the determination of FOFs and OCFs for cylindrical IC, we recommend the orientation with its stem parallel to the beam axis. Our recommendation arises from two main