ESTRO 2021 Abstract Book

S1317

ESTRO 2021

Conclusion As can be seen on the results, using a detector with a small grid size and low uncertainties enables to optimize the TPF parameters correctly. In addition, the parameters associated with the leakage differ accordingly between the 6 MV beam with flattening filter, and the 6 MV beam without the flattening filter. PO-1594 Implementation of a QC procedure based on absolute dosimetry of 192-Ir HDR sources by micro- ionization chamber M. Martínez Albaladejo 1 , I. Seedhouse 1 , P. Tudhope 1 , P. Birmpakos 1 , A. Coles 1 , L. Contoret 1 , L. Mcgowan 1 , A. Pointon 1 , C. Edwards 1 1 University Hospitals of North Midlands, NHS, Radiotherapy Physics, Stoke-on-Trent, United Kingdom Purpose or Objective To establish an end-to-end test to check the rescaling of treatment dwell times of a calibrated 192 Ir high-dose- rate (HDR) brachytherapy (BT) source based on the absolute dose measurement of a planned dose by means of a PinPoint 3D ionization chamber. After a literature review, a quality control (QC) procedure for the absorbed dose-in water was developed and included in the quality assurance (QA) of the GammaPlus iX HDR unit (Varian Medical Systems) belonging to UHNM. Materials and Methods The dose to water in water medium (D w ) can be determined from the detector measurement in a phantom material using the following proposed formalism [D. Baltas, L. Sakelliou, N. Zamboglou, The Physics of Modern Brachytherapy for Oncology, 2007, page 448 ff]:

where M is the measured charge during the integrated time; ACF is the air density correction factor and DCF, the dose correction factor.

Annual cross-calibrations have been performed between two field instruments (PTW 31016 PinPoint-UNIDOS electrometer) and a National Physics Lab secondary standard (NE2611) as reference detector, in a Gulmay D3300 kilovoltage machine by means of simultaneous irraditions in a bespoke Perspex phantom. For this aim, the calculation of the cross calibration factor for a Half Value Layer of 2.0mmCu is derived from the IPEM X- rays Code of Practice. For the absolute dose measurements in BT, single line plans are delivered with the normalization of 1.0Gy in the effective point of the chamber (nominally located at 4.8cm from the closest dwell position). A CT scan of the water equivalent slab phantom is used, including a rectangular insert which hosts consistently the mico- chamber and a 6F source guide. Results The data are recorded in an in-house program coded in Visual Basic 2010 and compared with the expected values estimated by BrachyVision Treatment Planning System (TPS) and Monte Carlo general purpose code Geant4. On average, the measured absorbed dose has been: 102.2 ± 1.0 cGy (k=2), as it is shown in Table 1 and Fig. 1. The expected point doses by the TPS and Geant4 are 100.0 and 100.5 cGy, respectively. The expanded uncertainty for every measurement has been 4.0% (k=2), estimated by quadratic propagation of the variance. This is slightly lower to the estimation found in the literature for TLDs, radio-chromic films, MOSFETs and diode detectors.