Dosimetric characterization and behaviour in small X-ray fields of a microchamber and a plastic scintillator detector.
OBJECTIVE
The aim of this work was to investigate the main dosimetric characteristics and the performance of an A26 Exradin ionization microchamber (A26 IC) and a W1 Exradin plastic scintillation detector (W1 PSD) in small photon beam dosimetry for treatment planning system commissioning and quality assurance programme.


METHODS
Detector characterization measurements (short-term stability, dose linearity, angular dependence and energy dependence) were performed in water for field sizes up to 10 × 10 cm2. Polarity effect (Ppol) was examined for the A26 IC. The behaviour of the detectors in small field relative dosimetry [percentage depth dose, dose profiles often called the off-axis ratio and output factors (OFs)] was investigated for field sizes ranging from 1 × 1 to 3 × 3 cm2.


RESULTS
Results were compared with those obtained with other detectors we already use for small photon beam dosimetry. A26 IC and W1 PSD showed a linear dose response. While the A26 IC showed no energy dependence, the W1 PSD showed energy dependence within 2%; no angular dependence was registered. Ppolvalues for A26 IC were below 0.9% (0.5% for field size >2 × 2 cm2). A26 IC and W1 PSD depth-dose curves and lateral profiles agreed with those obtained with an EDGE diode. No differences were observed among the detectors in OF measurement for field sizes larger than 1 × 1 cm2, with average differences <1%. For field sizes <1 × 1 cm2, the effective volume of ionization chamber and non-water equivalence of EDGE diode become significant. A26 IC OF values were significantly lower than EDGE diode and W1 PSD values, with percentage differences of about -23 and -13% for the smallest field, respectively. W1 PSD OF values lay between ion chambers and diode values, with a maximum percentage difference of about -10% with respect to the EDGE diode, for a 6 × 6-mm2field size.


CONCLUSIONS
The results of our investigation confirm that A26 IC and W1 PSD could play an important role in small field relative dosimetry. Advances in knowledge: Dosimetric characteristics of Exradin A26 ionization microchamber and W1 plastic scintillation detector for small field dosimetry.
1 Medical Physics Department, AO Ordine Mauriziano di Torino, Turin, Italy.
Massimo
M
Pasquino
1 Medical Physics Department, AO Ordine Mauriziano di Torino, Turin, Italy.
Claudia
C
Cutaia
1 Medical Physics Department, AO Ordine Mauriziano di Torino, Turin, Italy.
Lorenzo
L
Radici
1 Medical Physics Department, AO Ordine Mauriziano di Torino, Turin, Italy.
Serena
S
Valzano
1 Medical Physics Department, AO Ordine Mauriziano di Torino, Turin, Italy.
Eva
E
Gino
2 Medical Physics Department, University Hospital, Verona, Italy.
Carlo
C
Cavedon
1 Medical Physics Department, AO Ordine Mauriziano di Torino, Turin, Italy.
Michele
M
Stasi
D006801:Humans; D008962:Models, Theoretical; D010315:Particle Accelerators; D017785:Photons; D010969:Plastics; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D012680:Sensitivity and Specificity; D014965:X-Rays
D010969:Plastics