Background: Small megavoltage photon fields are increasingly used in modern radiotherapy techniques such as stereotactic radiotherapy. Therefore, it is important to study the reliability of dosimetry in the small-field conditions. The IBA Razor Nano Chamber (Nano chamber) ionization chamber is particularly intended for small-field measurements. In this work, properties of the Nano chamber were studied with both measurements and Monte Carlo (MC) simulations.

Material and methods: The measurements and MC simulations were performed with 6MV, 6MV FFF and 10MV FFF photon beams from the Varian TrueBeam linear accelerator. The source-to-surface distance was fixed at 100cm. The measurements and MC simulations included profiles, percentage depth doses (PDD), and output factors (OF) in square jaw-collimated fields. The MC simulations were performed with the EGSnrc software system in a large water phantom.

Results: The measured profiles and PDDs obtained with the Nano chamber were compared against IBA Razor Diode, PTW microDiamond and the PTW Semiflex ionization chamber. These results indicate that the Nano chamber is a high-resolution detector and thus suitable for small field profile measurements down to field sizes 2 x 2 cm(2) and appropriate for the PDD measurements. The field output correction factors K-Qclin,K- (f)(c)(l)(i)(n)(,)(fmsr)(Qmsr )( ) and field OFs Omega(Qclin,) (f)(c)(l)(i)(n)(,)(fmsr)(Qmsr )( ) were determined according to TRS-483 protocol In the 6MV FF and FFF beams, the determined correction factors K-Qclin,K- (f)(c)(l)(i)(n)(,)(fmsr)(Qmsr )( ) were within 1.2% for the field sizes of 1 x 1 cm(2)-3 x 3 cm(2) and the experimental and MC defined field output factors Omega(Qclin,) (f)(c)(l)(i)(n)(,)(fmsr)(Qmsr )( ) showed good agreement.

Conclusion: The Nano chamber with its small cavity volume is a potential detector for the small-field dosimetry. In this study, the properties of this detector were characterized with measurements and MC simulations. The determined correction factors K-Qclin,K- (f)(c)(l)(i)(n)(,)(fmsr)(Qmsr )( ) are novel results for the NC in the TrueBeam fields.