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Reference dosimetry and small-field dosimetry in external beam radiotherapy: Results from a Danish intercomparison study

A comparison of dosimetry methods at different clinics can be used as a means to uncover systematic uncertainties
in ra-diotherapy. To assess the current status of reference dosimetry and small-field dosimetry in clinical
practice, a collaborative comparison study involving several dosimetry methods was performed by DTU Nutech
at six Danish clinics. The first part of the intercomparison regarded the consistency of reference dosimetry. Absorbed
dose to water under reference conditions was measured using a Farmer ionization chamber, and was
found to agree within 1 % with the daily dose checks obtained routinely at each clinic. The second part of the
study concerned the accuracy of small-field dosimetry and dose calculations. The geometric size of small fields
down to 1 cm x 1 cm was measured using radiochromic film. Minor discrepancies were seen between the nominal
field sizes set by the collimators and the measured field sizes, although one clinic showed field dimensions
that were down to 21 ± 3 % smaller than expected. Small-field correction factors were estimated for a PinPoint
chamber and a diamond detector using a fibre-coupled organic scintillator as reference, after correcting for volume
averaging. The corrections were found to be within 2 % down to the 1 cm x 1 cm field size. Output factor
measurements performed with the three detectors were compared with the commissioning beam data originally
acquired by the individual clinics using their own detectors and protocols, and with dose calculations performed
using the treatment planning systems. Measured output factors agreed within 3 % with commissioning beam
data and within 2 % with dose calculations for small MLC-defined fields. The study demonstrated (i) consistency
of reference dosimetry and small-field dosimetry on a national level, and (ii) clinical applications of fibercoupled
plastic scintillators. The study also demonstrated that the estimation of detector-specific correction factors
in small fields is consistent among clinics and linac models, supporting the robustness and usefulness of the
proposed IAEA formalism for detector-specific correction factors for non-reference fields.

Anders R. Beierholm, Claus F. Behrens, Patrik Sibolt, Heidi S. Rønde, Susan B.N. Biancardo, Marianne C. Aznar, Jakob B. Thomsen, L. Præstegaard, L. Nyvang, Hans L. Riis, Kurt M. Pedersen, Jakob Helt-Hansen and Claus E. Andersen

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