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 compari-son study involving several dosimetry methods was performed by DTU Nutech at six Danish clinics.

The first part of the intercompa-rison 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 discre-pancies were seen between the nominal field sizes set by the colli-mators 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 cham-ber and a diamond detector using a fibre-coupled organic scintilla-tor as reference, after correcting for volume averaging. The correc-tions were found to be within 2 % down to the 1 cm x 1 cm field size. Output factor measurements performed with the three detec-tors 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 le-vel, and (ii) clinical applications of fiber-coupled plastic scintilla-tors.

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.