Dosimetric verification of a commercial 3D treatment planning system for conformal radiotherapy with a dynamic multileaf collimator

The dosimetric accuracy of a 3D treatment planning system (TPS) for conform al radiotherapy with a computer-assisted dynamic multileaf collimator (DMLC ) was evaluated. The DMLC and the TPS have been developed for clinical applications where dy namic fields not greater than 10 x 10 cm(2) and multiple non-coplanar arcs are required. Dosimetric verifications were performed by simulating conform al treatments of irregularly shaped targets using several arcs of irradiati on with 6 MV x-rays and a spherical-shaped, tissue-simulating phantom. The accuracy of the delivered dose at the isocentre was verified using an ioniz ation chamber placed in the centre of the phantom. Isodose distributions in the axial and sagittal planes passing through the centre of the phantom we re measured using double-layer radiochromic films. Measured dose at the iso centre as well as isodose distributions were compared to those calculated b y the TPS. The maximum percentage difference between measured and prescribe d dose was less than 2.5% for all the simulated treatment plans. The mean ( ISI) displacement between measured and calculated isodoses was, in the axia l planes, 1.0+/-0.6 mm, 1.2+/-0.7 mm and 1.5+/-1.1 mm for 80%, 50% and 20% isodose curves, respectively, whereas in the sagittal planes it was 2.0+/-1 .2 mm and 2.2+/-2 mm For 80% and 50% isodose curves, respectively. The resu lts indicate that the accuracy of the 3D treatment planning system used wit h the DMLC is reasonably acceptable in clinical applications which require treatments with several non-coplanar arcs and small dynamic fields.