Nondosimetric quality assurance of radiotherapy treatment planning system using multi-leaf collimator beam geometry phantom

Radiotherapy treatment planning system (RTPS) plays an important role in overall treatment delivery process. Nondosimetric quality assurance (QA) of the RTPS was carried out to assess the accuracy of nondosimetric parameters of the RTPS for regular/irregular fields obtained with jaws/multi-leaf collimator (MLC) using a dedicated MLC beam geometry phantom. Simulated radiation beams of field sizes 1 x 2 cm, 10 x 10 cm and 15 x 15 cm were created in RTPS using jaws for combinations of (i) 0° couch and gantry angles, and (ii) 323° gantry and 204° couch angles on computed tomography (CT) images of the phantom. Digitally reconstructed radiographs (DRR) for these setups were also generated. MLC co-ordinates were set in the RTPS corresponding to preset irregular field (formed by over travel of A or B bank of leaves) and resulting leaves positions were manually adjusted to fit the structures provided in the phantom. The dimensions of known geometries were measured and compared against the actual dimensions. The variation in measured and expected values of field sizes created by jaws was within 1.8 mm. In the case of DRR for 0° couch and gantry angles, the variation ranged from - 2.7 mm to 1.7 mm and for 204° couch and 323° gantry angles it was in the range of - 1.5-1.3 mm. The maximum variation between set leaf positions and manually adjusted positions for irregular field of MLC were found in the range of 2-4 mm. Nondosimetric QA of an RTPS was carried out, and results of the test provide confidence for its safe use for clinical practice. (author)