Development of the measurement method for MLC small field output factor in intensity modulated radiation therapy(IMRT)

Objective: To develop the methods for using 0.015 cc pinpoint chambers, 0.007 cc miniature chambers and diode detector to measure Multi-leaf collimator (MLC) small field in IMRT. Methods: MAX4000 and Unidos electrometers were connected with different types of small chambers and diode detectors. MLC shaped fields of 10 cm × 10 cm, 6 cm × 6 cm, 4 cm × 4 cm, 3 cm × 3 cm, 2 cm × 2 cm were defined at 100 cm SSD. The field sizes for the Varian accelerator were defined by the tertiary MLC, while the secondary jaws were kept at 10 cm × 10 cm field, with the monitor units of 250 MU. Each field was measured three times to obtain the average value. The readings of all small fields were normalized to 10 cm × 10 cm field values for comparison of measured and published output factors. Results: The relative deviations of the MLC small field output factors from the published outputs are 1.0%, 1.7%, 1.5% and 2.4%, respectively, for Unidos electrometer connected with 0.015 cc pinpoint chamber; 0.2%, 0.8%, 0.8% and 1.4%, respectively, for Unidos electrometer connected with 0.007 cc miniature chamber; and 0.1%, 0.5%, 0.5% and 0.9%, respectively, for MAX4000 electrometer connected with 0.007 cc miniature chamber. Conclusions: The 0.015 cc chamber-measured MLC output factors for 3 cm × 3 cm and 2 cm × 2 cm fields are excellent. As required by IAEA, the relative deviations of the measured output factor from the published output factor are within ±2% for 2 cm × 2 cm fields and ±3% for larger fields. The results measured using 0.007 cc chamber are better than those measured using 0.015 cc chamber. The measured results using the diode detector, normalized to the 10 cm × 10 cm field, are consistent with the minimum requirements and excellent when being normalized to the 4 cm × 4 cm field. For dosimetric consideration, MLC small field output factor should be measured using small chamber and diode detector. The method is accurate and reliable, therefore, all measured output factors for MLC small fields should be input into radiation treatment plan system. (authors)