Thomas LoSasso

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.

The delivery of intensity modulated radiation therapy (IMRT) using dynamic multileaf collimation (DMLC) accounts for about 40% of all treatments on eight multileaf collimator (MLC)‐equipped linacs currently at the central and satellite facilities of the Memorial Sloan Kettering Cancer Center (MSKCC). These numbers have been steadily growing for the past 6 years. Treatment with IMRT has been used for a variety of cancers, including tumors of the prostate, head and neck, breast, paraspinal regions, and brain. To ensure that patients are treated in an accurate, efficient, and safe manner, we have developed a comprehensive QA program including mechanical quality assurance, which monitors known potential weaknesses in the MLC device itself, and verification of patient‐specific treatments, to confirm the reliability of new software and to identify errors introduced through human interaction with individual patient's plans. This paper presents the reasoning, methodology, and results for mechanical, dosimetric, and electronic tests that are conducted at MSKCC.

Beam profiles along the nonwedged direction of a wedged field produced by a linear accelerator exhibit more "sagging" than that of an open field at the same depth. For large fields, the profiles of open and wedged fields can differ by as much as 7%. The extra "sagging" of wedged profiles is mainly due to the difference in penetration between on- and off-axis rays caused by the variation of beam quality across the field. An algorithm was developed to estimate an "effective" depth such that the profile of a wedged field can be approximated by the open-field profile at the effective depth. The algorithm was verified by measured beam profiles for 6- and 15-MV x-ray beams for 15 degree, 30 degree, 45 degree, and 60 degree wedges.