J. E. Bayouth

The objective of this work was to measure the performance characteristics of a double‐focus multileaf collimator (MLC) for intensity modulated radiation therapy (IMRT), specifically the variation in penumbra and leakage for narrow fields as a function of field position over a 20×27 cm space available for segmented MLC IMRT. Measurements were made with 6 MV x rays through a MLC containing 29 leaf pairs (27 pairs of 1 cm width), and EDR2 film at 10 cm depth in solid water at 100 cm SAD. Films were digitized with 0.17 mm resolution and converted to dose. Interleaf and intraleaf transmission were measured along 11 vertical profile locations. Leaf‐end transmission was measured along horizontal profiles for each of 9 different leaf abutments, traveling over a 20 cm range. In‐plane penumbra measurements were made through a single leaf retracted, for 7 different leaves. Cross‐plane penumbra (leaf‐end) measurements were made for all 27 leaf pairs, where the 1 cm field width was placed in 11 different off‐axis positions (20 cm range). Interleaf leakage (range 1.0%–1.5%), intraleaf transmission (range 0.6%–0.8%), and leaf‐end transmission (range 0.8%–2.7%) were consistent for all leaf pairs at a given abutment position. The penumbra for these 1‐cm‐wide fields was measured to be 0.36 cm±0.03 cm for 99% of the measurements. In conclusion, the penumbra and leakage of the double‐focus MLC were remarkably consistent for the range of leaf positions studied, producing dosimetric characteristics that are well suited for IMRT segments where opposing leaf pairs are often separated by 10 mm or less.

Intensity modulated radiotherapy (IMRT) requires extensive knowledge of multileaf collimator (MLC) leaf positioning accuracy, precision, and long-term reproducibility. We have developed a technique to efficiently measure the absolute position of each MLC leaf, over the range of leaf positions utilized in IMRT, based on dosimetric information. A single radiographic film was exposed to 6 MV x-rays for twelve exposures: one open field with a radio-opaque marker tray present, and eleven fields (1 x 28 cm strips via 1 cm gaps between opposed leaf pairs) separated by 2 cm center to center. The process was repeated while varying direction of leaf travel; each film was digitized using a commercial film dosimetry system. The digital images were manipulated to remove translation and rotation of the film data with respect to the collimator coordinate system by extraction of radiation dose profiles perpendicular to the MLC leaf motion and measuring the center of the x-ray leakage between leaves. Radiation dose profiles in the direction of leaf motion were acquired through the center of each leaf pair (leaves 2-28), which provided leaf position information every 2 cm with 0.2 mm precision. Nine separate leaf reproducibility studies over a 90 day period which evaluated 600 measurement points on each film show 0.3 mm precision for 95% confidence, while hysteresis studies show 0.5 mm precision. Absolute leaf position error measurements demonstrated a radial dependence, with a maximum of 1.5 mm at 16.4 cm from central axis, due to rotational error at calibration. Recalibration of the MLC leaves based utilizing this tool yields absolute leaf position measurements where 91.5% of all leaves/positions were within 0.5 mm, with a mean error of 0.1 mm and a maximum error less than 1.0 mm.