Concepedia

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy

Thomas LoSasso, Chen‐Shou Chui, C. Clifton Ling

Medical Physics · 1998 · 440 citations · 14 references

TL;DR

The use of a multileaf collimator in dynamic mode for intensity‑modulated radiotherapy began at our institution in 1995, but unlike static field treatments, dynamic therapy introduces significant dosimetric challenges that must be evaluated. The study aims to assess how the mechanical accuracy of the multileaf collimator, from calibration to delivery, influences the dose delivered in dynamic intensity‑modulated radiotherapy. The authors performed calculations and measurements—including head‑scatter, collimator and leaf‑end transmission, and ion‑chamber and film dosimetry for 6 and 15 MV beams—to evaluate the influence of MLC mechanical accuracy on dose delivery. Uncorrected head‑scatter, transmission, and leaf‑end effects can alter prostate dose by 5–20 % in a typical plan.

Abstract

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.