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

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.

IMRT clinical implementation: Prostate and pelvic node irradiation using Helios and a 120‐leaf multileaf collimator

“…CadPlan requires only one value for the MLC transmission per energy. It has been shown 6 that for 6 MV photons this value changes with depth and field size (range 1.8–2.4%). In our measurements the transmission through the leaves was found to be 1.6% and 1.8% at depths of 5 and 15 cm for 6 MV photons.…”

“…, 6 The degree of accuracy of the calculated dose distribution with measurements depends on the configuration of the pencil beam algorithm and the MLC parameters entered into the planning system. The accuracy of the pencil beam algorithm kernels in modelling the scatter and field edges has an impact on the small field segments encountered in IMRT.…”

“…There are two ways of deriving this parameter. One method is a variation of that proposed by LoSasso et al , 6 to verify the data by plotting net dose against gap width by moving slits of different leaf gaps and measuring the transmission. Dose is measured using a sliding window, which produces a uniform field, and an ionization chamber with a build‐up cap positioned off the end of the couch.…”

Determining the optimal dosimetric leaf gap setting for rounded leaf‐end multileaf collimator systems by simple test fields

“…Individual patient quality assurance (QA) is required for MLC‐involved intensity‐modulated plans (i.e., intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT)) to verify agreement between the planned and delivered doses, which is critical for patient care (1) . Over two decades, MLC properties, their uncertainties, and their potential clinical effects have been extensively investigated 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 . With recently developed narrow leaf MLC systems and their applications in hypofractionated treatment, it is necessary to more accurately determine the properties and associated uncertainties.…”

Verification of dynamic and segmental IMRT delivery by dynamic log file analysis

“…Most often the effects of leaf motion are inferred from dose deviations on film or an electronic portal imaging device 1 5 Likewise, the dosimetic effects of leaf position uncertainty may be determined from variations in ionization measurements 6 . These techniques provide dosimetric information but do not provide detailed information for diagnosing delivery problems.…”