Ionization chamber‐based reference dosimetry of intensity modulated radiation beams

Abstract

The present paper addresses reference dose measurements using thimble ionization chambers for quality assurance in IMRT fields. In these radiation fields, detector fluence perturbation effects invalidate the application of open-field dosimetry protocol data for the derivation of absorbed dose to water from ionization chamber measurements. We define a correction factor C(Q)IMRT to correct the absorbed dose to water calibration coefficient N(D, w)Q for fluence perturbation effects in individual segments of an IMRT delivery and developed a calculation method to evaluate the factor. The method consists of precalculating, using accurate Monte Carlo techniques, ionization chamber, type-dependent cavity air dose, and in-phantom dose to water at the reference point for zero-width pencil beams as a function of position of the pencil beams impinging on the phantom surface. These precalculated kernels are convolved with the IMRT fluence distribution to arrive at the dose-to-water-dose-to-cavity air ratio [D(a)w (IMRT)] for IMRT fields and with a 10x10 cm2 open-field fluence to arrive at the same ratio D(a)w (Q) for the 10x10 cm2 reference field. The correction factor C(Q)IMRT is then calculated as the ratio of D(a)w (IMRT) and D(a)w (Q). The calculation method was experimentally validated and the magnitude of chamber correction factors in reference dose measurements in single static and dynamic IMRT fields was studied. The results show that, for thimble-type ionization chambers the correction factor in a single, realistic dynamic IMRT field can be of the order of 10% or more. We therefore propose that for accurate reference dosimetry of complete n-beam IMRT deliveries, ionization chamber fluence perturbation correction factors must explicitly be taken into account.

Clinical implementation and error sensitivity of a 3D quality assurance protocol for prostate and thoracic IMRT

“…Although there are certain advantages of both methods, there are commonly acknowledged drawbacks. A drawback for point measurement is its unreliability when the tumor size is similar to the chamber's active volume or the chamber is placed in high‐dose gradient areas (3) . In addition, a single ion chamber measurement is a poor surrogate for analysis of a complex type of treatment such as IMRT.…”

Small field dosimetry: What have we learnt?

“…However, when field size deceases to smaller the range of charged particle, electronic equilibrium cannot be established and Bragg-Cavity theory cannot be used. There had been surge of literature (12)(13)(14)(15)(16)(17)(18)(19) searching the solution in such situation. We will not dwell on cavity theory in this paper, however we refer to various published papers on this subject.…”

Reference dosimetry using radiochromic film

“…The Farmer chamber measurement gives a gross estimate of the systematic error either introduced during the delivery or due to the limitations of the TPS in calculating the dose accurately. However, it is important to also consider that ionization chambers are subject to errors when measuring IMRT fields, (29) even when placed in low‐gradient regions within the plan, such as was the case here. The corrections were carried out under the assumption that the measurements performed with the Farmer chamber were done under ideal conditions.…”