Some limitations in the practical delivery of intensity modulated radiation therapy.

Abstract

The resolution characteristics of intensity modulated beam (IMB) profiles produced by milled compensators and by multileaf collimators (MLCs) are independently investigated with respect to the primary fluence. It is shown that both methods have different characteristics in the longitudinal and lateral direction and, as a consequence, the resolutions of the longitudinal and lateral delivered IMB profiles differ. For both methods, the restrictions are identified. For compensators, the maximum slopes in the machining process, which should not be exceeded, are quantified. For MLCs, emphasis is given to the direction perpendicular to leaf movement. A number of test modulations were created and the effect of different size MLCs on the intensity profile revealed that unacceptable errors can be introduced if the profiles are heavily modulated. The production of intensity modulated radiation therapy (IMRT) beams by both machined compensators or by MLCs is limited by physical constraints. Having identified these constraints, some steps should now be taken to accommodate them either in the objective function for the calculation of the beam profiles or in the delivery system.

Compensators: An alternative IMRT delivery technique

“…In the worstcase scenario, where the field dimension perpendicular to the milling path is 25 cm, the resolution perpendicular to the milling path is 3.9 mm, less than the drill bit diameter of 6 mm. A detailed theoretical analysis on the influence of the drill size and other factors on the accuracy of compensator-generated intensity modulation was carried out by Meyer et al (2) The finite drill size is not considered in the dose optimization. However, the error in compensator-generated intensity map due to milling limitation is reflected and evaluated in the IMRT QA procedure.…”

“…MLC-based IMRT techniques have shown several drawbacks in clinical application. (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13) For instance, the total monitor units (MUs) required for a segmental MLC-IMRT treatment are often much higher than that of the corresponding nonintensity modulated treatment. As a result, the treatment delivery time is often considerably extended, and concerns about radiation contamination of the prolonged beam-on time are also raised.…”

“…In the last decade compensator techniques have been used for delivering IMRT treatments designed by dose optimization algorithms. (2,(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26) Customized compensators are shaped to attenuate the open-field photon fluence such that the transmitted fluence map is as designed by the dose optimization algorithm. The obvious advantage of this IMRT delivery method is simplicity.…”

IMRT with Compensators for Head-and-Neck Cancers

“…In the study by Meyer et al [19], some limitations are reported because of a maximum slope of the machining process and the use of spherical cutter. These effects depend on the machine used.…”

The delivery of IMRT with a single physical modulator for multiple fields: a feasibility study for paranasal sinus cancer

“…It can be used on any linear accelerator, it has improved spatial resolution, and the simultaneous beam intensity modulation avoids the potential interplay of MLC leaf motion and internal organ motion. The largest delivery limitations with MLC-IMRT occur because of the width of the leaves (perpendicular to leaf motion) (1). Reducing leaf width reduces this inaccuracy, but modulator-IMRT has the capability of spatial resolution higher than any current MLC.…”