Purpose: We describe the experimental demonstration of the delivery of a three-dimensional conformal radiotherapy dose distribution using in-field modulation of nine fixed-gantry fields.
Methods and materials: Two-dimensional in-field modulation profiles, varying from field to field, were realized by quasi-dynamic multileaf collimation using the prototype of a commercially available multileaf collimator installed on a medical linear accelerator. The profiles were calculated to deliver an optimal dose distribution for a patient with a prostate carcinoma. The target volume surface was invaginated and bifurcated. The calculated dose distribution was delivered to a homogeneous polystyrene phantom consisting of 1 cm thick slices that were cut to match the patient's outer contour. Seven therapy verification films were placed between the phantom slices.
Results: Analysis of the films revealed a degree of conformation of the high-dose region to the target shape that would not be possible with unmodulated conformal therapy. However, small observed spatial displacements of the dose distribution confirm the need for very accurate positioning.
Conclusions: It is feasible to deliver clinically relevant, three-dimensional dose distributions that conform to invaginated and bifurcated target volumes using fields modulated by multileaf collimators.