Compensation of x-ray beam penumbra in conformal radiotherapy.

In radiotherapy, the gross tumor volume is surrounded by a clinically defined margin to allow for the presence of undetected malignant cells. Additional margins are added to accommodate positioning uncertainties and organ motion, creating a planning target volume, or PTV. Finally, a margin is included in the beam apertures surrounding the PTV to account for the dose fall-off at the beam edges (i.e., the "penumbra"). For higher energy beams and for low density tissues adjacent to the PTV, the beam aperture margin should be increased to account for the increased range of scattered photons and electrons. However, increased margins also increase the volume of normal tissue irradiated. In this work, the beam aperture margin is reduced by using filters and multileaf collimator (MLC) techniques to create compensating rinds of increased beam intensity. These compensation techniques were evaluated for 6 and 18 MV x rays by calculating penumbral widths as a function of the increased beam intensity in the rind, the rind width, and tissue density. Dose calculations were performed using a 3D superposition algorithm, which includes an extrafocal source model. Calculations were validated experimentally with film dosimetry. Results show the distance between the 95%-50% isodose lines is reduced from 11 mm to 4 mm for 6 MV x rays in the lung phantom, when the beam intensity is increased by 20% in a 10 mm wide rind. At 18 MV, this distance is reduced from 16 mm to 6 mm with a 20% increase in rind intensity, but a 15 mm wide rind is required. In all cases, penumbra compensation did not result in any appreciable increase in scatter dose outside the field boundaries. These results suggest that penumbra compensation is a practical means of controlling the beam aperture margin.