Compensation of x-ray beam penumbra in conformal radiotherapy

In radiotherapy, the gross tumor volume is surrounded by a clinically defin ed margin to allow for the presence of undetected malignant cells. Addition al margins are added to accommodate positioning uncertainties and organ mot ion, creating a planning target volume, or PTV. Finally, a margin is includ ed in the beam apertures surrounding the PTV to account for the dose fair-o ff at the beam edges (i.e., the ''penumbra"). For higher energy beams and f or 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 ti ssue irradiated. In this work, the beam aperture margin is reduced by using filters and multileaf collimator (MLC) techniques to create compensating r inds of increased beam intensity. These compensation techniques were evalua ted 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 densi ty. Dose calculations were performed using a 3D superposition algorithm, wh ich includes an extrafocal source model. Calculations were validated experi mentally 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 ph antom, 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 c ompensation did not result in any appreciable increase in scatter dose outs ide the field boundaries. These results suggest that penumbra compensation is a practical means of controlling the beam aperture margin. (C) 2000 Amer ican Association of Physicists in Medicine. [S0094-2405(00)02408-1].