Beam intensity modulation to reduce the field sizes for conformal irradiation of lung tumors: A dosimetric study

Purpose: In conformal radiotherapy of lung tumors, penumbra broadening in l ung tissue necessitates the use of larger field sizes to achieve the same t arget coverage as in a homogeneous environment. In an idealized model confi guration, some fundamental aspects of field size reduction were investigate d, both for the static situation and for a moving tumor, while maintaining the dose homogeneity in the target volume by employing a simple beam-intens ity modulation technique. Methods and Materials: An inhomogeneous phantom, consisting of polystyrene, cork, and polystyrene layers, with a 6 x 6 x 6 cm(3) polystyrene cube insi de the cork representing the tumor, was used to simulate a lung cancer trea tment. Film dosimetry experiments were performed for an AP-PA irradiation t echnique with 8-MV or 18-MV beams. Dose distributions were compared for lar ge square fields, small square fields, and intensity-modulated fields in wh ich additional segments increase the dose at the edge of the field. The eff ect of target motion was studied by measuring the dose distribution for the solid cube, displaced with respect to the beams. Results: For the 18-MV beam, the field sizes required to establish a suffic ient target coverage are larger than for the 8-MV beam. For each beam energ y, the mean dose in cork can significantly be reduced (at least a factor of 1.6) by decreasing the field size with 2 cm, while keeping the mean target dose constant. Target dose inhomogeneity for these smaller fields is limit ed if the additional edge segments are applied for 8% of the number of moni tor units given with the open fields. The target dose distribution averaged over a motion cycle is hardly affected if the target edge does not approac h the field edge to within 3 mm. Conclusions: For lung cancer treatment, a beam energy of 8 MV is more suita ble than 18 MV. The mean lung dose can be significantly reduced by decreasi ng the field sizes of conformal fields. The smaller fields result in the sa me biological effect to the tumor if the mean target dose is kept constant. Intensity modulation can be employed to maintain the same target dose homo geneity for these smaller fields. As long as the target (with a 3 mm margin ) stays within the field portal, application of a margin for target motion is not necessary, (C) 1999 Elsevier Science Inc.