Radiotherapy to targets with concave surfaces: planning and clinical execution by intensity modulation using a static-segmentation technique

When the target volume has concavities in its surface, it may be impossible to find 'all-in' beam incidences that isolate the target volume from the tissues at risk. In order to obtain a 3D-conformal dose distribution, beam intensity modulation may be required. We have developed a heuristic planning procedure that allows to obtain such dose distributions that selectively exclude those tissues at risk indenting into the target. During the planning process, a number of beam incidences circumscribing the tissues at risk, are defined. Dose homogeneization at the target is obtained by calculation of intensities given to beam segments of specific, predetermined geometry. This specific geometry is required to maximize the area of each segment and than reduces the number of segments. Treatments were planned using the GRATIS[reg] planning system developed by Sherouse and were executed using the MLC Philips multileaf collimator in a static, sequential mode. Generally, 6-8 beam incidences are required to obtain a 80-85 % dose homogeneity at the target. The number of segments is proportional to the volume of the target. Treatments executed required 30-50 segments. The planning, dosimetry and clinical execution of actually treated cases will be discussed