The role of CT simulation in whole-brain irradiation

Purpose: Evidence is growing that incorrect field-shaping is a major cause of treatment failure in whole-brain irradiation (WBI). To evaluate the pote ntial benefits of CT simulation in WBI we compared field-shaping based on 3 D CT simulation to conventional 2D simulation Methods: CT head scans were obtained from 20 patients. Conventional 2D plan ning was imitated by drawing the block contours on digitally reconstructed radiographs (DRR) by four radiotherapists. Critical parts of the target and the eye lenses were subsequently marked and planning was repeated using 3D information ("3D planning"). The results of both methods were compared by evaluation of the minimal distance from the field edge according to each si te. Results: In conventional planning using DRR, major geographic mismatches (< -3 mm) occurred in the subfrontal region and both eye lenses with 1% each location. Minor mismatches (-3 to 0 mm) predominantly occurred in the contr alateral lens (21%), ipsilateral lens (10%), and subfrontal region (9%). Cl ose margins (0-5 mm) were most frequently noted at the contralateral lens ( 49%), ipsilateral lens (35%), and the subfrontal region (28%). When 3D plan ning was used, mismatches were not found. However, close margins were inevi table at the ipsilateral lens (5%), subfrontal region (30%), and contralate ral lens (70%). Conclusions: CT simulation in WBI is significantly superior to conventional simulation with respect to complete coverage of the target volume and prot ection of the eye lenses. The narrow passage between the ocular lenses and lamina cribrosa represents a serious limitation. These patients are safely identified with CT simulation and can be referred for modified irradiation techniques. (C) 1999 Elsevier Science Inc.