COLLISION-AVOIDANCE IN COMPUTER-OPTIMIZED TREATMENT PLANNING

Of major concern in fully automated computerized treatment delivery is the possibility of gantry/couch or gantry/patient collisions. In this work, software has been developed to detect collisions between gantry and couch or patient for both transaxial and noncoplanar treatment fi elds during the treatment planning process. The code uses the gantry a ngles, turntable angles, and position of the couch surface relative to the isocenter supplied by the planner for the prescribed radiation fi elds. In addition, the maximum patient anterior-posterior and lateral separations are entered in order to model the patient outline by a con servative cylindrical ellipse. By accessing a database containing the precise mechanical dimensions of the therapy equipment, 3D analytical geometry is used to test for collisions between gantry/patient and gan try/couch for each treatment field. When collisions are detected, the software inspects the use of an extended distance treatment, by recalc ulating and testing for collisions, with the couch at a greater distan ce from the collimator along the direction of the central axis. If a c ollision is avoided at extended distance, the lateral, longitudinal, a nd vertical motions of the couch are recorded for entry into the treat ment plan, or else a warning message is printed, together with the nea rest permissible collision-free gantry angle. Upon inspection, the pla nner can either elect to use the calculated closest permissible gantry angle or reject the plan. The software verifies that each proposed tr eatment field is safe, but also that the transition between fields is collision-free. This requires that the sequence of the treatment field s be ordered, preferably into a sequence which minimizes the delivery time compatible with patient safety. A separate program, referred to a s a segment manager, has been developed to optimize the sequence of th e treatment planner's radiation fields. When collisions are possible o n the transition between treatment fields, intermediate couch position s are inserted into the treatment delivery procedure to ensure a conti nual safe distance between both the couch and patient and the moving g antry. Treatment segments are organized according to gantry angle, beg inning at 180-degrees, and sequentially stepping clockwise around the patient. The use of extended distance treatment is proposed as a means of increasing the safety zone between the patient and the gantry. As the gantry rotates, one proposal to maximize patient safety would main tain the couch at extended distance. In this case, the couch prescribe s a circle around the isocenter, which is maintained at 180-degrees ou t of phase with the gantry motion.