Clearance assurance for stereotactic radiosurgery and radiotherapy

The nature of stereotactic radiotherapy (SRT)/radiosurgery (SRS) requires the use of oblique non-coplanar beams to avoid critical structures and maximize target coverage. These beams are delivered via a combination of gantry, collimator, and couch rotations. Such beam orientations could result in the gantry colliding with the patient or couch. The outcome can be patient injury, damaged equipment, and unrealized treatment. Our objective in this work was to create a treatment planning tool that utilizes each unique patient geometry to quantify clearance for stereotactic beams. Emphasis was placed on developing a general platform that can completely, yet easily, define any user system. Gantry components were described by providing component dimensions to software that generates thousands of surface points. Table components were described as a combination of boxes and measured surface points. During the treatment planning process isocenter coordinates, patient dimensions and beam orientation were specified. Gantry components were then transformed into the table reference frame. The shortest distance between the gantry and patient or couch was computed and compared to a safety margin. This clearance assurance algorithm was developed in response to the need to reduce patient setup time, and to increase the range of potentially useful beams. The software was verified by measuring minimum gantry-table distances at multiple beam orientations and comparing to calculations. Differences between calculated and measured clearances were on the order of 1 cm. The software enabled the safe delivery of noncoplanar oblique beams that are difficult to visualize. The software was used successfully to assure clearance for 50 patients (366 beams). This useful clinical tool became an integral part of the stereotactic quality assurance protocol at St Luke's-Roosevelt Hospital Center.