FINE-TUNING OF LINEAR-ACCELERATOR ACCESSORIES FOR STEREOTAXIC RADIOTHERAPY

Purpose: Experience with the University of Wisconsin's stereotactic ra diotherapy (SRT) accessory system was applied to build a new system, f acilitate alignment of linac photon beams with a Brown-Roberts-Wells ( BRW) stereotaxy, and increase the versatility and stability of the ste reotaxy. Methods and Materials: High tensile strength stainless steel was used in the floor stand to increase the range of gantry rotation r elative to ranges allowed by truss-mounted stands. The collimator asse mbly and floor stand were each fitted with two-axis gimbal and transla tion adjustments in addition to the floor stand's three-axis adjustmen ts. The head ring positioning assembly was fitted with two braces to p revent the head ring from deforming with patient motion. Six MV linac photon beam characteristics were measured with a computer-controlled s canning system and a diode in water, at source to surface distances (S SD) of 80 and 100 cm, and for 13 divergent collimators ranging in diam eter from 1-4 cm at 100 cm SSD. Quality assurance software was applied to screen data for questionable consistency or symmetry. Integrity of the stereotaxy was evaluated with target simulation films and repeate d measurements which were part of the quality assurance of clinical tr eatments. A method was developed using a glass etched contact reticle to obtain average simulated target to beam center distances (deltaav) from target simulation films. Results and Conclusion: New aspects of t he current system have improved the ability to fine tune and analyze s tereotactic alignment. Beam characteristics met stringent output crite ria and penumbral widths were the same or narrower than penumbral widt hs reported elsewhere. The precision of measuring deltaav was 0.1 mm, and deltaav averaged over 50 target simulation films was 0.7 +/- 0.1 m m. Results suggest that it may be useful to determine deltaav from tar get simulation films with the method described here.