AbstractID: 9701 Title: Comparison of Peripheral Dose Measurements to those Calculated by the Peridose Software Program for 6 MV X-rays from a Varian 2100 EX Accelerator Peridose1,2 , a free software calculation program, assists users in approximating the dose to critical organs at large distances from external beam treatment fields. Such an estimate is very useful for instance when a pregnant patient or a patient with a pacemaker is being considered for treatment. The software’s calculation is based on measured data and largely takes into account X-ray energy, depth of critical organ, distance from the treatment fields, field geometry, scatter from cerrobend blocks, scatter from the collimator, wedge attenuation and scatter, head leakage, and couch attenuation. The program distinguishes between treatments involving either orthogonal or tangential beams. We have tested the software for distances up to 40 cm from a 6 MV, 10x10 cm of a Varian 2100EX accelerator and depths up to 12 cm. Calculated results agreed with measured results to within 30% at distances greater than 10 cm from an open beam. This agreement is well within the 60% (two standard deviations) accuracy claimed by the author of the software. 1. van der Giessen, Radiotherapy and Oncology: 58, 2, (2001), pages 209-213 2. Peridose is currently available from the Medical Physics Software Exchange list maintained by Carter Schroy.