Small field relative dosimetry using a silicon diode of new generation

Introduction Small field dosimetry is increasingly important due to the development of complex treatment techniques. Standard dosimetric procedures are mandatory to ensure the same treatment in each radiotherapy center. Purpose A multicentric study on small fields dosimetry for the major linear accelerator manufacturers, using two IBA unshielded silicon diode RAZOR in 30 different institutions in Italy is reported. Data were collected for each linac manufacturer and model with the main objective to provide indications to beginner institutions. Materials and methods In and cross-plane beam profiles and Relative Output ratios were measured for each field size at depth = 10 cm and at SSD = 90 cm. The long axis of the diode detector was placed parallel to the beam with the active volume positioned at the isocenter. The profiles were compared in terms of 80–20% penumbra. Output factors were calculated with respect to a jaw-collimated square field of 3 × 3 cm 2 and as a function of effective field size EOF. Results EOF of all linacs were in agreement within a few per mille from 5.6cm to 1cm FS. For FS below 1cm it is evident a different trend depending on different linac models. Data were fitted with a polynomial showing coefficients of determination R 2 > 0.98 . Penumbra measurements were in agreement each other for each FS and accelerator model. Conclusion Multicentric data have been collected with the same instrumentation and protocol setup, providing good accuracy in the measurements and allowing to create gold standard values to be used by those institutions who intend to implement new performing treatment techniques.