Evaluation of dosimetric performance of "Siemens ONCOR" linear accelerator with small monitor unit values and small segment sizes and consistency with treatment planning system

OBJECTIVES The aim of our study was to evaluate the performance of the "Siemens ONCOR Avant-Garde" linear accelerator under intensity modulated radiation therapy (IMRT) conditions. The beam stability for small monitor units in terms of dose linearity, inter-segment variations and profiles was investigated. Validation of doses at small size segments was done to compare with measurement and calculation dose values. METHODS Measurements of linearity and beam stability with small monitor units were done using 0.6 cc "Farmer" ion chamber and PTW-LA48 linear detector with 47 diodes. Validation of doses for small segment sizes was done for comparison of the values obtained with the PTW Si diode detector and the calculated values on the planning system. RESULTS Dose linearity of small monitor units was found to be <3% for all monitor unit settings. Although inter-segment variations for >2 monitor unit setting were measured at an acceptable value, 3%, <2 monitor unit setting was measured as larger than 3%. Comparison of doses at small segment sizes showed unacceptable value for <2x2 cm(2). Measurements of doses for >2x2 cm(2) were consistent (<3%) with calculated doses from the planning system under the same conditions. CONCLUSION The dose characteristics of Siemens ONCOR Avant-Garde linear accelerator with small monitor units and small segment sizes were found to be suitable for "Step & Shoot" IMRT.