Verification of Buildup Region Dose Calculation of a Treatment Planning System

Buildup region dose calculation of Pinnacle3 (version 9.0), a treatment planning system (TPS) commissioned using cylindrical ionization chamber measurements, have been verified experimentally. Dose values measured using Attix parallel plate ionization chamber were compared with those calculated by Pinnacle3 for a variety of clinical setups involving: 6 MV and 15 MV photon beams, open fields, enhanced dynamic wedges, physical wedges, block tray, 85, 100 and 120 cm source‐to‐surface distances, and square field sizes ranging from 3 × 3 to 30 × 30 cm2. The dose difference (DD) and distance‐to‐agreement (DTA) techniques were used to evaluate the discrepancy between measured and calculated dose values. Based on the criteria of DD less than 2% or DTA less than 2mm, 93.7% of 1,710 dose points for the 6 MV and 96.1% of 2,244 dose points for 15 MV passed. Dose points that failed were mostly for open fields, block tray fields, and physical wedges (15 and 30 degrees) fields; this is attributed to high electron contamination (EC) associated with these fields. The levels of discrepancies between measured and calculated dose values were greatly reduced after remodeling the EC in Pinnacle3 using Attix chamber measurements, an indication that the EC equation in Pinnacle3 may be adequate for modeling EC in the dose buildup region, and the commissioning of a TPS using cylindrical ionization chamber measurements may not provide accurate buildup region dose calculation. Attix chamber measurements were validated using GafChromic EBT2 film; the disagreement was less than 3% for 89.9% of dose values compared. © 2012, American Association of Physicists in Medicine. All rights reserved.