Absolute dose verification of static intensity modulated radiation therapy (IMRT) with ion chambers of various volumes and TLD detectors.

Abstract

This study aims at examining absolute dose verification of step-and-shoot intensity modulated radiation treatment (IMRT) of prostate and brain patients by use of ion chambers of two different volumes and thermoluminescent detectors (TLD). The volume of the ion chamber (IC) is very important for absolute dose verification of IMRT plans since the IC has a volume average effect. With TLD detectors absolute dose verification can be done measuring the dose of multiple points simultaneously. Ion chambers FC65-P of volume 0.65cc and semiflex of volume 0.125cc as well as TLDs were used to measure the central axis absolute dose of IMRT quality assurance (QA) plans. The results were compared with doses calculated by a treatment planning system (TPS). The absolute doses of off axis points located 2cm and 4cm away from the isocenter were measured with TLDs. The measurements of the 0.125cc ion chamber were found to be closer to TPS calculations compared to the 0.65cc ion chamber, for both patient groups. For both groups the root mean square (RMS) differences between doses of the TPS and the TLD detectors are within 3.0% for the central axis and points 2cm away from the isocenter of each axis. Larger deviations were found at the field edges, which have steep dose gradient. The 0.125cc ion chamber measures the absolute dose of the isocenter more accurately compared to the 0.65cc chamber. TLDs have good accuracy (within 3.0%) for absolute dose measurements of in-field points.