An approach to dose measurement for total body irradiation

Abstract

An approach is proposed to allow the interpretation of diode measurements on patients receiving total body irradiation (TBI) in opposed lateral fields as midplane dose measurements. This technique consists of making measurements on both entrance and exit sides of any ray path along which the midplane dose is desired. The diode on the entrance side is calibrated so that its output is indicative of the dose delivered at the depth of maximum dose along the ray. The diode on the exit side is calibrated so that its output represents the dose that would have been delivered to that depth had the medium (the patient) been semi-infinite. Because these two measurements lie along the same ray, they are related by a simple depth-dose equation from which the effective absorption coefficient (mueff) can be determined. The dose at the midplane is calculated using this mueff in a similar equation. The validity of this approach was checked with measurements at isocenter and at the TBI distance using a polystyrene phantom. In both cases, the calculated midplane dose was within reasonable experimental error (range approximately +/- 2% on average) of the measured dose. Measurements on eight patients showed the excellent reproducibility of entrance surface measurements (standard deviation [SD] approximately +/- 1%), compared to that of midplane measurements (SD approximately +/- 4%) and exit surface measurements (SD approximately < or = 8%). Midplane doses determined from these equations confirmed that dose inhomogeneity using this opposed lateral technique is on the order of +/- 10% with highest doses delivered to the calves and lowest to the lungs. Recent measurements made on TBI patients have shown that a commercial eight-diode system can be used successfully to measure the dose delivered to the midplane within an accuracy of approximately +/- 4%. The reproducibility of entrance surface measurements (approximately +/- 1%) shows that such measurements provide the accuracy required for quality assurance purposes.