Jr. Greig et al., AN APPROACH TO DOSE MEASUREMENT FOR TOTAL-BODY IRRADIATION, International journal of radiation oncology, biology, physics, 36(2), 1996, pp. 463-468
Citations number
7
Categorie Soggetti
Oncology,"Radiology,Nuclear Medicine & Medical Imaging
Purpose: An approach is proposed to allow the interpretation of diode
measurements on patients receiving total body irradiation (TBI) in opp
osed lateral fields as midplane dose measurements. Methods and Materia
ls: This technique consists of making measurements on both entrance an
d 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 in
dicative 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 repre
sents the dose that would have been delivered to that depth had the me
dium (the patient) been semi-infinite. Because these two measurements
Lie along the same ray, they are related by a simple depth-dose equati
on from which the effective absorption coefficient (mu(eff)) can be de
termined. The dose at the midplane is calculated using this mu(eff) in
a similar equation. Results: The validity of this approach was checke
d with measurements at isocenter and at the TBI distance using a polys
tyrene phantom. In both cases, the calculated midplane dose was within
reasonable experimental error (range similar to+/-2% on average) of t
he measured dose, Measurements on eight patients showed the excellent
reproducibility of entrance surface measurements (standard deviation [
SD] similar to+/-1%), compared to that of midplane measurements (SD si
milar to+/-4%) and exit surface measurements (SD similar to less than
or equal to 8%). Midplane doses determined from these equations confir
med that dose inhomogeneity using this opposed lateral technique is on
the order of +/-10% aith highest doses delivered to the calves and lo
west to the lungs. Conclusion: Recent measurements made on TBI patient
s have shown that a commercial eight-diode system can be used successf
ully to measure the dose delivered to the midplane within an accuracy
of similar to+/-4%. The reproducibility of entrance surface measuremen
ts (similar to+/-1%) shows that such measurements provide the accuracy
required for quality assurance purposes. Copyright (C) 1996 Elsevier
Science Inc.