[Dosimetric verification in intensity modulated radiation therapy].
As part of dosimetric verification for IMRT intensity modulated radiation therapy, we examined the selection of a dosimeter in accordance with the purpose of physical measurement and the process of data analysis. Because of the high dose conformation in the target volume and minimum dose in the organs at risk (OAR) in IMRT, dosimetric verification is essential. Because the performance of dosimetric verification in a patient is not allowed, a physical phantom and dosimeter must be used. Dose verification using a physical phantom, from which the beam data optimized for a patient slated for IMRT are transferred, may cause latent error as a result of change in the depth of each beam toward an isocenter. This effect may change the dose distribution and prescription dose. The basic methods of dosimetric verification with physical measurement are point dosimetry, when the reference dose is given at a point by planning software, and volumetric dosimetry, when planning software gives the dose as a volumetric configuration. While the most accurate dosimetry is done using a calibrated ionization chamber, IMRT requires volumetric dosimetry using some kind of portal film or a polymer gel dosimeter, because of the need for dosimetric verification for an irregular dose distribution in IMRT. The importance of indirect dosimetry using these methods is to provide calibration as a dosimeter, absolute dose, and preservation of calibration. In our study, the verification of dose distribution for IMRT using portal film and a RANDO phantom could be performed with an error of less than 2% in all cases. The measurement error for the central dose using a JARP-type ionization chamber and MixDP was less than 3% in all cases except for the case with the maximum error. At the moment, IMRT requires a great deal of effort in the processes of planning, dosimetric verification, and isocenter checking in every fraction to maintain high accuracy. Although the need for a large amount of effort in the service of maintaining accuracy may be reasonable, it could be enough to inhibit the spread of IMRT. It is hoped that an easy method of dosimetric verification that still maintains a high level of accuracy will develop as a result of this great effort.
Narita
Yuichiro
Y
Department of Radiation Therapy Physics, Chiba Cancer Center, Japan.
Hatano
Kazuo
K
Shimizu
Takayuki
T
Shimizu
Hideki
H
Iwase
Tsutomu
T
Utagawa
Kimio
K
Ishigaki
Hideyo
H
Okazaki
Yukio
Y
D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D050397:Radiotherapy, Intensity-Modulated / Q000592:standards*; D012306:Risk; D012680:Sensitivity and Specificity