A novel method for dose distribution registration using fiducial marks made by a megavoltage beam in film dosimetry for intensity-modulated radiation therapy quality assurance.
OBJECTIVE
Photographic film is widely used for the dose distribution verification of intensity-modulated radiation therapy (IMRT). However, analysis for verification of the results is subjective. We present a novel method for marking the isocenter using irradiation from a megavoltage (MV) beam transmitted through slits in a multi-leaf collimator (MLC).


METHODS
We evaluated the effect of the marking irradiation at 500 monitor units (MU) on the total transmission through the MLC using an ionization chamber and Radiochromic Film. Film dosimetry was performed for quality assurance (QA) of IMRT plans. Three methods of registration were used for each film: marking by irradiating with an MV beam through slits in the MLC (MLC-IC); marking with a fabricated phantom (Phantom-IC); and a subjective method based on isodose lines (Manual). Each method was subjected to local γ-analysis.


RESULTS
The effect of the marking irradiation on the total transmission was 0.16%, as measured by a ionization chamber at a 10-cm depth in a solid phantom, while the inter-leaf transmission was 0.3%, determined from the film. The mean pass rates for each registration method agreed within ± 1% when the criteria used were a distance-to-agreement (DTA) of 3 mm and a dose difference (DD) of 3%. For DTA/DD criteria of 2mm/3%, the pass rates in the sagittal plane were 96.09 ± 0.631% (MLC-IC), 96.27 ± 0.399% (Phantom-IC), and 95.62 ± 0.988% (Manual).


CONCLUSIONS
The present method is a versatile and useful method of improving the objectivity of film dosimetry for IMRT QA.
Division of Clinical Radiology Service, Okayama Central Hospital, 6-3 Ishimakitacho, kitaku, Okayama 700-0017, Japan.
Shinichi
S
Nakayama
Division of Clinical Radiology Service, Okayama Central Hospital, 6-3 Ishimakitacho, kitaku, Okayama 700-0017, Japan; Department of Radiation Oncology, Graduate School of Medicine, Kinki University, Osaka, Japan. Electronic address: hmon@kuhp.kyoto-u.ac.jp.
Hajime
H
Monzen
Division of Clinical Radiology Service, Okayama Central Hospital, 6-3 Ishimakitacho, kitaku, Okayama 700-0017, Japan.
Yuuichi
Y
Oonishi
Division of Clinical Radiology Service, Okayama Municipal Hospital, Okayama, Japan.
Rika
R
Mizote
Department of Nuclear Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Japan.
Hiraku
H
Iramina
Department of Radiology, Okayama Central Hospital, Okayama, Japan.
Souichirou
S
Kaneshige
Department of Radiation Oncology and Image-Applied Therapy, Graduate School of Medicine, Kyoto University, Kyoto, Japan.
Takashi
T
Mizowaki
D057918:Fiducial Markers; D005373:Film Dosimetry / Q000592:standards*; D019047:Phantoms, Imaging; D011785:Quality Assurance, Health Care; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000592:standards*; D050397:Radiotherapy, Intensity-Modulated
Dose distribution; Film dosimetry; Quality assurance; Radiochromic film