Routine individualised patient dosimetry using electronic portal imaging devices.
OBJECTIVE
To analyse the results of routine EPID measurements for individualised patient dosimetry.


METHODS
Calibrated camera-based EPIDs were used to measure the central field dose, which was compared with a dose prediction at the EPID level. For transit dosimetry, dose data were calculated using patient transmission and scatter, and compared with measured values. Furthermore, measured transit dose data were back-projected to an in vivo dose value at 5 cm depth in water (D(5)) and directly compared with D(5) from the treatment planning system. Dose differences per treatment session were calculated by weighting dose values with the number of monitor units per beam. Reported errors were categorised and analysed for approximately 37,500 images from 2511 patients during a period of 24 months.


RESULTS
Pre-treatment measurements showed a mean dose difference per treatment session of 0.0+/-1.7% (1 SD). Transfer errors were detected and corrected prior to the first treatment session. An accelerator output variation of about 4% was found between two weekly QC measurements. Patient dosimetry showed mean transit and D(5) dose differences of -0.7+/-5.2% (1 SD) and -0.3+/-5.6% (1 SD) per treatment session, respectively. Dose differences could be related to set-up errors, organ motion, erroneous density corrections and changes in patient anatomy.


CONCLUSIONS
EPIDs can be used routinely to accurately verify treatment parameter transfer and machine output. By applying transit and in vivo dosimetry, more insight can be obtained with respect to the different error sources influencing dose delivery to a patient.
Nijsten
Sebastiaan M J J G
SM
Department of Radiation Oncology (MAASTRO), GROW, University Hospital of Maastricht, Maastricht, The Netherlands. bas.nijsten@maastro.nl
Mijnheer
Ben J
BJ
Dekker
André L A J
AL
Lambin
Philippe
P
Minken
André W H
AW
D001943:Breast Neoplasms / Q000532:radiotherapy; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy; D009369:Neoplasms / Q000532:radiotherapy; D010315:Particle Accelerators; D010386:Pelvic Neoplasms / Q000532:radiotherapy; D011878:Radiotherapy / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D013679:Technology, Radiologic