Speaker
Fatemeh Akbari
(Department of medical physics, Reza Radiation Oncology Center, Mashhad, Iran)
Description
INTRODUCTION
Leakage radiation evaluation, as a part of acceptance tests during linac commissioning, is of paramount importance regarding patient and staff radiation protection. In this study, leakage tests were performed for a newly installed linac according to IEC 6061.
METHODS
I) Photon beam
a) Photons leakage from head and beam transport system:
In order to measure the potential leakage from the linac waveguide and the source,EDR-2 films were used to wrap around the linac head, covering all the sides of linac head except the collimator.The films should be marked to permit the determination of theirposition on the machine after they are exposed and processed. Next, a long enough exposure of 1000 MU was delivered using the 15MV photon beams in a situation that the jaws and MLC leaves were fully closed, Gantry=0, Collimator=0.In the next step, after developing the films, the leakage was measured inany area with density greater than the background or fog using an ionization chamber-style survey meter.
b) Photon leakage from beam limiting system and reaching outside the useful beam:
First, a 0.6 cc Farmer ion chamber(withappropriate buildup caps) was placed at SSD=100 in a 10×10 cm² field size to be exposed 1000 Monitor Unit for three times using the highest available energy. The ion chamber should be positioned along an arc of 1 meter radius circle to repeat delivering the same exposure. Finally, the results compared with the reading collected at the reference condition.
II) Electron beam
Cone leakage:
In order to evaluate the leakage from electron cone, a ROOS ion chamber was positioned in center of radiation field at SSD=100 cm. The gantry, couch and collimator angles were set zero. The position of the ion chamber should be changed in a line 2 cm and 4 cm outside the periphery of the geometrical radiation field. 100 MU was delivered using 18 MeV exposure in each position for each measurement point. This procedure should be performed for all available applicators.
RESULTS
According to the head leakage test, there were no hot spots on films.Dose due to photons leakage outside the useful beam, ranged from 0.01 percent to 0.03 percentof maximum absorbed dose measured at isocentre plane in a 10×10 radiation field. As far as electron beam tests are concerned, the maximum ratio of dose at 2 cm and 4 cm from useful beam edge to dose at isocenterwere5.01% for applicator 20×20 and 2.16% for 15×15 applicator, respectively.
DISCUSSION
From this study, it has been found that:
1- Leakage radiation at 1 meter along the path of the electrons between gun and target not exceed 0.1 percent of the maximum dose measured at isocentre of a reference radiation field.
2- Leakage radiation at any point outside the maximum useful beam and in a circular plane of radius 1 m from isocenter also not exceed 0.2% of the isocenter.
3- Absorbed dose at 2cm outside the 18 MeV useful beam area for all applicators not Exceed 10% max absorbed dose on the reference axis at SSD=100.
CONCLUSION
The newly installedARTISTE linac was allowed to be used for clinical applications asall the radiation leakageparameters were within the tolerance limit specified by theIEC60-61 requirements. It is suggested that measurement of radiation leakage should be necessary during acceptancetesting of a linac machine, especially for new type machines such as the ARTISRE, toensure patient and staff safety.
| Institution | Reza Radiation Oncology Center |
|---|---|
| Country | Iran |
Primary author
Fatemeh Akbari
(Department of medical physics, Reza Radiation Oncology Center, Mashhad, Iran)
Co-authors
Masoume Akbari
(1)Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran 2)Technical Department of Matlab Medical Group)
Mohammad Mohammadi
(a)Department of Medical Physics,Reza Radiation Oncology Center,Mashhad, Iran b)Department of Medical Physics, Royal Adelaide Hospital, Australia)
