Radiant energy – Calibration or standardization methods
Patent
1990-01-19
1991-01-29
Howell, Janice A.
Radiant energy
Calibration or standardization methods
250389, 250374, 25037006, G01T 1161, A61N 510
Patent
active
049888668
DESCRIPTION:
BRIEF SUMMARY
In treatment of cancer tumours treatment machines for radiotherapy, e.g. linear accelerators, betatrons or cobalt units are often used to give a certain radiation dose to the tumour and its border zone. The radiation from such machines is usually photons or electrons in the energy range 0.5-45 MeV and the size of the radiation field is determined by adjustable collimators that can be matched to the extent of the intended treatment volume. The treatment machines are also equipped with an internal dose monitor (cobalt units usually have a timer instead) to control the radiation does that is given to a patient during a treatment.
The effect of radiation therapy is based on the fact that many types of tumour tissue under certain circumstances have a higher sensitivity to ionising radiation than normal tissue. The margin between the dose required to completely eliminate the tumour and the dose level that gives severe radiation damage in the normal tissue is however very small. For that reason it is crucial for the result of the treatment that the dose given to every part of the tumour volume is according to the therapist's prescription, and large resources are spent in measuring and calculating doses and dose distributions so that they agree with the treatment plan.
As planning and calculation of the radiation dose for the treatment is based on measured radiation field data for each treatment machine it is of course also important that the treatment machines in all respects maintain their radiation properties, and for that reason the most important radiation parameters are controlled regularly. If these parameters during a control are outside certain tolerances the treatment machine may not be used for treatment of patients before it has been repaired or adjusted so that it meets the set requirements again.
The most important radiation parameters are: the dose monitor calibration, the radiation energy, the radiation field homogeneity and the correspondence between the radiation field and the alignment tools that are used as for example field lights, cross-hairs or laser lines.
For measurements of these parameters the existing method is to use combinations of different types of measuring equipment such as ionisation chambers with electrometers, water phantoms or simple detector scanners with ionisation chambers or semiconductors as well as x-ray film with film evaluation equipment.
The dose monitor calibration is usually checked with an ionisation chamber or semiconductor detector placed at a specified depth in a plastic block. The block is placed at a fixed distance from the radiation source and irradiated with a certain number of monitor units and the dose monitor calibration is checked by comparing the measured dose with the reference value from the initial machine calibration.
The energy is usually checked by dose measurements similar to the dose monitor calibration procedure but with two measurements at different depths in the plastic block, e.g. 10 and 20 cm, and then calculating the ratio between these values and comparing with the reference value from the initial machine calibration. The energy can also be determined by scanning a detector in a water phantom, thereby obtaining a depth dose distribution, and calculating the dose ratio between two different depths from these values.
The homogeneity of the radiation field is usually checked by scanning a detector across the field at a specified depth in a water phantom and plotting the detector signal against position. The field profile curve that is obtained is then read at certain points between the field centre and the edge, and the relative percentage dose values in these points are used to characterise the dose homogeneity and these values are compared with the tolerance limits. This procedure is done for both main axes of the field, perpendicular to the beam axis.
The field profile curve may also be obtained with a scanner where the detector is mounted inside a build-up block and the entire block is scanned in air.
X-ray film may also be used so that
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Dunn Drew A.
Howell Janice A.
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