Conclusion

1]. They are both often seen as an unnecessary intrusion into the routine treatment of patients. It can become very frustrating for treatment personnel when the performance of a treatment machine needs to be checked and confirmed following the repair after a breakdown. It is, however, apparent from previous incidents that measurement of a physical parameter could have averted incorrect treatment over a lengthy period before clinical indicators indicated a problem.

It is important to be able to put the checks on performance into context. This can be done in relation to the treatment planning and dose prediction undertaken for the treatment of a patient. For example, considerable effort is used on occasions to achieve a dose distribution satisfying the ICRU 50 [16] criteria of −5% to +7% across the target. The performance of the machine can compromise this due to several factors. A non-uniform beam due to energy change or tilted beam due to poor steering could change this by ±6%. The variation in the factors used for calculation of the monitor units in combination with the dose calibration of the machine could also change this by up to ±6%. Geometric misalignment between the radiation beam and the beam indicators can result in dose differences, particularly in regions of high dose gradient such as wedged or modulated fields. It is worth bearing in mind that, typically, within 5 mm of a radiation field’s geometrical edge, a dose variation of up to ±10% can result from the order of a 1 mm discrepancy.

A random combination of these many factors throughout the treatment cannot be guaranteed and the performance variation that might result without adequate quality control and planned maintenance could well negate or even swamp the variation expected from the planned dose prediction.

A comprehensive system of quality control for all the equipment used in radiotherapy will ensure that accurate and reproducible treatment is being delivered.

References

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[13] The Exeter District Health Authority. The Report of the Committee of Enquiry into the overdoses administered in the Department of Radiotherapy in the period February to July 1988. 28 November. 1988.

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[16] ICRU Report 50 Prescribing, recording, and reporting photon beam therapy. International Commission on Radiation Units and Measurements. 1993.

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