QUALITY ASSURANCE OF THE DOSE DELIVERED BY SMALL RADIATION SEGMENTS

The use of intensity modulation with multiple static fields has been s uggested by many authors as a way to achieve highly conformal fields i n radiotherapy. However, quality assurance of linear accelerators is g enerally done only for beam segments of 100 MU or higher, and by measu ring beam profiles once the beam has stabilized. We propose a set of m easurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose d elivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either p hotographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the th ree radiotherapy centres on a set of five different Philips SL acceler ators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monit or unit over the range of 1 to 100 MU was found to be accurate to with in +/-5% of the nominal dose per monitor unit as defined for the deliv ery of 100 MU for all the energies. For four out of the five accelerat ors the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some case s found to be larger for small segments by a maximum of 9% of the flat ness/symmetry for large segments. The result of this study provides th e dosimetric evidence that the delivery of small segment doses as top- up fields for beam intensity modulation is feasible. However, it shoul d be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement sho uld be performed for each machine before the delivery of small dose se gments is approved. In some cases it may be advisable to use a low pul se repetition frequency (PRF) to obtain more accurate dose delivery of small segments.