Dosimetric problems at low monitor unit settings for scanned and scattering foil electron beams

Abstract

Electron beam dosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for different types of linear accelerators. It is observed that for the scattering foil units, the dose/MU is a smooth function of MU for all beam energies. Discrepancies in dose/MU are highest at the lowest MU. Significant variation (5%-245%) in dose linearity is observed among various linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units tested. On the contrary, for the scanned beam, there is no predictable pattern as dose/MU is random in nature and varies with time and beam energy. The maximum dosimetric error is observed for the highest energy beam where the beam width is most narrow. Using film, the beam uniformity was noticed to be very poor at low MU and high energy for scanned beams. The beam uniformity and dose linearity are random at low MU due to the random nature of the scan cycle. Under the adverse conditions, the deviation in dosimetric parameters was observed up to 200 MU.

Dose Delivery Accuracy of Therapeutic Photon and Electron Beams at Low Monitor Unit Settings

“…The reasons for the difference in variation in dose delivery noticed are mainly due to the change in design and the difference in start-up behavior of the linacs. Though it was reported that the variation in dose delivery is energy-dependent [3], no definite conclusion of such variation is observed in our study for electron beams.…”

“…Dose linearity, beam flatness and the variation in beam energies were studied for different dose rate electron beams. The dose delivery accuracy for scanned electron beams was found to be random in nature [3]. Higher dose linearity ratios were reported in literature for high-energy electron beams at higher dose rates [22].…”

“…As the variation in energy for the scattering foil-produced electron beam is within +2% [3], same values of K Q,Qo were considered for the testing and calibrating MU. Hence the equation for the RD now gets reduced to the ratio of corrected dosimeter readings (MR/MU) for the testing and calibrating conditions.…”

Linearity of the Dose Monitor System at Low Monitor Units

“…This problem has been noticed and a number of publications are available [1,2,4,5,10,11,13,17,18,20,22] which analyze the linearity of the dose monitor system of medical electron linear accelerators (linac) down to 1 MU which is the lowest MU setting which can be used at most linacs. The available results show drastic variations: whereas Hansen et al [11] report minor changes of the order of 1-2% of the relative dose at low MU, very recently Ravikumar et al [18] have measured relative doses which are increased by about 20-40% at low MU.…”

Suppression of dark current radiation in step‐and‐shoot intensity modulated radiation therapy by the initial pulse‐forming network

“…Each individual segment is defined by a certain multileaf pattern generated by an IMRT algorithm. It is known that for small ͑Ͻ10͒ monitor unit ͑MU͒, characteristics of every type of machine differ significantly in normal treatment mode as noted by various investigators [8][9][10][11] and in IMRT mode. [12][13][14] In the Siemens step-and-shoot technique, an intensity modulated ͑IM͒ group comprising of all the segments in a static field first must be created with PRIMEVIEW, a proprietary software ͑Siemens Oncology Systems, Inc., Concord, CA͒ which interfaces to a Siemens Primus accelerator.…”