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

Abstract

The effect of the initial pulse forming network (IPFN) on the suppression of dark current is investigated for a Siemens Primus accelerator. The dark current produces a spurious radiation, which is referred to as dark current radiation (DCR) in this study. In the step-and-shoot delivery of an intensity modulated radiation therapy (IMRT), the DCR could be of some concern for whole body dose along with leakage radiation through collimator jaws or multileaf collimator. By adjusting the IPFN-to-PFN ratio to >0.8, the DCR can be measured with an ion chamber during the "PAUSE" state of the accelerator in the IMRT mode. For 15 MV x rays, the magnitude of the DCR is approximately equal to 0.7% of the dose at dmax for a 10 x 10 cm2 field. The DCR has a similar central axis depth dose as a 15 MV beam as determined from a water phantom scan. When the IPFN-to-PFN ratio is lowered to <0.8, no DCR is detected. For low energy x rays (6 MV), no DCR is detected regardless of the IPFN-to-PFN ratio. Although the DCR is studied only for the Siemens Primus model accelerator, the same precaution applies to other models of modern accelerators from other vendors. Due to the large number of field segments used in a step-and-shoot IMRT, it is imperative therefore, that dark current evaluation be part of machine commissioning and annual calibration for high-energy photon beams. Should DCR be detected, the medical physicist should work with a service engineer to rectify the problem. In view of DCR and whole body dose, low-energy photon beams are advisable for IMRT.

Characteristics of flattening filter free beams at low monitor unit settings

“…13 The intensity modulated (IM) mode, with which Siemens linacs regulate beam delivery for stepand-shoot IMRT, is unique. 14,15 In IM mode, the Artiste TM accelerator does not switch off between subfield deliveries, but the injector that feeds electrons to the waveguide is desynchronized from the radio-frequency (RF) pulse. To evaluate the characteristics of low-MU beams for use in IMRT, beams produced in IM mode were measured.…”

Investigation of the reliability, accuracy, and efficiency of gated IMRT delivery with a commercial linear accelerator

“…During the beam hold-off of the step-and-shoot IMRT the IPFN ͑Initial Pulse Forming Network͒ signal is detuned to prevent any dark current generation. 26 IPFN is not detuned during the beam hold-off of the gating on the Siemens Primus accelerator. Therefore, the dark current for the beam hold during gating may be higher than that for the beam hold during the step-and-shoot IMRT.…”

Dose linearity and uniformity of a linear accelerator designed for implementation of multileaf collimation system‐based intensity modulated radiation therapy

“…7 With a different beam switching mechanism, it should be of concern whether the dose delivered is linear, reproducible, and uniform. [8][9][10] The recent work of Cheng and Das 9,10 indicated that the dose is nonlinear for small monitor units. Using film dosimetry, they also indicated that the dose is not reproducible and nonuniform.…”