The influence of angular misalignment on fixed‐portal intensity modulated radiation therapy

Abstract

A method has been developed to estimate potential dose errors due to linear accelerator angular setting misalignments of Intensity Modulated Radiation Therapy (IMRT) treatments. A first-order approximation to the dose error at a point is modeled as the dot product of the dose gradient and the shift vector of the point due to the rotational error. The analysis method is applied to a previously published set of optimized fluences for a 50 MV IMRT pelvis irradiation. Three of the published cases exhibiting a wide range of modulation are presented; a rectangular open field, a field optimized for a static multileaf collimator defining the portal outline coupled with a single broad bremsstrahlung profile modulation, and a fully modulated field using a physical modulator. To examine the energy dependence of angle setting errors, the study is repeated using the same fluence distributions, but with a dose-spread kernel appropriate for a 6 MV photon beam. The collimator angle error is set to 2 degree, and the dose error determined with both a centrally located isocenter and an isocenter chosen to model a split-field geometry. The dose error due to a 2 degree gantry setting error is assessed at a plane 10 cm distal to the isocenter. The mathematical form of the dose error due to couch motion is similar to the other two errors, so the dose error resulting from a couch angle missetting is not presented. The magnitude of the errors is largest for the 6 MV beam, while the volume encompassed by the errors is greater for the 50 MV beam. The gantry error yields the largest dose error values, with the 6 MV modulated case presenting dose errors of greater than 40%.

Is RapidArc more susceptible to delivery uncertainties than dynamic IMRT?

“…The dosimetric impact of random and systematic changes in gantry angle during IMRT delivery has been reported by few authors. In their study of angular misalignments on fixed-portal IMRT, Low et al 40 presented a method that estimates dose errors caused by unintended collimator, gantry and couch setting errors. Xing et al 41 noted that although angular setting misalignments play a smaller role than patient positioning errors, they found that a 5 • gantry error in only one of nine coplanar beams resulted in a 1.5% decrease in the minimum target dose or 5.1% in the maximum cord dose.…”

“…In their study of angular misalignments on fixed-portal IMRT, Low et al 40 presented a method that estimates dose errors caused by unintended collimator, gantry and couch setting errors. Xing et al 41 noted that although angular setting misalignments play a smaller role than patient positioning errors, they found that a 5…”

Technical Note: A method to evaluate dosimetric effects on organs-at-risk for treatment delivery systematic uncertainties

“…The dosimetric effects of various RT treatment uncertainties have been reported in a number of studies . Random treatment delivery errors (MLC leaf errors, gantry rotation errors, etc.)…”

Assessing the Dosimetric Consequence of Inter-fractional Setup Shifts on Helical TomoTherapy Plans with Independent Dose Calculation

“…Detrimental dose impacts to the target volume as well as to the normal tissues or organs at risk are the primary concern. Dosimetric effect due to machine misalignment has been investigated by different groups for various modalities including tomotherapy (Su et al, 2008) and Linac (Low et al, 1997;Xing et al, 2000) machines. From these studies, it was shown that machine misalignment in clinical practice could yield significant dose differences, and that damage to organs at risk might be profound.…”