Practical guidelines for routine intensity-modulated radiotherapy verification: pre-treatment verification with portal dosimetry and treatment verification within vivodosimetry

Abstract

ABSTRACT. The purpose of this work is to provide guidelines for the routine use of portal dosimetry and in vivo diode measurements to verify intensity-modulated radiotherapy (IMRT) treatments. To achieve tolerance levels that are sensitive enough to intercept problems, both the portal dosimetry and the in vivo procedure must be optimised. Portal dosimetry was improved by the introduction of an optimised twodimensional (2D) profile correction, which also accounted for the effect of backscatter from the R-arm. The scaled score, indicating the fraction of points not meeting the desired gamma evaluation criteria within the field opening, was determined as the parameter of interest. Using gamma criteria of a 3% dose difference and 3 mm distance to agreement, a ''scaled score'' threshold value of 1.5% was chosen to indicate excessive tongue and groove and other problems. The pre-treatment portal dosimetry quality assurance (QA) does not encompass verification of the patient dose calculation or position, and so it is complemented by in vivo diode measurements. Diode positioning is crucial in IMRT, and so we describe a method for diode positioning at any suitable point. We achieved 95% of IMRT field measurements within ¡5% and 99% within ¡8%, with improved accuracy being achieved over time owing to better positioning. Although the careful preparation and setup of the diode measurements can be time-consuming, this is compensated for by the time efficiency of the optimised procedure. Both methods are now easily absorbed into the routine work of the department. In many radiotherapy departments, routine treatment verification for conventional static treatment fields is performed during the first treatment session. This verification often consists of in vivo measurements with diodes (mostly on the beam axis) in combination with portal imaging of the treatment-field shapes superimposed on the patient's anatomy. Before the first treatment session, the treatment parameters are carefully reviewed by a qualified person and independent monitor unit (MU) calculations may be performed as an additional check.For dynamic intensity-modulated radiotherapy (IMRT) fields, a more extensive quality assurance (QA) protocol is considered to be good practice, the reasons for this being at least fourfold:(1) Both delivery and dose calculations are considerably more complex for dynamic IMRT fields than for conventional static fields.(2) Conventional portal imaging of the treatment fields provides images that are difficult to interpret because the modulated fluence is superimposed on the patient's anatomy.(3) Because of the presence of many dose gradients within the IMRT fields, conventional in vivo dosimetry on the beam axis is prone to large deviations, or can be meaningless when little dose is delivered on the beam axis.(4) Dosimetric treatment parameters, such as MU and multileaf collimator (MLC) shape (or movement pattern), depend on the individual patient plan and can vary substantially among patients as a function of the modulation. Hence,...

Comparison between an in‐house 1D profile correction method and a 2D correction provided in Varian's PDPC Package for improving the accuracy of portal dosimetry images

“…While commissioning the method available in our clinic (Varian's Portal Dose Image Prediction (PDIP) algorithm (Varian Medical Systems, Inc., Palo Alto, CA)), an asymmetric radial response in the portal imager due to backscatter from the imager arm was observed. It led to differences on the order of 2%–3% for simple square fields ($<20\times 20\hspace{0.17em}{\text{cm}}^2$) when comparing the measured to predicted portal fluences, which has also been noted by other authors 15 , 16 , 17 , 18 , 19 , 20 . However, the asymmetric response due to the backscatter from the arm is just one problem.…”

“…Many authors have suggested methods to correct one or both of these problems by either changing the prediction model 16 , 19 or by modifying how the images are acquired 15 , 17 , 18 , 20 . Greer et al (16) provided a 2D image of the backscatter component of the Varian Exact arm that they determined from measurements with the EPID on and off the support arm.…”

Implementing RapidArc into clinical routine: A comprehensive program from machine QA to TPS validation and patient QA

“…The dosimetric calibration of the imager panel was performed at a source imager distance of 100 cm and included a two dimensional profile correction 64,65 instead of the official profile correction based on a diagonal 40 Â 40 cm 2 field profile measurement at d max .…”

“…Images obtained on real patient plans yield similar results, albeit on more complex looking images. The gamma distribution was calculated with 3%, 3mm criteria and-adapting previously established acceptance scores for IMRT fields 65 to the new gamma calculation options in the 8.9 software versionrequesting 95% of the in-field points to be within these limits. Whereas some images yield near-perfect gamma scores [e.g., Fig.…”

In vivo dosimetry in UK external beam radiotherapy: current and future usage

“…Alaei et al 17 cite diode agreement to within 10% of the planned dose on 76% of 300 treatment beams. Vinall et al 18 cite the use of diodes as a complementary check to EPID dosimetry in a study involving 80 patients and 437 IMRT fields. Measured diode doses were within 5% of those expected for 95% of IMRT fields, representing excellent agreement, but the technique required considerable preparation which may not be considered sustainable in many departments.…”