Commissioning and quality assurance for intensity‐modulated radiotherapy with dynamic multileaf collimator: Experience of the Pontificia Universidad Católica de Chile

Abstract

The objective of this paper is to present our experience in the commissioning and quality assurance (QA) for intensity-modulated radiotherapy (IMRT) using the dynamic multileaf collimator (dMLC), sliding window technique. The connectivity and operation between all IMRT chain components were checked on the Varian equipment. Then the following tests were performed: stability of leaf positioning and leaf speed, sensitivity to treatment interruptions (acceleration and deceleration), evaluation of standard field patterns, stability of dMLC output, segmental dose accuracy check, average leaf transmission, dosimetric leaf separation, effects of lateral disequilibrium between adjacent leaves in dose profiles, and multiple carriage field verification. Standard patterns were generated for verification: uniform field, pyramid, hole, wedge, peaks, and chair. Weekly QA protocol includes sweeping gap output, garden fence test (narrow bands, 2 mm wide, of exposure spaced at 2-cm intervals), and segmental dose accuracy check. Monthly QA includes sweeping gap output at multiple gantry and collimator angles, sweeping gap off-axis output, picket fence test (eight consecutive movements of a 5-cm wide rectangular field spaced at 5-cm intervals), stability of leaf speed and leaf motor current test (PWM test). Our patient QA procedure consists of an absolute dose measurement for all treatment fields in the treatment condition, analysis of actual leaf position versus planned leaf position (dynalog files) for each treatment field, film relative dose determination for each field, film relative dose determination for the plan (all treatment fields) in two axial planes, and patient positioning verification with orthogonal films. The tests performed showed acceptable results. After over one year of IMRT treatment, the routine QA machine checks confirmed the precision and stability of the IMRT system.

MLC positional accuracy evaluation through the Picket Fence test on EBT2 films and a 3D volumetric phantom

“…In the literature there are two proposed methodologies to conduct the test: either by creating a uniform pattern with abutting fields (7,8) or by using specified intervals to irradiate a series of narrow bands. (9,10) In the first proposed method, the resulting image is checked for missed or overirradiated spots. In the second one, the width of the narrow bands is measured and checked for discrepancies.…”

EPID‐based verification of the MLC performance for dynamic IMRT and VMAT

“…The conventional method for two-dimensional (2D) checking of MLC leaf positions was to use film images of a dynamic MLC (DMLC) leaf pattern. 13,[20][21][22][23][24] Film images were either visually inspected or scanned, digitized, and contrast enhanced. 2 Visual inspection is a subjective and inaccurate method for checking the leaf pair alignment and uniformity of the gap widths.…”

“…Dynalog files have been tested 34 and used as reference in many studies on the leaf positioning accuracy. 8,22,23,[36][37][38] However, the validity of the data in Dynalog files strongly depends on the accuracy of leaf position readouts. Any possible drift in the encoder readings could be temporarily improved by the MLC re-initialization, 9 but it must be routinely checked by independent imaging methods.…”

Commissioning and quality assurance for VMAT delivery systems: An efficient time‐resolved system using real‐time EPID imaging

“…One popular approach to treatment delivery system quality control is the use of machine log files (e.g., Varian Dynalog files). 5,[15][16][17][18][19][20][21][22] Machine log files offer a number of advantages such as the ability to streamline and automate QA processes, however, a number of recent studies have shown that log files are insensitive to some types of delivery errors. 16,23,24 This insensitivity arises from the fact that the parameters recorded by log files (i.e., MLC positions, gantry angle and dose fraction) are sourced from the linac control system itself and hence are not independent measurements.…”