For radiotherapy, Linear Accelerator (Linac) quality assurance is necessary to provide precise and accurate radiation treatment. Besides treatment plans and patient positioning, machine status is another vital issue that a ects radiotherapy. In this paper, we implemented and discussed the Isocenter and Field of View Accuracy Measurement Software for Linac. All data and images were taken by Brainlab Vero Linac of Erlangen University Clinic.
In general, quality assurance means all planned or systematic actions for
guaranteing the given requirements. In radiotherapy, quality assurance refers to all
procedures that ensure consistency of medical prescriptions and practical ful
llment [
According to radiation therapy work ow, treatment planning system TPS plans, the treatment details based on assumption of real Linac parameters; also, during irradiation, patient positions are compared to radiation isocenter for patient positioning. For accuracy considerations, consistency of treatment planning assumption, patient positioning, and actual machine status are critical, which also means that regular assurance of machine quality is necessary.
Besides dosimetric accuracy, the machine quality assurance can also increase the probability of recognition and recti cation accidents or fault in case when they do occur. Minor incidents can occur anytime, as well as by prompt recognition, we can modify sequential fractions and, thus, reduce overall consequences.
The objective of machine QA is to ensure exposure of normal tissue during irradiation be kept ALARA (as low as reasonably achieved), so as to integrate patient safety by reducing machine uncertainty.
Usually, quality assurance includes a certain types of measurements: isocenter accuracy, eld of view (FOV) accuracy, multileaf collimator MLC leakage, dynamic MLC banks position, leaf position, one picket test, and MLC leakage ratio. The rst two types of measurements are the most important for delivering a certain radiation dose to the object.
In this article, we use EDR2 (Extended Dose Range) lm for evaluation of dose exposure. The EDR2 lm developes in case exposed in normal environment, thus, during measurement, the lm should be covered by opaque paper, and in this article, we make use of such property for eld reference. Before irradiation, we puncture reference point (light eld vertex or longitudinal direction) with needle on cover, so as to develop a small dot for reference for the analysis.
The EDR2 lm needs developing procedure before analysis, and we have developed the lm by automatic developing device, but considering average cost, it is quite worth to substitute it by instant self-developing for the EDR2 lm. 2.2
EPID is used for several processes. For example, patient positioning, veri
cation and dosimetric veri cation of IMRT, and Linac quality assurance. Quality
assurance is based on EPID and o ers high spatial resolution, fast image
acquisition, and digital output. It is considered as an accurate tool for both patient
and machine QA in radiation therapy [
In this article, we measure the accuracy of beam isocenter with rotation of gantry
and rings. We set the MLC window width to be identical 10 mm and irradiate
from di erent direction and compare the area of isocenter sphere with expected
value [
Isocenter sphere for gantry-rotation. We clamp the lm between two plastic blocks to x and place it corresponding to the expected beam isocenter by laser (Fig. 1). Then we irradiate with the MLC window size of 10 50 mm2 and from gantry angle of 0, 70, 140, 210, 280, and 350 degree. After that, lm is scanned and analyzed.
Isocenter sphere for ring-rotation. Similarly to the gantry-rotation test, we make the lm positioning direction towards the beam portal. We change the MLC window size to 10 150 mm2 and irradiate from di erent ring angles. Film is also scanned for the analysis step. 2.4
In this test, we measure correlation of the light- eld, as well as the irradiation eld and we choose 50 50 mm2 and 100 100 mm2 eld for test. First, we place the lm towards beam portal and puncture four corners corresponding to the light- eld on lm, then we change the MLC window size to the same and irradiate with dose of 300MU. The lm is later analyzed by scanner after being developed. 3
Based on the smallest hitting circle coming from algorithm in Fig. 3d, we have calculated the diameters of isocenter sphere are 1.56 mm for gantry rotation and 0.54 mm for ring rotation. In isocenter accuracy assurance test, diameter of the gantry-rotation isocenter sphere is 13% excessive to expected value and in the ring-rotation isocenter sphere is 27% excessive. This implies slight o set of beam isocenter from gantryrotation isocenter and ring-rotation isocenter. In the eld correlation test, difference of edge length of actual irradiation eld and light- eld are within 2%, so, we can conclude that irradiation eld conforms with to the light- eld quite well.
Thus, in the paper, we have implemented and discussed the Linac quality assurance software of two important parameters. Irradiation eld (50
50mm2)
All data and images were taken by Brainlab Vero Linac of Erlangen University Clinic. The experiments, which is shown in this paper were supported by Prof. Dr. rer. nat. Christoph Bert of Friedrich-Alexander University ErlangenNurnberg, Erlangen.