Imaging doses from the Elekta Synergy X-ray cone beam CT system

Abstract

The Elekta Synergy is a radiotherapy treatment machine with integrated kilovoltage (kV) X-ray imaging system capable of producing cone beam CT (CBCT) images of the patient in the treatment position. The aim of this study is to assess the additional imaging dose. Cone beam CT dose index (CBDI) is introduced and measured inside standard CTDI phantoms for several sites (head: 100 kV, 38 mAs, lung: 120 kV, 152 mAs and pelvis: 130 kV, 456 mAs). The measured weighted doses were compared with thermoluminescent dosimeter (TLD) measurements at various locations in a Rando phantom and at patients' surfaces. The measured CBDIs in-air at the isocentre were 9.2 mGy 100 mAs(-1), 7.3 mGy 100 mAs(-1) and 5.3 mGy 100 mAs(-1) for 130 kV, 120 kV and 100 kV, respectively. The body phantom weighted CBDI were 5.5 mGy 100 mAs(-1) and 3.8 mGy 100 mAs(-1 )for 130 kV and 120 kV. The head phantom weighted CBDI was 4.3 mGy 100 mAs(-1) for 100 kV. The weighted doses for the Christie Hospital CBCT imaging techniques were 1.6 mGy, 6 mGy and 22 mGy for the head, lung and pelvis. The measured CBDIs were used to estimate the total effective dose for the Synergy system using the ImPACT CT Patient Dosimetry Calculator. Measured CBCT doses using the Christie Hospital protocols are low for head and lung scans whether compared with electronic portal imaging (EPI), commonly used for treatment verification, or single and multiple slice CT. For the pelvis, doses are similar to EPI but higher than CT. Repeated use of CBCT for treatment verification is likely and hence the total patient dose needs to be carefully considered. It is important to consider further development of low dose CBCT techniques to keep additional doses as low as reasonably practicable.

Difference in performance between 3D and 4D CBCT for lung imaging: a dose and image quality analysis

“…The usual computed tomography dose index (CTDI) (9) is not valid when considering CBCT geometry, and it was therefore replaced by the cone‐beam dose index (CBDI) defined as: 7 , 9 , 10 $CBDI=\frac{1}{L}\underset{-Y/2}{\overset{Y/2}{\int }}Dfalse(yfalse)dy$ …”

Optimal usage of cone beam computed tomography system with different field of views in image guided radiotherapy (IGRT)

“…Since addition of dose to the area wider than target zone may increase the probability of secondary cancers risk 9 , it is better to use the methods where scan area is less. Other interesting observation is, one can put any kV collimator irrespective of the preset, as there is no interlock/indication.…”

“…Since dose is added over a field wider than target zone, this has already influenced the debate on incidence of secondary cancers. 9 Daily CBCT potentially increase secondary cancer risk up to 2-4%. 10 CBCT volume scan can be obtained with different field of views in XVI system namely small, medium and large.…”

Variation of patient imaging doses with scanning parameters for linac-integrated kilovoltage cone beam CT

“…However, due to the X-ray radiation nature of the CBCT process, dose deposited in patient during the daily procedures in patient repositioning has become a technical issue to recognize the extra dose received on top of treatment dosage [10][11][12][13][14][15][16]. In an attempt to determine what level of optimization of scanning parameters is taking place, the main purpose of the current study is to evaluate the clinical impact of these changes on the doses received by patients undergoing IGRT and to assess the associate image quality.…”