THE EFFECTS OF METAL IMPLANT AND METAL ARTIFACT ON THE DOSE DISTRIBUTION DURING RADIATION THERAPY OF THE PELVIC REGION

In some cancer patients, there are metal implants in pelvic and femoral regions.
Due to the interactions of the photon with matter and location of treating region and metal implant, such
high atomic numbered elements can influence absorbed dose compared to predicted values. Also, metal
implants cause metal artifacts in CT images due to their highly effective atomic number compared to
body texture. The aim of this study was to evaluate the effect of metal implants and metal artifacts on
the dose distribution in the treatment volume.
Materials & Methods: In this study, CT images of seven prostate cancer patients who were referred to
Imam Khomeini hospital, Tehran for treatment with titanium metal implant in femur region were
investigated. In these patients, initially dose distributions were calculated by Monaco treatment planning
system considering the effects of metal artifacts (plan A), correcting CT images and modifying electron
density of artifact regions to soft tissue (plan B), transmission of photon through metal (plan C) and
modifying electron density of metal to bone (plan D). The obtained results from Monaco treatment
planning system were then transferred to Verisoft software. The quantitative differences of plans A and
B were analyzed using a gamma index of 3%/3mm in this software. Also, the effects of metal implant
in beam attenuation (Plans C and D) were analyzed quantitatively.
Results: This study showed that the difference of calculated monitor units in corrected and not-corrected
electron density of metal artifact regions ranged between 0.81-3.78 monitor units per fraction. Also the
presence of metal in beam path can lead to a 3% difference compared to beam passing through bone.
Conclusion: Therefore, for the precise implementation of the treatment, necessary corrections on CT
images should be considered before the treatment planning to minimize the errors related to the monitor
unit calculations