CLINICAL SOLUTIONS - PROSTATE CANCER

Image Guided Radiotherapy Study for Prostate

A. J. Nederveen, J. J. W. Lagendijk and P. Hofman - University Medical Center Utrecht, Utrecht, The Netherlands

Feasibility of automatic marker detection with an amorphous silicon flat-panel imager

Goal

Marker guided position verification to improve conformal delivery by avoiding errors from set-up and organ motion.

Equipment requirements

Imaging of implanted markers using portal imaging for daily tumor targeting has been proposed as a method for on-line position verification. The requirements that need to be met are:

• clinically acceptable marker size (<1.2 mm diameter)
• automatic detection (detection success rate >90%)
• high accuracy (<1mm)
• low imaging dose (1 to 2MU)
• fast analysis (1s)

All these requirements could be met with a clinically used EPID except for the minimization of the marker size. Markers are generally not smaller than 1.6mm which is not yet clinically acceptable, however flat panel imagers (FPI) with increased contrast resolution using a-Si technology makes the use of 1.0mm markers feasible.

Application

Test images were obtained by placing a template containing markers on the skin of a patient during an irradiation session. Lateral prostate fields at 18MV were used as a worst case scenario. Just before the irradiation started, the panel was switched on with a frame time of 200ms. For each patient about 20 images of 1.5MU were obtained.

Template with 2 markers of 2mm diameter and 1 marker of 1.2mm diameter at the left and two markers of 1.0mm diameter at the right

Essential for automatic position verification is the determination of a fiducial reference point for detecting gold markers. This reference point is related to the field boundary in order to use the actual dosimetric field shape as a basis for position verification.

Results

Detection rates of 0.99, 0.9 and 0.95 were found for markers of 1.2mm diameter x 5mm long, 1.0mm diameter x 5mm long and 1.0mm diameter x 10mm long respectively.

These detection success rates were based on a search in the total area within the field boundaries. In a clinical situation it is more likely to search for the marker within a tighter margin around its expected position. This will result in even higher success rates.

The localization accuracy appeared to be better than 0.3mm and reference points could be found with an accuracy equal to 1 pixel.

Conclusion

There is ample evidence from the literature to show that FPIs have a superior imaging performance compared to other commercially available EPIDs. Markers of 1.0mm diameter can be used for marker implantation and can be automatically detected relative to a stable reference point related to the field boundary.

Based on the following published work

References

A.J. Nederveen, J.J.W. Lagendijk, and P. Hofman. Feasibility of automatic marker detection with an a-Si flat-panel imager. Phys. Med. Biol. 2001; 46(4) 1219-30

A.J. Nederveen, J.J.W. Lagendijk and P. Hofman. Detection of feducial gold markers for automatic on-line megavoltage position verification using a marker extrection kernel (MEK). Int.J.Radiat. Oncol. Biol. Phys. 2000;47 (5) 1435-1442.

“Thanks to the increased contrast resolution of the a-Si Flat-panel imager, a clinically acceptable marker size results in a clinically acceptable detection rate”Aart Nederveen