The role of fiducial markers

The need for target reference points in radiation therapy

Precision to target is essential in external beam radiation therapy.

Methods to secure target reference points have a long history and were initially in the form of a cross penciled or tattooed on the skin of the patient to guide the entry point of the radiation beam. Later, when Image Guided Radiation Therapy was introduced, bony structures near the tumor were used as landmarks on images for patient set-up at the point of treatment and as a guide for better target precision. Most of the imaging modalities available at the point of treatment are however not able to differentiate sufficiently between different soft tissues, so taking into account the movement of organs and tumors for precise delivery of the radiation dose remains a challenge in many types of tumors.

Fiducial markers as target reference points

Since the mid 1960’ies, fiducial markers have been used to act as tumor target reference points to guide for high precision in radiation therapy.

Fiducial markers are small objects with visibility on one or more imaging modalities. Fiducial markers are placed under visual guidance or in combination with ultra sound (US) near or in the tissue targeted for radiation therapy.

The fiducial markers help:

To identify tumor target location with greater accuracy for better treatment planning, treatment and follow-up
To maximize radiation to the tumor and minimize radiation to healthy surrounding tissue.

The benefits of fiducial markers

The use of fiducial markers in radiation therapy has several advantages, including the following:

Fiducial markers help limit the volume of healthy tissue being exposed to radiation therapy
Fiducial markers make it possible to locate the tumor from day to day on the treatment unit and help over-come the challenge of inter-fractional target movability
Fiducial markers make it possible to monitor live tumor motion during a fraction of radiation treatment and help overcome the challenge of intra-fractional target movement
Fiducial markers enable automatic patient set-up matching and positioning by use of artificial intelligence (AI)
Fiducial markers allow the precise re-identification of treatment area during follow-up

Fiducial marker performance features

For a fiducial marker to be a relevant tool through the entire radiation therapy work flow and across cancer types, the following performance features are demanded:

Treatment planning

Marker implantation:

Fast and easy to implant
Injectable through thin needles

Radiation planning:

Visibility on relevant imaging modalities
Positional stability

Treatment

Visibility throughout the entire treatment
Positional stability throughout the entire treatment
No interaction with the radiation therapy treatment (no dose perturbation)
Visibility on relevant imaging modalities

Follow-up

Long term positional stability for follow-up