Cesium-131 & Sirius MRI Marker

Your path to better prostate implants

Brachytherapy seed placement is critical for prostate cancer outcomes. Cesium-131 paired with the Sirius® MRI Marker gives you a clear picture of every implant for highly precise prostate cancer treatment.

The Sirius Advantage

Ultrasound, CT, and X-ray imaging don’t visualize soft tissue well, while MRI doesn’t always accurately identify brachytherapy seeds. MRI-assisted radiosurgery (MARS) using the Sirius positive-signal MRI Marker allows for superior visibility of soft tissue and precise seed localization.

“Permanent prostate brachytherapy dosimetry based only on MRI using positive-contrast MRI markers [Sirius] is feasible, accurate, and reduces the uncertainties arising from CT-MRI fusion, abating the need for postimplant multimodality imaging.1”
Steven J. Frank, MD
Past President of the American Brachytherapy Society.

Cesium-131 Treatment Areas

Cesium-131 Products

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Better quality, better outcomes

Improved seed detection and localization dramatically impacts prostate cancer outcomes: 8-year prostate-specific antigen (PSA) relapse-free survival rates of 93% for high-quality implants versus 76% for low-quality implants have been reported.2

For my patients, I use Cesium-131 instead of Iodine because most of the effective isotope life is spent in 4-5 wks, compared to the 60-day half-life of Iodine,” he said. “Iodine will still be putting out a pretty good dose of radiation 8 or 9 months later. My patients have a shorter duration of acute uropathic complaints because of that.
Dr. Bradley Prestidge, Radiation Oncologist
Dr. Bradley Prestidge
Radiation Oncologist

About Cesium-131

Isoray’s proprietary isotope, Cesium-131, has over 10 years of proven patient outcomes for the treatment of prostate cancer. Additionally, Cesium-131 offers a powerful option for treating other cancers throughout the body.


9.7 Day Half-Life

Cesium-131 has demonstrated shorter duration of bothersome voiding symptoms.

30.4 keV

Higher energy may lead to greater
coverage of the prostate.

33 days for 90% Dose

Rapid dose delivery rate.

Sirius MRI Marker

About Sirius

The Sirius MRI Marker consists of a sealed biocompatible, polymer capsule containing a unique MRI agent. The length of each capsule is 5.5mm, and the diameter is 0.8mm. The MRI marker is intended to be implanted as a brachytherapy seed spacer that facilitates the anatomical post-implant localization of seeds.

Sirius® MRI Marker

The First Positive Signal MRI Marker

Sealed biocompatible polymer capsule containing a unique MRI agent.

Seed Localization with MRI Alone

Sirius® Markers are optimally visualized using a standard fully balanced steady-state free precession MRI sequence, and can positively identify the location of Cesium-131 seeds.

Improved Prostate Contouring

The superior image quality of MRI could improve the accuracy of prostate contouring.

A strand of Cesium-131 paired with the Sirius MRI Marker delivers a clear picture of every implant without changing your surgical procedure.

CS-131 source specification data

Find a Doctor Near You

A diagnosis is just the first step toward treating your cancer. We are here to help you make an informed treatment decision with your doctor so you can begin living your life again.  Use the search (below) to find a Doctor that specializes in Brachytherapy.

See the difference

MRI allows for visualization of critical intraprostatic structures and soft tissue differentiation.

Want to know more about the versatility of Cesium-131

Do you have questions? We can help with any issues you may have


  1. Frank SJ, et al. American College of Radiology Appropriateness Criteria: Permanent Source Brachytherapy for Prostate Cancer. Brachytherapy. 2011;10(5):357–362
  2. Zelefsky MJ, et al. Multi-institutional analysis of long-term outcome for stages T1-T2 prostate cancer treated with permanent seed implantation. Int J Radiat Oncol Biol Phys.2007;67(2):327–333
  3. Lee WR, et al. Interobserver variability leads to significant differences in quantifiers of prostate implant adequacy. Int J Radiat Oncol Biol Phys. 2002;54(2):457–461
  4. Cesium Blu Instructions for Use
  5. Murphy MK, Piper RK, Greenwood LR, Mitch MG, Lamperti PJ, Seltzer SM, Bales MJ, Phillips MH. Evaluation of the new cesium-131 seed for use in low-energy x-ray brachytherapy. Med Phys 31(6):1529-38, 2004
  6. Sirius Marker Instructions for Use
  1. Cesium-131 brachytherapy seeds are indicated for the treatment of malignant disease (e.g., head and neck, eye, brain, breast, prostate, etc.) and may be used in surface, interstitial, and intracavitary applications for tumors with known radiosensitivity. The seeds may be used as a primary treatment or in conjunction with other treatment modalities, such as external beam radiation therapy, chemotherapy or as treatment for residual disease after excision of primary tumors. Contraindications: As with other brachytherapy sources, treatment of tumors in generally poor condition (e.g., ulcerated) is not recommended with Cesium-131 brachytherapy seeds due to the potential for source migration. Prior to use, please reference the Cesium-131 Instructions for more information on indications, contraindications, warnings, all precautions, and adverse events.

    Caution: Federal law restricts this device to sale by or on the order of a physician.

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