Demystifying Y-90 Dosimetry for Early Career Interventional Radiologists

1. What is dosimetry

2. Common terminology

3. Determining goal of Y-90 therapy

4. Pre-treatment planning

5. Dose models and their benefits/pitfalls

6. Dosimetry optimization by treatment strategy

7. Step-by-Step example of dosimetry planning

8. Limitations and future directions

Background:

Microsphere radiation therapy (MRT) with Yttrium-90 (Y-90) is a treatment for hepatocellular carcinoma (HCC) and liver metastasis. It involves administration of radiation to the liver tumor using Y-90, which is a radioactive isotope embedded onto glass or resin microspheres. Often considered to be a palliative therapy, MRT has been shown to have improved outcomes or curative potential when dosimetry is optimized{1}. With a proven dose-effect relationship, the goal of treatment is to deliver a tumoricidal radiation dose to the liver lesion(s) while keeping dose to normal liver at a minimum {2,3}. There are several dosimetry models which can create confusion among inexperienced users, primarily trainees. Therefore, the aim of this abstract is to educate users on the essentials of Y-90 dosimetry to optimize outcomes of MRT.

Clinical Findings/Procedure Details:

MRT can be performed with curative intent or local disease control when lesions are confined to ≤2 segments. Palliative intent is reserved for multifocal or bi-lobar disease and/or portal vein invasion. Dosimetry is performed based on clinical indication for treatment, estimated lung shunt fraction (using Tc-99m MAA planar or SPECT imaging) and several other factors that separate the existing dosimetry models{4}. Currently three dosimetry models are routinely used: body surface area model (BSA), medical internal radiation dose model (MIRD), and the partition model {5-6,7-9}. This exhibit will provide case-based examples illustrating step-by-step dosimetry calculation using each model (including comparison of contrast-enhanced intra-procedural and diagnostic preoperative CT imaging), including advantages and disadvantages of each model. Intra-procedural CT enables accurate measurement of perfused liver and tumor volume, thereby allowing maximal dosimetry optimization. It also helps in accurate assessment of tumor perfusion and any variant anatomy.

Conclusion and/or Teaching Points:

• Sound understanding and application of dosimetry and a clear treatment intent are essential for good treatment response with MRT.


• Intra-procedural CT imaging improves accuracy in volume calculations, assessment of tumor perfusion, and evaluation of variant anatomy when compared to diagnostic preoperative CT imaging