Radiosynovectomy treats an inflamed synovial joint lining by injecting a…

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Nuclear medicine, dosimetry and radiotherapy · Nuclear medicine, dosimetry and radiotherapy

The Nucleus Resolved, Ch What it is used for: medicine, dating and analysis

Problem

Radiosynovectomy treats an inflamed synovial joint lining by injecting a beta-emitting colloid directly into the joint cavity. Three joint sizes are treated, each characterised by the distance from the joint cavity to the overlying skin, which sets the largest particle range that can be used without burning the skin: Knee (large joint): distance to skin

\approx 15\,\mathrm{mm}

. Wrist or ankle (medium joint): distance to skin

\approx 6\,\mathrm{mm}

. Finger joint (small joint): distance to skin

\approx 3\,\mathrm{mm}

. To treat the joint, the particle range must also reach at least across the inflamed lining, which is about

5\,\mathrm{mm}

thick in the knee,

2\,\mathrm{mm}

in the wrist or ankle and

0.5\,\mathrm{mm}

in a finger joint. Three candidate colloids, in order of decreasing maximum beta range in tissue:

^{90}\mathrm{Y}

: maximum range

11\,\mathrm{mm}

T_{1/2} = 64.1\,\mathrm{h}

^{186}\mathrm{Re}

: maximum range

3.7\,\mathrm{mm}

T_{1/2} = 3.7\,\mathrm{d}

^{169}\mathrm{Er}

: maximum range

1.0\,\mathrm{mm}

T_{1/2} = 9.4\,\mathrm{d}

. Before any therapeutic injection, a leak check is performed with

^{99m}\mathrm{Tc}

-labelled colloid (

T_{1/2} = 6.0\,\mathrm{h}

, one

140.5\,\mathrm{keV}

photon, no particle emission), imaged the same day to confirm the colloid stays inside the joint rather than draining to lymph nodes. ((a)) Give one nuclear-property reason why

^{99m}\mathrm{Tc}

, rather than one of the three therapy colloids, is used for the leak check. ((b)) For each joint, compute the ratio of the colloid's maximum range to the distance-to-skin for the colloid you judge correctly matched to that joint, and for one mismatched pairing. Use the ratios to justify matching each colloid to a joint. ((c)) List two decisions in this scheme forced by a nuclear property and one that is not.

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