Info

Figure 3. A 20 year old female who presented with locally invasive and surgically unresectable papillary carcinoma. Each image was obtained 48 hours after administration of 70-370 MBq of 131I. In (a) the pretreatment image shows intense uptake into thyroid tissue; (b) partial response following first treatment with a persisting focus of disease in the right neck and (c) ablation of all iodine-avid tissue. She has now been disease-free for a decade.

Figure 3. A 20 year old female who presented with locally invasive and surgically unresectable papillary carcinoma. Each image was obtained 48 hours after administration of 70-370 MBq of 131I. In (a) the pretreatment image shows intense uptake into thyroid tissue; (b) partial response following first treatment with a persisting focus of disease in the right neck and (c) ablation of all iodine-avid tissue. She has now been disease-free for a decade.

Lung metastases are generally treatable with RAI, especially if they are larger than 1-2 mm in size. Microscopic metastases may not be treatable because the mean path length of the beta particle is then much larger than the tumor diameter (Fig. 4). Most of the radiation dose is then deposited in normal lung and may cause pulmonary fibrosis. Consideration has been given to the use of 125I whose low-energy Auger electrons have a much shorter path length (less than 72 microns) and would deposit more energy within the small dimensions of the tumor but this approach is still investigational.

Bone metastases from papillary carcinoma are very resistant to treatment and they tend to progress even if they concentrate RAI. In such cases, treatment with RAI should be followed with external beam radiotherapy to the metastatic site. For a suspected solitary site, surgical excision might be attempted, but most cases are associated with other sites of blood-borne metastases. Bone metastases from follicular carcinoma may respond well to RAI alone as their uptake tends to be greater (Fig. 5).

The historical model for radiation safety concerning RAI treatment considered the patient as an iodine point source and modeled the radiation safety protocol accordingly. In fact, the patient is an attenuated, distributed source, which greatly reduces the gamma dose to others. Nor did the historical model take into account the concept of the effective dose equivalent (see Chapter 2). These three factors, taken together, reduce the risk posed by the patient by a factor of about 20 relative to the model. The historical modeling also did not take account of the ability of patients and families to follow instructions to further reduce exposures from the patient and the possibility of individualizing instructions to the patient. It was from this historical model that a fixed limit of 1.1 GBq (30 mCi) for out-patient therapy was determined.

The assumptions inherent in this model came to be questioned as a result of recommendations to reduce public exposures from 5 to 1 mSv/year. Mere intensification of past practices would have greatly increased the need for hospitalization for both malignant and benign therapies. The re-evaluation has served to justify modification of the approach to these treatments. In many countries,

0 0

Post a comment