Combined modalities

Targeted radiotherapy using p-emitters inevitably results in a whole-body radiation dose because of limited targeting specificities (cross-

Table 40.2 Radionuclides for targeted radiotherapy

Radionuclide

Half-life

Emitted particles

Particle range

90y

2.7 days

ß

5mm

131I

8 days

ß

0

8 mm

67Cu

2.5 days

ß

0

6 mm

199Au

3.1 days

ß

0

3 mm

211At

7 hours

a

0

05 mm

212Bi

1 hour

a

0

05 mm

125I

60 days

Auger electrons

1 fj,m

123I

15 hours

Auger electrons

1 fj,m

targeting to normal tissues) and because of radionuclide in the general circulation. Radiobiological modelling suggested that systemic targeted radiotherapy might best be regarded as a kind of non-uniform TBI—with higher doses being given to tumour cells than to normal tissues. Therefore, systemic targeted radiotherapy might be most appropriate for patients for whom conventional TBI, with marrow rescue (or other forms of haematological support), was already an option.

These considerations suggest the possible advantages of a combined modality regime incorporating targeted radiotherapy, TBI, and marrow rescue. Computer simulation studies of the effect of combined modality treatment have suggested that disseminated tumours of differing size may be optimally treated by different components of the combination. For example, distributed single cells and very small micrometastases are effectively treated by TBI (and by chemotherapy), targeted 131I is ideal for treatment of larger micrometastases, whilst macroscopic tumours are best dealt with by local modalities (radiotherapy, surgery).

These concepts are now being applied in the targeted radiotherapy of neuroblastoma using 131I-MIBG in combination with TBI or with high-dose chemotherapy. Combined modality treatment of B-cell lymphoma using radio-labelled antibodies, TBI, or systemic chemotherapy and haemopoetic rescue may be an appropriate next step.

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