Radiation therapy of malignant tumours can be delivered by external beam radiation (RT) or radioimmunotherapy (RIT), using nuclides attached to monoclonal antibodies (mAbs). These treatment modalities have now been combined in order to investigate putative therapeutic advantages and elucidate the biological responses involved. Nude mice were transplanted subcutaneously on the back with human HeLa Hep2 tumour cells. RT (3x5 Gy) and/or 100 microg (131)I-labelled mAb H7, against placental alkaline phosphatase, or (131)I-labelled mAb TS1, against cytokeratin, was administered separately or in combination (specific activity of 120-200 MBq/mg antibody). Significant tumour growth retardation was observed both with RT alone and with RIT alone. Combining these regimens enhanced the therapeutic effects further, and a significant reduction in tumour volume could be demonstrated. The tumours were subjected to extensive histochemical and immunohistochemical investigations in order to elucidate changes in biology and histology within them. The following stainings were used: haematoxylin-eosin (morphology), Ki67 (proliferation), M30 (apoptosis), TUNEL (apoptosis) and endoglin (vascularisation). Tumours in the control group grew fast, with an average tumour doubling time of 9 days. These tumours contained large viable tumour cell masses displaying vast proliferation zones of Ki67-positive tumour cells, as well as necrotic regions and small amounts of connective tissue. Apoptotic cells could be identified both with M30 and TUNEL staining. When RT was applied, the growth rate was significantly reduced (doubling time 19 days) and typical alterations in morphology were seen, with a relative increase in connective tissue and a decrease in necrotic regions. Apoptotic cells were identified and a decrease in cell density was also observed. When RIT alone was applied, the growth parameters indicated a longer lasting growth reduction, especially when TS1 was used separately or in combination with H7. The histological appearances of these tumours were somewhat different from the RT-treated tumours, with a larger portion of intratumoural cysts. These tumours also presented a reduced tumour cell density. Dramatic effects were observed when RT was combined with RIT, with a pronounced growth reduction seen in all combination treatment groups. Pronounced tumour volume reduction was also evident in both the RT + RIT ((131)I-TS1) group and RT + RIT ((131)I-TS1/(131)I-H7) group, and in some animals no tumour remained at all. The morphology of the tumour remnants at day 22 was chaotic with a drastically changed histology, with presence of abundant cysts, low fractions of Ki67-positive cells, reduction in cell density, increased amounts of connective tissue and a decrease in necrotic regions. Again, apoptotic cells could be identified, scattered throughout the viable regions. Combining RT and RIT seems to generate an efficient treatment with convincing and long-lasting tumour growth inhibition, which is reflected in a highly aberrant histology withi...