Locoregional therapies of liver metastases in a rat CC531 coloncarcinoma model results in increased resistance to tumour rechallenge

Background: Locoregional treatments like photodynamic therapy (PDT), radiofrequency ablation (RFA) or hepatic artery infusion (HAI) of chemotherapeutics may be applied for unresectable colorectal liver metastases. We evaluated the effect of these treatments on the immune response in a rat colon tumour liver metastases model. Method: Wag/Rij rats were inoculated at day 0 with CC531 tumour cells at two sites in the liver. At day 15, one of two tumours was treated with RFA or PDT, or the liver was treated by HAI. Twelve days later (day 27), rats were rechallenged locally with CC531 cells in the liver or systemically with CC531 cells in the femoral vein. At day 42, tumour growth in liver and lungs was determined. Results: RFA, PDT and HAI were very effective in liver tumour eradication, but following RFA or PDT there was no inhibitory effect on untreated nearby liver tumours. Outgrowth after local rechallenge was, however, significantly inhibited in RFA-, PDT- and HAI-treated rats, whereas all control rats showed outgrowth of a third liver tumour. After systemic rechallenge, control rats developed lung metastases whereas treated rats did not, but this difference was not statistically significant. Conclusion: These results show that following PDT, RFA and HAI resistance to local and possibly systemic tumour rechallenge is increased. This may be partly due to the induction or enhancement of a cellular immune response.