Impact of the tumour bed effect on microenvironment,radiobiological hypoxia and the outcome of fractionated radiotherapy of human FaDu squamous-cell carcinoma growing in the nude mouse

Purpose : To investigate the impact of the tumour bed effect (TBE) on histological parameters of the micromilieu, radiobiological hypoxic fraction and local control after fractionated irradiation in FaDu squamous-cell carcinoma in the nude mouse. This tumour has previously shown a clear-cut TBE caused by increased necrotic cell loss at a constant cell production rate in the viable tumour compartment. Materials and methods : Human FaDu tumours were studied in the NMRI nude mouse. Tumours were transplanted either into unirradiated subcutaneous (s.c.) tissues (controls) or s.c. tissues pre-irradiated with 12.5 Gy (TBE group). In both groups we measured the volume doubling time (VDT), potential doubling time (T pot) , relative necrotic area, and in the viable tumour compartment the relative vascular area (9F1 mAb), relative hypoxic area (NITP or pimonidazole), relative perfused area (Hoechst 33342), and the perfused fraction of vasculature. The tumour control dose 50% (TCD 50), radiobiological hypoxic fraction (rHF) and dose-modifying factors (DMF) for the comparison of tumours in the TBE and control groups were determined from local tumour control data after treatment with single doses under ambient conditions or under clamp hypoxia, and after irradiation with 30 fractions under ambient conditions within 6 weeks using maximum-likelihood analysis. Results : A clear-cut TBE (VDT = 4.0 days (95%CI 2.9;4.4) for the control group versus 7.2 days (6.4;8.9) for the TBE group; p <0.0001) caused by increased necrosis (mean relative necrotic area of 12% (5;20)) versus 33% (10;41); p = 0.07) at a constant cell production rate (T pot = 2.2 days (1.4;2.3) versus 2.2 days (1.7;2.6); p = 0.30) was confirmed. Histological analysis of the micromilieu within the vital subarea revealed no systematic differences between the TBE and control groups. The rHF of 2% (0.1;27) for control tumours was lower than the 15% (95% CI 2;91) for the TBE group, but this difference was nonsignificant (p = 0.12). Compared with control tumours, the TCD 50 for irradiation under clamped hypoxia was in a statistical trend lower for tumours in the TBE group (DMF 1.11 (0.98;1.28), p = 0.09). After fractionated irradiation, tumours of the TBE group were significantly more radiosensitive (TCD 50 56.6 Gy (46;70) versus 78.7 Gy (63;100); p = 0.003). Conclusions : The results on FaDu tumours growing in pre-irradiated tissues indicate that increased necrosis caused by impairment of the vascular supply may increase the radiosensitivity of tumours treated by fractioned irradiation.