KEY POINTS
- Investigators performed longitudinal histology in two Dunning R3327 rat prostate carcinoma sublines: the faster-growing, hormone-insensitive and more hypoxic HI model, and the slower, hormone-sensitive H model. Five tumors per treatment/time-point group were collected at 8 hours, 20 hours, 3, 7, 14 and 21 days after irradiation.
- Radiation doses were selected from previous tumor-control experiments to be biologically isoeffective. In HI tumors, curative single doses were 75 Gy photons, 64 Gy protons and 37 Gy carbon ions; subcurative doses were 45 Gy photons and 25 Gy carbon ions. H tumors received curative 58 Gy photons or 30 Gy carbon ions.
- All modalities caused an early rise in γH2AX-positive cells and strong suppression of BrdU-positive proliferation. In HI tumors, however, DNA-damage signaling decreased more slowly after carbon ions, particularly at curative doses, whereas photons and protons behaved more similarly.
- Oxygenation substantially altered response. After curative treatment, proliferation remained suppressed in oxic HI regions but rebounded in hypoxic regions at 14–21 days. The late BrdU/γH2AX double-positive signal was particularly pronounced after curative carbon ions despite those dose levels being associated with complete tumor control.
- Gross vascular density and perfusion were relatively stable, but carbon ions altered the type of hypoxia. HI tumors shifted from predominantly perfusion-limited hypoxia toward more cyclic and diffusion-limited hypoxia, suggesting persistent changes in oxygen delivery even without major structural vessel loss.
- CD68-positive macrophages increased over time after all modalities. Carbon ions showed a smaller early macrophage increase but a late accumulation within chronically hypoxic regions after curative doses, temporally overlapping with the proliferative rebound. CD68 staining could not distinguish pro- versus antitumor macrophage phenotypes.
- CD3-positive T cells in HI tumors showed a biphasic depletion followed by recovery, largely independent of radiation modality and oxygenation; H tumors showed much smaller immune changes. The study used very high single doses, followed tumors for only three weeks and did not phenotype T-cell subsets, limiting translation to clinical fractionated therapy.
CLINICAL TAKEAWAY
Carbon ions produced a more persistent biological injury pattern than isoeffective low-LET irradiation, but the study also shows that hypoxic niches remain biologically active even after curative high-LET treatment. The late proliferative and macrophage rebound may help explain microenvironmental adaptation, but these are mechanistic animal data rather than evidence for changing prostate cancer treatment.