Early-responding tissues (skin, mucosa, bone marrow)

  • React within days to weeks.

  • Higher α/β (~10 Gy).

  • Less sensitive to fraction size.

Late-responding tissues (spinal cord, rectum, kidney)

  • Damage appears months to years later.

  • Low α/β (~2–3 Gy).

  • Very sensitive to fraction size.

Tissue tolerance

  • Maximum dose a normal tissue can receive without unacceptable risk of complication.

  • Historical Emami tables (1991) gave TD 5/5 values (5% risk at 5 years).

  • Modern guidance: QUANTEC (Quantitative Analyses of Normal Tissue Effects in the Clinic).

Retreatment principles

  • Must account for prior BED/EQD2 delivered.

  • Assume partial recovery of tolerance over time (not complete).

  • Careful balance needed, especially for spinal cord, brainstem, rectum.

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