Section 2 – Physical Factors & Dose-Time Effects

Physical Factors Affecting Radiosensitivity

  • Linear Energy Transfer (LET)

    • LET=energy depositedpath lengthLET = \frac{\text{energy deposited}}{\text{path length}}

    • Higher LETLET ⇒ more energy per unit length ⇒ greater biological damage.

    • α\alpha-particles & protons: high LETLET; X-rays & electrons: low LETLET.

  • Relative Biological Effectiveness (RBE)

    • RBE=D<em>250 kVp X-raysD</em>test radiationRBE = \frac{D<em>{250\,\text{kVp X-rays}}}{D</em>{\text{test radiation}}} (doses give the same biological effect).

    • RBE > 1 ⇒ test radiation is more damaging than 250 kVp X-rays; RBE < 1 ⇒ less damaging.

    • Efficiency (“damage per dose”) can be desirable (tumor therapy) or undesirable (diagnostic exposure).

  • Dose-Time Relationship

    • Spreading dose over longer time → less damage (allows cellular repair).

    • Acute exposure: high (or low) dose delivered rapidly.

    • Chronic exposure: dose spread over long duration.

Techniques to Modify Dose-Time Profile

  • Fractionation

    • Same dose rate, total dose divided into multiple fractions separated by ≥24 h.

    • Widely used in radiotherapy to let normal tissue recover.

  • Protraction

    • Same total dose, delivered continuously at a lower dose rate over extended time.

    • Repair occurs during exposure.

  • Example summary (mouse model)

    • 6 Gy6\,\text{Gy} in 3 min3\,\text{min} (2 Gy min−1^{-1}) → lethal.

    • 12 fractions of 0.5 Gy0.5\,\text{Gy} at same rate or 6 Gy protracted at 10 mGy h−1^{-1} (≈600 h) → survival.

Risk & Protection (preview)

  • Key risk metrics to follow: excess, absolute, relative risk.

  • Protection strategies (to be detailed later): shielding, distance, barriers.