LII:Radiation Oncology/Physics/Dosimetry

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  • Biologic damage to DNA and other structures depends on the amount of ionizing (directly or indirectly) energy absorbed from the radiation beam and deposited into the tissue
  • Absorbed dose is defined as energy absorbed per unit mass of tissue. It is essentially the measure of liberated electrons
  • It is difficult to measure absorbed dose directly in tissues; instead, ionization is measured in air (exposure) and then converted to absorbed dose tissue equivalent
  • Bragg–Gray cavity theory

Radiation Exposure

  • Dose of radiation absorbed correlates directly with the energy of the beam
  • Gray = basic unit of absorbed dose, in energy (J) per unit mass (kg). Replaces rad (100 rad = 1 Gy)
  • to be continued ...

CBCT Exposure

  • Vanderbilt; 2009 PMID 19147025 -- "Radiation dose from kilovoltage cone beam computed tomography in an image-guided radiotherapy procedure." (Ding GX, Int J Radiat Oncol Biol Phys. 2009 Feb 1;73(2):610-7.)
    • Monte Carlo simulation, adult and pediatric
    • H&N scan, half-fan mode: adult vs. peds eye 7 vs 8 cGy, cord 5 vs 6 cGy, brain 5 vs 6 cGy, vertebra 18 vs 23 cGy. Full-fan mode 10-20% lower dose
    • Abdomen, half-fan mode: prostate 3 and 7 cGy, femoral heads 7 and 17 cGy
    • Conclusion: Doses can total 3 Gy over entire treatment course



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