Radiation Oncology/Physics/Dosimetry

Dosimetry

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

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 ...

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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