281 - Radiation Oncology

Question 1

How does fractionating radiation allow for better cell kill and sparing of normal tissues?

Answer 1

Fractionating radiation = spreads radiation treatment out over several weeks.

• Spares normal tissue
  
  • Allows normal tissue to repair and re-populate itself
  • Often, malignant cells do not have these repair mechanisms
• Better cell kill
  
  • Reassortment: Allows cells in radio-resistant phase to get to a more radio-sensitive phase before the next tx
  • Reoxygenation: Tumors that are hypoxic are radio-resistant; they may reoxygenate (radio-sensitive) in between fractions which allows for a better cell kill

Question 2

What is the mechanism by which radiation induces a biologic event?

Answer 2

• Directly ionizing (~1/3 of damage)
  
  • Ionizing radiation causes DNA breaks
• Indirectly ionizing (~2/3 of damage)
  
  • Ionizing radiation generates free radicals, which also causes DNA breaks
  • Oxygen is important for this!

Question 3

What is brachytherapy?

When is it indicated?

Answer 3

Brachytherapy: Focal delivery of a radioactive isotope via implant for the purpose of cancer therapy

Example: curative therapy in locally advanced cervical cancer

Vs. external beam = shooting x-rays or gamma rays at the tumor from an external source

Question 4

What is “gamma knife” radiation?

When is it used?

Answer 4

Gamma knife = stereotactic radiosurgery. Uses advanced image guidance

Used when there are small, well-defined targets (not for microscopic spread)

Often used for intra-cranial treatment (single fraction)