283. Radiation Oncology

Question 1

Explain the types of scenarios where RT is utilized

Answer 1

CURATIVE

• primary (RT alone)
• with chemotx (concurrent or sequential)
• pre-operative (neoadjuvant, with surgery): improve chance of complete margin resection, reduce risk of local recurrence, shrink tumor to allow surgery
• post-operative (adjuvant, with surgery): eradicate subclinical disease
• SALVAGE: eliminate recurrence after surgery (can be curative)

PALLIATIVE
- pain control

Question 2

What are the steps of RT?

- describe a special type of RT tx

Answer 2

1. Consultation
2. Treatment PLANNING
   - Simulation: immobilize pt and use laser guided positioning coordinates or specialized CT scanner for 3D reconstruction
   - Target Definition
   - Planning/Optimization (to be as focal yet as effective as possible)
3. TREATMENT
   - laser guided positioning
   - CT verification
   - daily tx
   - regular clinical monitoring
   - STEROTACTIC: for cranial/extracranial lesions: high dose, VERY targeted for small, well-defined targets (not microscopic spread), uses multiple beams (ex: Gamma Knife - 192 beams - accumulation of many low dose frames for high dose focal tx while preserving surroundings)

Question 3

Radiation Physics

• what is radiation
• two types of radiation
• how does it spare other tissue?

Answer 3

Radiation = photon production

1. X-rays: extra-nuclear photons made from powerful electrical devices that accelerate electrons to high energy (LINAC)
2. Gamma-rays: intranuclear (atomic) photons made by radioactive isotope decay (only use Co-60 isotope, which has long half life and is dangerous for clinicians)

3D conformational tx, use multiple beams to shape out tumor - computerize simulations to modulate beam SHAPE and INTENSITY to better model shape, dose, and intensity for optimal SPECIFIC tumor tx - highly computational to be tissue-sparing

Question 4

Radiation biology

• what is the mechanism of RT in cancer tx? Which is most biologically significant?
• what factors improve RT or hinder RT?

Answer 4

MoA: many ssDNA breaks (easily repaired), many damaged bases (easily repaired), 40-50 dsDNA preaks per cell/per Gy (MOST BIO SIGNIFICANT)
1/3 direct damage - direct RT-induced ionization of DNA
2/3 indirect damage - ionization leading to free radical chain rxn

Factors

• OXYGEN: more oxygen = more ROS to damage tumor (tumors with higher Hb have more oxygenation and are more sensitive to RT = better pt survival)
• ANEMIA: anemia = more tumor survival (less ROS) = important to CORRECT anemia
• CELL CYCLE: maximum sensitivity to M and G2 phase, decreased sensitivity to S phase

Question 5

Radiation Biology

• what are 4 factors that favor fractionation of RT?
• which is more effective: fractionation or single dose? why?

Answer 5

1. Re-oxygenation (of tumor = more tumor damage)
2. Re-distribution (of tumor = more in G2/M phase)
3. Repair (of good tissue)
4. Repopulation (of good tissue)

Fraction is MORE important than total dose!

• tumor cells behave linearly - killed early by RT
• normal cells behave quadratic - killed late by RT
• Lower doses (fractions) preferentially DAMAGE tumor over normal tissue!
• best RT: low doses over multiple intervals (determines biologically equivalent dose - safest dose to kill tumor and not kill normal)

Question 6

What are the toxicities assoc with RT?

Answer 6

Acute: epithelial, hematocytopenia, inflammation
- Skin: Grade 1 (erythema), G2 (dry desquamation), G3 (moist desquamation)

Late: vascular, fibrosis, parenchymal cell death

• Telangiectasia
• Fribrosis (retraction)