Radiation Oncology Flashcards
How does RT work?
Causes cell death by DNA damage
How does RT cause DNA damage?
- Damages DNA through direct interaction
2. Creates free radicals by hitting a water molecule that then damage DNA
What is RT dosed in fractions?
- This allows time for normal tissue to repair its DNA
- Caveat is that tumor cells can also repair its DNA during the break
What determines whether a tumor is radiosensitive or radioresistant?
- Capacity for DNA repair
2. Oxygenation (under hypoxic conditions, free radicals can be scavenged by the hypoxia-induced acidic environment)
What is the most commonly used form of RT?
Photons
What is electron RT good for?
Skin cancer bc it has short depth of penetration (90% deposits energy w/in 2 cm)
How is RT produced?
Linear accelerator
What is 1 gray (Gy)
- The amount of energy dose absorbed per unit mass
- 1 J/kg
- Equal to 100 rad
What is the typical total dose for adjuvant RT for HNC
60-66 Gy
What is the typical total dose for definitive RT for HNC
70-74 Gy
What is “radiobiologically equivalent dose”
It is the total dose of RT tolerated and is dependent on the fractionation chosen
-e.g. may be 30 Gy at 3 Gy fractions vs 45 Gy at 2 Gy fractions
What are the different fractionation patterns available?
- Conventional/standard: QD Mon-Fri
- Hypofractionation: QD
- Accelerated fractionation or concomitant boost: once daily until last 12 days of Tx, then >QD
Which fractionation pattern results in the best local control?
- Hyperfractionation or accelerated
- Risk is increased acute toxicity (but no significant increase in late toxicity)
What are the 2 ways to improve RT outcome?
- Hyperfractionation
2. Chemotherapy
When should you choose chemotherapy or hyperfractionation?
- Only use hyperfractionation for pts who can’t get chemo and must be treated with RT alone
- Chemo + hyperfractionation vs Hyperfractionation alone –> 70% vs 44% locoregional control
- Chemo + standard fractionation had no difference in outcome vs chemo + hyperfractionation
Why should treatment breaks be avoided during RT?
Local failure increases when total treatment time (from surgery to completion of RT) exceeds 11 wks
How does one reduce setup error between and during treatment?
Pts have a plastic mask to reproduce positioning
What is the difference between
- Gross tumor volume (GTV)
- Clinical tumor volume (CTV)
- Planning tumor volume (PTV)
- GTV: The areas of actual tumor (this should be zero for adjuvant RT bc it follows surgery)
- CTV: Areas at risk for harboring microscopic dz
- PTV: an extra 3-10mm expansion on the CTV or GTV to account for errors in daily setup
What is an “organ at risk (OAR)”?
-Normal tissue not involved w/ cancer that needs to be protected from RT (spine, parotid, mandible, pharyngeal constrictors)
How much radiation can normal parotids take?
25 Gy to 50% of the gland
How much radiation can the mandible take b4 getting ORN
70 Gy
What is a dose volume histogram (DVH)?
- A plot of percent volume of a structure vs the radiation dose.
- Used to evaluate a radiation plan and determine if its safe for normal structures (OARs)
Indications for adjuvant RT
- margins
- nodal dz
- Large tumor size (T3, T4)
- PNI
Indications for adjuvant chemo
- ECS
- + margins
What is IMRT?
- Intensity-modulated radiation therapy
- Uses multiple radiation beams (beamlets) from different directions all of which can have different intensities of radiation
- Allows radiation dose to “bend” around nl structures
- Requires a multileaf collimator (MLC) but its cheap and easy to implement
Advantage of IMRT?
- Protect OARs whereas all structures in the 3D plans path would get high dose
- Thus, reduces likelihood of late radiation-induced toxicity (xerostomia 39% vs 82% in conventional RT)
- Reported improved QOL
Disadvantages of IMRT?
- Lot of planning required (not ideal for rapidly growing or bleeding tumors that need plans quickly)
- More expensive
- While the high-dose RT is very focused, a much broader area gets low-dose RT in comparison to 3D planning (can result in nausea from RT through the brainstem)
- ?higher chance of radiation-induced cancer since broad low-dose volume
When does acute toxicity from RT typically appear? plateau?
- At the 2 wk mark
- Plateaus at 5-6 wk mark
Acute toxicity from RT
- PAIN (sore throat/mouth) from mucositis
- Taste change
- Loss of facial hair (in the field)
- Skin irritation
- Thick mucous
- Dry mouth
When do the acute toxicities from RT abate?
- Skin breakdown heals in about 2 wks
- Thick mucus resolves around 3-4 wks
- Pain improves around 8-12 wks
- Most can return to work at 8 wks post-RT
What are the late toxicities from RT
- Xerostomia
- Taste change (resolves by 3 months usu)
- Hypothyroidism
- Neck fibrosis
- Carotid artery stenosis
- ORN
- Dysphagia and aspiration
What is the role of PET with RT
PET-based contouring allows for a significantly smaller target volume with good survival rates
Why is proton radiation better than photon radiation?
- Photons lose energy slowly as they travel through tissue so energy gets deposited in nl tissue in front of and behind tumor
- Protons have very low energy loss until reach specific depth at which point all their energy is released (thus don’t affect nl tissue as bad)
What is “Bragg peak”
The sudden peak in dose at a specific depth seen with proton radiation
What is radiosurgery?
- Requires very stringent immobilization and alignment techniques allowing for high doses of RT to be delivered in a highly conformal manner
- Much higher doses over much fewer fractions
