Radiation Oncology Flashcards
What is the lower limit of detection of tumor cells?
What is a lethal volume of CA ?
Tumors grow EXPONENTIALLY
Lower limit of detection: 1 x 10^9 cells (1cc^3)
A lethal volume of CA is about 10^12 (1 Kg)
Most CA have therefore completed about 2/3 of their natural life span before they detected
What is radiation?
electromagnetic radiation @ higher frequency / higher energy
–> causes DNA damage –> cell killing
What is external beam RT?
Aim ionizing radiation at a tumor using beams that come from the outside
(ionizing radiation = energy strong enough to ionize an atom)
Historical method: gamma radiation with Co-60
gamma rays = photons emitted from radioactive materials as they decay
Current method: megavolutage x-ray radiation using a linear accelerator
e-rays are photons produced by machine- high energy photons are produced by a linear accelerator when tungsten target bombarded by high energy electrons
kV vs MV
Kilovoltage - diagnositic
photoelectric effect creates sharp distinction btw tissue type
energy btw 25 kV -150 kV
Magavoltage (MV) - radiation therapy
Intentionally ionizing
energy bt 4mV - 20 mV
What is 3D conformal?
type of external beam radiation therapy
Radiation plan based on CT scan of pt in the radiation position
What are different types of external beam radiation therapy (3)
3D conformal
Conventional (simple fields, planning based on bony landmarks)
IMRT - intesity modulated radiation therapy, computer-aided* planning using multiple ~100 *overlapping fields; intensity of radiation beam varies across the radiation field; inverse treatment planning
- if used as treatment, ~ 70 Gy to regions of gross disease
(concurrent with q 3 weekly cisplatin chemo; post treatment imaging at 8 weeks to assess reponse to therapy)
How is a 3D conformal dose determined:
How is it measured?
What is a typical dose per day?
What is a typical dose to a whole breast?
Unit of dose = gray (Gy)
1 Gy = i Joule/kg, measure of energy deposited in the tissue treated
Typiecal dose per day = 2Gy
Typical total dose to whole breast = 50 Gy
50 Gy/ (2 Gy/day) = 25 treatments
~5 weeks of treatment
What is a boost?
How does this differ from normal radiation therapy?
How long is this typical treatment done for?
The region of the tumor bed is the area at highest risk for recurrence thus addiational radiation is given to the lumpectomy cavity
Boost uses a different delivering technique vs radiation therapy to the neoplasti tissue. Boost is typically done with electrons (rather than photons) to keep dose to other tissues low - don’t penetrate as much.
~1 more week of therapy
Why is radiation given typically?
Reduces risk of recurrence
Improves survival
Usually adjuvant therapy = in additional!
How is an IMRT plan done?
Radiation oncologist defines regions to treat and doses to give to both GROSS tumor and PROPHYLACTIC areas
+
Define regions to avoid (parotid glands, spinal cord, eyes..)
–>
Radiation physicist creates plan
Max treatment, minimize side effects
What is brachytherapy?
Placement of radioactive materials into or immediately adjacent to the cancer
Requires an invasive procedure
May be done alone or in combo with external beam radiation therapy
What are some options for prostate ca?
Radical prostatectomy
radiation therapy
- For prostate CA: external beam radiaiton therapy is given at aprox 78Gy = aprox 40 treatments*
- VS*
- Brachytherapy- low dose rate (permanent), high dose rate (temp)*
watchful waiting
What is the difference between HDR and LDR:
Temp or perm?
How long?
What are some typical isotopes used (4)?
Low dose:
sources could be temp or permanently
Radiation is delievered over days to weeks
typical isotope for gyn cases = Cs-137
typical isotopes for PPI cases = I-125 and Pd-103
High Dose:
Radiation source is temp
source is VERY HOT so given within minutes
typical isotopes Ir-192
How is a permanent prostate implant delievered?
radiactive seeds are inserted into the prostate and remian there forever
Seeds are loaded into needles –> needles directed thru the perinueum and into the prostate under US guidance
What are the typical implant dose given for PPI when using..
1-125?
Pd- 103?
What are their half life?
I-125 –> 144 Gy
half life: 60 days
Pd- 103 –>125
half life: 17 days
dose is high because radiation is given off slowly
