Dose Grid-Brachytherapy

HDR vs .LDR

-HDR has increased risk of normal tissue complications -reduced need for hospital stay -reduced radiation exposure to staff -makes source inventory easier since only one source used -dose distribution can be optimized

What is an example of a secondary calc checked used for an HDR plan?

-Mobius -performs a point dose comparison to any point in the plan -should be within 5%

Quimby System

-a uniform distribution of sources of equal linear activity -higher dose in the middle of the implant

Details on prostate implants:

-can be temp. or permanent -use long half life sources to deliver dose in 3-4 days then source is removed -permanent= short half life, low energy, source is left in pt

Paterson-Parker System

-designed to implant sources of various strength/configuration -uniform dose within 10%

Intraluminal Implant

-for esophagus/bronchus treatment -dose distribution looks cylindrical

Manchester System-Rectal point

-frontal radiograph= midpoint of the ovoid sources -lateral radiograph= line drawn from the middle of the ovoid sources, 5mm behind the posterior vaginal wall

Manchester System- Bladder point

-localized by using a foley cath (contrast) -on frontal radiograph=the bladder point is marked at the center of the balloon -lateral radiograph= point is placed in the middle of the ant/post directions of the balloon at the posterior edge of balloon

Manchester System-Point A

-point of dose prescription * 2 cm sup of external cervical os and 2cm lat of cervical canal -where the uterine vessels cross the ureter -the tolerances of these structures is the main limiting factor in irradiating the uterine cervix

When is leak testing done on the pigs and how?

-semi annually -using a scintillation well counter pigs are used to transport sources because they are lead lined and on wheels

After you change out a HDR source with a new one, what tests must be done?

-source strength determination -source positioning accuracy -dose linearity/timer accuracy -wipe test of package -the calibrations must be traceable to the NIST

What does prostate HDR doses depend on?

-source used -if there is a boost or not

Intravascular Implant

-temp (sealed sources or liquid filled balloons) or permanent ( radioactive stents) -used for restenosis ( artery blockage at tx site) -target vol= region of angioplasty -depth of dose rx is 2mm from the center of the source and for peripheral arteries its 2mm beyond the average lumen radius -15-20 Gy to a 2-3 cm length of the arterial wall -high dose rate beta sources are preferred

External Applicator/molds

-treats skin lesions -surface mold applicators were loaded with gamma emitters and placed on skin

Conversion of mg-Radium equivalents:

-used to convert radium to the source being used mgRaeq= isotope/radium

NRC licensing requirements for HDR:

-written procedures -pre tx safety checks -HDR operating procedures -emergency procedures

4 Dose specifications for HDR Intracavitary of cervical cx:

1. 100% of dose tapered along the tandem; and 140% along the ring/ovoid surface 2. 100% along tandem; 100 % along ring/surface 3. 100% of dose to point A without other applicator specific points 4. non optimized point using dwell weighting to simulate LDR loading with dose spec. at point A

What are the different types of ways to deliver brachytherapy?

1. External Applicators/molds 2. Interstitial implants 3. Intracavitary implant 4. Intraluminal implant 5. Intravascular implant

What are 3 historic systems of brachy implants?

1. Paris System 2. Quimby System 3. Paterson-Parker System

Radium sources are specified by what 4 things?

1. active length 2. physical length 3. activity/strength of source 4. filtration

Radiation monitoring systems for brachy?

1. scintillation well counter: leak testing of brachy sources 2. Geiger Mueller counters: much more sensitive then ion chamber, not very specific but good for looking for leaks and surveying

Radium needles can have their activity where in the needle?

1. uniform linear intensity (full intensity=0.66mg/cm or half intensity=0.33mg/cm) 2. higher activity at one end= club 3. high activities at both ends= dumbbell 4. tube

Manchester System-Point B

3 cm lateral to point A

What is the exposure rate constant of radium?

8.25Rcm^2/mg-h or 0.825 mR*m^2/mghr

What sources are used in permanent HDR to treat early stage prostate cancer?

I-125 or Pd-103

What is the most commonly used brachytherapy technique?

Manchester System (LDR radium brachytherapy) -tandem and ovoid

Prostate HDR dose using Pd-103

Mono= 115 Gy Boost= 90 Gy

Prostate HDR dose using I-125

Mono= 145 Gy Boost (45gy EBRT)= 120Gy

What 4 points does the Manchester system use?

Point A Point B Bladder point Rectum point

Some protection guidelines when handling brachy sources (NCRP Report 40)

Source prep: forceps, after loading technique Storage: leadlined, exhaust to outdoors, sink with filter Shielded drawers Shielded pigs Shielded barriers

Intracavitary Implant

Uterine, cervix, vagina= tandem and ovoids Uterine corpus= Heyman capsules Vaginal cylinder

Afterloading

a technique where sources are loaded into tubes previously implanted into the tissues; reducing exposure to personnel

Specific Activity

activity per unit of mass (mCi/g) -a high specific activity means you have a small but powerful source

Apparent activity of a brachy source:

determined by dividing the measured exposure rate at 1m with the exposure rate constant of an unfiltered source at 1m

How can you tell exactly where your radioactive sources are within the patient?

either two radiographs are taken and reconstructed or the film coordinates are put in manually using a digitizer (both older techniques)

What can HDR implants be used for?

interstitial or intercavitary -most HDR implants contain a single Ir-192 source

What is HDR defined as?

prescription dose rates of 20cGy/min or higher

Interstital Implants

radioactive sources in needles, wires, seeds, and directly inserted into tissue ex. prostate

Historically, what was the most common radioactive source?

radium

Paris System

removable implants of long line sources of Ir-192

White I label for packing of brachy sources:

surface <0.5 mrem/hr non detectable at 1m

Yellow II label for packing of brachy sources:

surface <50mrem/hr 1mrem @ 1m

Yellow III label for packing of brachy sources:

surface >50 mrem/hr 1mrem/hr @ 1m

What does an autoradiograph test and how is it done?

tests the uniformity of activity distribution -it is done by placing a source on a x-ray film and leaving it there until the darkness is displayed

Physical length

the length of the actual source from end to end

Active length

the length of the radioactive source from end to end

Air Kerma Strength

the product of air kerma rate in "free space" and the square of the distance of the calibration point from the source center along the perpendicular bisector (similar to exposure, but the exposure term is not used in brachy)

When should an HDR source be replaced?

there are no requirements requiring frequency of source replacements

What does an isotropic source mean?

where the intensity is equal in all directions