Brachytherapy Exam 5

What is the acceptable level of agreement in the delivery of prescribed dose in brachytherapy?

+/= 15%

How many sources are used in an HDR unit?

1

Name three radiation safety rules for radioactive patient visitors.

1. Any person under the age of 18 and/or any person who is pregnant is NOT permitted to visit patients with radioactive implants 2. Visitors should remain six feet from the patient and behind bedside shields 3. Visits should be limited to 30 minutes per day

Name the three basic principles for safety in handling brachytherapy sources, and how you would accomplish those.

1. Time - work quickly but safely when handling radioactive materials 2. Distance - Use long handled forceps and scissors to increase the distance between the sources and the operator's hands 3. Shielding - leads shields at the workbench, preferably with a leaded-glass viewing panel built in

Name 3 places to post radioactivity signs for brachytherapy treatment.

1. Wrist band on the patient stating the radioisotope in use 2. Place a cautionary radioactive material tag on the patient's hospital room door 3. Place a cautionary radioactive material chart tag on the front surface of the patient's chart stating the mR/hr at 1 meter, date of implant, number of sources, strength of sources

3 principal QA endpoints/clinical goals:

1. accuracy of source selection 2. spatial positioning 3. control of treatment time

3 important safety procedures to perform upon completion of HDR treatment

1. check radiation monitor before entering the room 2. survey (Geiger counter) of the afterloading device 3. survey the patient

4 disadvantages or concerns with HDR brachytherapy

1. expensive 2. additional cost if not located in an existing shielded facility 3. no significant improvements over LDR 4. frequent source change (every 3-4 months) and complexity of equipment require greater QA

6 advantages to HDR brachytherapy

1. low exposure to staff 2. small source and varying dwell times allows greater flexibility in creating dose geometry 3. eliminates cost of source inventory 4. eliminates multiple shielded rooms 5. income stays in radiotherapy 6. outpatient treatment, suited for treating large patient populations

5 daily HDR QA

1. monitors and interlocks checked 2. unit statement of source strength correct 3. test run for source positioning, timing 4. check applicator connections to unit 5. availability of emergency equipment (long forceps, geiger counter, lead receptacle)

List 4 description of sources

1. physical and chemical form 2. source encapsulation 3. radionuclide distribution and source uniformity 4. source identification

Brachytherapy QA procedures

1. source identification and description 2. leak testing 3. autoradiography for uniformity 4. user calibrates source activity 5. sources kept in locked shielded safe 6. shielded containers for transportation 7. Geiger counter

During calibration verify:

1. source uniformity 2. correct number/spacing of seeds 3. reproducible position in chamber 4. ADCL chamber calibration 5. compare to calibration source

What is the activity of the source used in an HDR unit?

10 Ci = 370 GBg

How many seeds should you calibrate when using secondary traceability by statistical inference?

10%

Dose rate ranges for high dose rates

1200 cGy/hr or higher

Dose rate ranges for medium dose rates

200-1200 cGy/hr

Dose rate ranges for low dose rates

40-200 cGy/hr

What is Sk and what are its units?

Air kerma strength cGycm2h-1 = 1U

G(r,theta) is a factor that you multiply times the inverse square to correct for activity distribution in a linear source. What do you think the G(r,theta) factor would be for a point source?

G(r,theta) would be 1 for a point source because there is not distortion of the inverse square law from a point source like at close distances to a linear source.

What is the most common source used in an HDR unit?

Ir-192

If the anisotropy function shows how the dose at equal distances around the source differ, how can you have an anisotropy constant and just use one factor to account for anisotropy at whatever angle?

Theoretically, the seed implants will end up with seeds pointing in all directions rather than in straight lines, which means that with a large number of seeds when you average the anisotropy function for each one of the different angles, you could come up with one anisotropy function.

How can the dose rate constant be different for two sources of the same activity, same radioisotope, but different manufacturers?

Two sources of the same isotope and activity could have different (dose rate constants) due to different source construction (source construction and distribution of radioactivity within the source).

TG40 recommends that source strength be stated in what term?

air kerma strength

source or calibrator has been calibrated at NIST or an ADCL

direct traceability

How can varied shaped isodose distribution be produced with only one source?

programming dwell position and variable dwell times per step

Explain r and theta and state the standard values.

r = distance from the center of an interstitial brachytherapy source, standard value = 1cm theta = angle with respect to the long axis of the source, standard value = 90 degrees

institution relies on the manufacturer's calibration

remote traceability

source is calibrated in comparison with a source of the same design and similar strength which has direct traceability

secondary traceability

established for a group of sources from which a suitable random sample has been calibrated with secondary traceability

secondary traceability by statistical inference

What type of chambers measure radiation intensity at a distance are preferred for high dose rate sources?

thimble chambers

What type of chambers are preferred for conventional strength brachytherapy sources?

well ionization chamber