Khan Outline: Ch. 15 Brachy Therapy

Front 1

List the sources with half lives on page 316 and also include the clinical uses.

Back 1

Cesium-137: used mostly for low-dose-rad (LDR) intracavitary brachytherapy such as in the treatment of uterine cervix.

Iridium-192: Seeds contained in nylon ribbons for interstitial implantation. High-intensity iridium sources, welded at the end of a flexible cable, are used for HDR brachytherapy.

Gold-198: Used mostly in eye plaques for treating intraocular tumors such as chorodial melnoma.

Iodine-125: Used for permanent implants such as in the treatment of early-stage prostate cancer.

Palladium-103: Have the same clinical application as that of I-125 seeds

Front 2

What does Uniform mean?

Back 2

The Paterson-Parker system specifies uniform distribution of source activity to attain "uniform dose" as within +- 10% in the specified plane of treatment.

Front 3

What does Indian Club mean?

Back 3

Needles with higher activity at one end.

Front 4

What does Dumbbell mean?

Back 4

Needles with higher activity at both ends.

Front 5

What does Tube mean?

Back 5

The tube is sealed container that holds the radioactive seeds.

Front 6

What is the purpose and characteristics of well type ion chambers?

Back 6

The well-type ionization chamber is used to calibrate the chamber. It serves as the working standard for calibrating Ir -192 seeds.

Front 7

What does the Sievert integral account for?

Back 7

The Sievert Integral is used to calculate the dose distribution around a brachytherapy source.

Front 8

What is isotrophy?

Back 8

Having the same measured dose in different directions.

Front 9

What is TG-43?

Back 9

TG-43 uses a modular approach to determine dose distributions. Calculations involve the Dose Rate constant for the source, the Air Kerma Strength, The Geometry Factor, The Radial dos function, and the Anisotropy function.

Front 10

Describe the Paterson-Parker system of implant dosimetry.

Back 10

Planar Implants: Stated dose to be 10% higher than the minimum dose in a parallel plan 0.5 cm away.

Volume Implants: Stated dose to be 10% higher that the minimum dose within the implanted volume.

Front 11

Describe the Quimby system of implant dosimetry.

Back 11

Planar Implants: Stated dose to be the maximum dose in the plane of treatment (e.g., 0.5 cm away)

Volume Implants: Stated dose to be the minimum dose within the implanted volume.

Front 12

Describe the Paris system of implant dosimetry.

Back 12

Stated dose to be the value of the reference isodose surface or 85% of the basal dose.

Front 13

Describe the Computer system of implant dosimetry.

Back 13

Stated dose to be the value of the isodose surface that just covers the implant boundary (which should correspond to the CTV boundary).

Front 14

Describe the different implant techniques

Back 14

Surface Molds:

Plastic molds are prepared to conform to the surface to be treated and the sources are securely positioned on the outer surface of the mold. The distance between the plane of the sources to the skin surface is chosen to give a treatment distance of usually 0.5 to 1.0 cm.

Interstitial Therapy:

Radioactive sources are fabricated in the form of needles, wires or seeds, which can be inserted directly into the tissue. There are two types: temporary and permanent implants.

Intracavitary Therapy:

Intracavitary therapy is mostly used for cancers of the uterine cervix, uterine body and vagina. A variety of applicators have been designed to hold the sources in a fixed configuration.

Front 15

What is meant by Milligram hours?

Back 15

The product of total source strength and the implant duration.

Front 16

Define the locations of Points A, B, Pelvic wall, bladder and rectum.

Back 16

Point A: 2 cm Superior to the external cervical os and 2 cm lateral to the cervical canal.

Point B: 3 cm Lateral of point A.

Bladder Point: The bladder point is localized by using a Foley catheter, with the balloon filled with a contrast material. On the fronta radiograph, the bladder point is marked at the center of the balloon; on the lateral radiograph , the bladder point is obtained on a line drawn anteroposteriorly through the center of the balloon, at the posterior surface.

Rectal Point: The rectal point is identified on the frontal radiograph at the midpoint of the ovoid sources. On the lateral radiograph, the rectal point is located on a line drawn from the middle of the ovoid sources, 5 mm behind the posterior vaginal wall.

Pelvic Wall Points: On the anteroposterior radiograph, the pelvic wall points are located at the intersection of a horizontal tangent to superior aspect of the acetabulum and a vertical line touching the medial aspect of the acetabulum. On the lateral view, these points are marked as the highest mid distance points of the right and left acetabulums.

Front 17

Define total reference Air Kerma?

Back 17

The total air kerma strength of sources times the implant duration.

Front 18

Define reference volume

Back 18

The Reference volume is the volume of the isodose surface that just surrounds the target volume.

Front 19

Describe the advantages and disadvantages of remote afterloading units.

Back 19

Remotely controlled afterloading devices are now available that eliminate the direct handling of the radioactive sources. In addition, the sources can be instantly loaded and unloaded, making it possible to provide patient care with the sources retracted into their shielded position.

Advantages:

1. Major advantage is the elimination or reduction of exposure to medical personnel.
2. Optimization of dose distributions beyond what is possible with manual afterloading.
3. Treatment techniques cam be made more consistent and reproducible.
4. Sources can be retracted into a shielded position to allow better patient care under normal as well as emergency conditions
5. Permits treatment on an outpatient basis, using multiple fraction regimens.
6. Suited for treating large patient populations that would otherwise require prolonged hospitalization.

Disadvantages:

1. Expensive and require a substantial capital expenditure for equipment acquisition.
2. Additional costs mus be considered for room shielding and installing ancillary imaging equipment.
3. Locating HDR in and existing radiation therapy room compounds the problem of patient scheduling unless the room is dedicated to HDR brachytherapy.
4. No significant improvements are expected in clinical outcome over state-of-the-art conventional LDR brachytherapy.
5. Quality assurance requirements for remote afterloading devices are significantly greater because of the greater complexity of the equipment and frequent source changes.

Front 20

How does HDR compare to LDR?

Back 20

HDR is superior in respect to radiation protection, reproducibility of treatment, the ability to treat large patient populations and convenience of treating on an outpatient basis. However, the outcome data in terms or survival ratess and early and late tissue complications rates for the HDR technique to date are not sufficient to draw definitive conclusions.