Brachytherapy
'Brachytherapy', is derived from the Greek words
'brachy, meaning 'short' and 'therapy', meaning 'treatment'
S sub k = U
1 U = 1 cGy/hr (cm x cm)
Compton Scatter can create 3 types of hits:
1. Direct hit on the target atom 2. Grazing hit on the target atom 3. 90 degree scatter
Temporary Breast Implants
1. Interstitial catheter-based implants 2. balloon-based implants (US - most popular) a. simplicity b. favorable cosmetic results c. patient comfort 3. IORT
HDR remote afterloading units use specially designed Ir - 192 sources with typical activities of 370 GBq (10 Ci).
10 [3.7 x 10(10) Bq] = 10Ci
Point A
2 cm superior to the top of the ovoids and 2 cm lateral to the tandem. Approximates where the uterine arteries and ureters interesect.
Curie = 1 Ci =
3.7 x 10(10) Bq; 37 x 10(9) Bq; Bq = Becquerel
Point B
5 cm lateral from the midline at the same level as Point A, representing the Parametrial Nodes.
The QUANTITY used to describe the Strength of a RADIOACTIVE SOURCE is known as
ACTIVITY
Tandem and Ovoids, T&O
Afterloader used for Cervical Cancer, another word for Ovoids is colpostat. All Ovoids, except the smallest have small shields placed anteriorly and posteriorly to help shiel the bladder and rectum. Placed into female anatomy and stabilized with packing.
Unit recommended for source strength specification
Air Kerma Strength; S sub k
Patterson-Parker
Also know as the Manchester System
Mean Life
Average time it will take atoms w/in sample to decay.
Paris System
Based on clinical work by Drs. Pierquin and Chassange in the 1960's. Follows the development of Remote Afterloading and the introduction of Ir-192 by Dr. Henschke a decade earlier. Similar to the Quimby System, the Paris System uses Uniform distribution of the sources. However, basal points have been introduced to to avoid toxicity associated with high doses at the center of the implant.
Unit of Activity, recommended by International System of Units (SI)
Becquerel = One Disintegration per second Bq = 1 d.p.s. named after French Scientist, Henri Becquerel
In Brachytherapy, where are the doses usually higher?
Brachytherapy doses are very high close to the radioactive source.
Compton Scatter Direct Hit
Compton Electron goes straight forward (i.e. same direction as incident photon) and carries away most of of the E, whereas the Scattered Photon will travel backward from the atom and carry away a minimum of E. This effect is called BACKSCATTER. At high photon energies (such as those from the typical Linac), the E from the secondary Photon approaches a Maximum value of 0.255 Mev, and the number of photons that scatter backwards is very small.
Historic unit of Activity is the ...
Curie (Ci), named after Marie Curie, and assigned the activity of 1 gram of radium.
Quimby/Memorial System
Distribution of activity within implant is uniform, resulting in non-uniform dose distribution. Rationale: when implanting a solid tumor, cells at center of the tumor are hypoxic and therefore more radioresistent, therefore requiring a higher dose at the center. It was recognized; however, that for resected tumors the dose at the center of the tumor should be lowered AND rather than prescribing at the edge of the target (as done for volume implants) the prescription for the planar implant should be defined at the center of the target.
'tele'
Greek word for 'long'
HDR
High Dose Rate; > 12 Gy/Hr; duration is short compared to cell proliferation rate
Compton Scatter Grazing Hit
Hit on the atom by the Incident (incoming) photon will cause very little E loss; thus most of the E is carried away by the Scattered Photon, which as a result is nearly the same E as the Incident Photon.
Inverse Square Law Formula
I1/I2 = (d2/d1)2 Use for Intensity or dose rate (change from isocenter) I1 = I2 (d2/d1)2 I = Intensity OR Dose Rate d = distance
Inverse Square Law
INTENSITY of a radiation beam in a non-absorbing medium increases/decreases as the square of the distance from the source changes due to beam divergence.
Manchester System
In 1930's in Manchester Hospital R. Patterson and H. M. Parker developed a series of guidelines and dosimetry methods. Goal = deliver uniform dose to plane/volume NON-UNIFORM SOURCE DISTRIBUTION = UNIFORM DOSE DISTRIBUTION, resulting in +/- 10% dose w/in targeted area or volume. It has been shown that source arrangement and milligram-hours values predicted by the Manchester System closely approximate computer-optimized plans. This makes the Manchester system an effective tool to perform independent verifications of brachytherapy treatment plans.
Classic Brachytherapy
Initial experience gained with the use of Radium -226
Photons emitted from the Low Energy radioactive brachytherapy source list (photon E < 30 kev)
Iodine - 125, Palladium - 103
Compton 90 Degree Scatter
Is important for radiation protection purposes to look at what occurs when the Scattered Photon emerges at 90 degrees to the Incident Photon. The E of the Scattered Photon reaches a Maximum value of .511 Mev and is essentially Independent of the E of the Incident Photon, even at very high photon energies.
LDR
Low Dose Rate, < 2 Gy/Hr; duration of treatmnet is long compared to cell proliferation rate
MDR
Medium Dose Rate; 2-12 Gy/Hr
Compton (Incoherent) Scatter
Most common photon interaction that occurs in the Energy range used in XRT. Incident Photon interacts w an OUTER Shell Electron (i.e. very loosely bound electron to the atom, sometimes known as a FREE Electron). The Electron gains energy in the interaction, creating a Scattered Photon and a Compton Electron. The Scattered Photon is on a different path and has differing E and Wavelength compared to the Incident (Incoming) Photon.
Pair Production
Occur at high energies. The Incident Photon E must be > 1.022 Mev. In Pair Production the Incident Photon passes close to the NUCLEUS. When the photon interacts with the EM field of the nucleus, it is absorbed and INSTANTLY the E is RE-EMITTED as an ELECTRON - Positron Pair (B- and B+), which is then ejected from the atom. Both the electron and positron will have E of 0.511 Mev and then any leftover E from the Incident Photon will be divided evenly between B- and B+.
HDR Examples
PROSTATE (4 fx x 9.5-10 Gy = 38-40 Gy); CERVIX 5 weekly fx x 6 Gy = 30 Gy OR 4 weekly fx of 7 Gy = 28 Gy; STS/Breast 10 BID fx of 3.4 Gy = 34 Gy OR Single IORT treatment 15-20 Gy
Temporary Implant
Placement of radioactive sources in/next to target volume for a limited amount of time. Afterloading techniques are used.
Permanent Implant
Radioactive Sources or seeds are left in the targeted tissue to decay. Most common use - I-125 for Prostate of 144 Gy or Pd-103 of 125 Gy
Photon emitting radioactive Isotopes, also known as the high-energy group (photon E > 300 kev)
Radium - 226, Co - 60, Cesium - 137, Iridium - 192, Gold - 198
Pulsed Dose Rate Brachytherapy
Reproducing the biologic effectiveness of low dose-rate treatments with remote afterloading technology. A remote afterloader similar to HDR is used to deliver short pulses of radiation, ~ 1x/hr.
The high E group of radioactive sources emitting PHOTONS and interact w matter primarily via COMPTON SCATTER.
Sources in this group are similar to Classic Radium Sources, and thus, this dose profile approximates the Inverse Square Law.
Beta emitting radioactive Isotopes
Strontium - 90, Ytrium - 90, Phosphorous - 32, Ruthenium - 106, Rhodium - 106
Brachytherapy dose distribution is inherently inhomogenous.
The Radiation Oncologist uses the brachytherapy "hot spot" to increase the therapeutic ratio of the treatments.
Volume Implant
The shape of the implanted volume resembles a 3D shape more than a plane. Shapes defined by Patterson - Parker include cylinders, ellipsoids, spheres and cubes.
A sources activity, even though easy to calculate, is difficult to measure because some of the emitted radiation interacts with the source material and encapsulation. True or False
True
AAPM TG-43 recommends the use of Source Strength for calculating brachytherapy doses. True or False
True
As the distance from a brachytherapy source increases, the absorbed dose (in tissues) decreases. True or False
True
Dose distribution from Radium-226 is the basis of all dose calculations in Classic Brachytherapy. True or False
True
MRI is the imaging modality of choice for GYN Implants. True or False
True
The amount of radiation emitted from a source is directly proportional to the absorbed dose around it. True or False
True
The dose computation system developed at Memorial Hospital formed the basis for the modern AAPM TG-43 formalism used today. True or False
True
Intravascular Brachytherapy, IVBT
Widely used in the past to reduce the rate of restenosis after angioplasty and the placement of stents in blood vessels. It has ceded popularity to drug alluding stents.
The Inverse Square Law is used to predict
absorbed dose in high energy photons and as distance increases from radioactive sources.
Source Activity
disintegrations per unit of time; d.p.s./time unit
Electronic Brachytherapy advantages
less shielding, less exposure to personnel and general public, less cost for shielded rooms, shipping & handling,
Electronic Brachytherapy
makes use of miniature x-ray devices v radioactive sources. (e.g. Xoft)
Tandem and Ring
newly designed applicator to avoid artifacts w Ct and MRI
Intraluminal Brachytherapy
placement of radiation sources w/in body tubes, e.g. esophogus, trachea, and bronchus via catheters
Interstitial Brachytherapy
placement of radioactive sources directly into a tumor/tumor bed, using rigid needles or flexible tubes.
Surface Brachytherapy
places radioactive sources on top of the area to be treated, e.g. non-melanoma skin cancer External applicators - Oral cavity, nasal cavity, orbital cavity (ocular melanoma), hard palate Moulds can be designed to incorporate shielding for adjacent sensitive structures.
Sealed sources
radioactive source is encased and sealed w/in titanium or stainless steel tubes that are welded shut at the ends. Today, an isotope may be embedded in plastic or a similar material.
Intracaviatry Implants
radioactive source9s0 are placed withing a body Cavity, using an applicator, e.g. tandem and ovoids for Cervical CA.
Teletherapy
refers to radiotherapy in which the source of radiation is located away from the patient
Decay Constant describes
the fraction of atoms that will disintegrate during the time interval
Describe Brachytherapy treatment.
treatment at a short distance during which radioactive sources are placed in close proximity to, or directly in the treatment volume.
Traditional Brachytherapy
use of radioactive materials as radioactive sources.
Electronic Brachytherapy disadvantages
x-ray generator requires cooling, source assembly is still too large to fully replace traditional sources in all clinical applications.
