Wk08 LDR Brachytherapy

Question 1

What are isotopes?

Answer 1

Atoms of the same element with the same no of protons but different number of neutrons (i.e differ in mass number)

Question 2

What are the difference among isotopes?

Answer 2

• different in number of neutrons
• atomic masses
• position in the nuclide chart
• physical properties determined by the mass of atom
  Eg. Densities, mass spectrometry, radioactivity, chemical reaction rates

Question 3

What are the decay mode and half life of I-125 and I-131 respectively?

Answer 3

I-125: electron capture ; 60days
I-131: beta decay ; 8.04 days

Question 4

Both brachytherapy and teletherapy obey the ___________________.

Answer 4

Inverse square law
(One very close to source and one quite far away)

Question 5

What is the normal dose rate for low dose rate brachytherapy?

Answer 5

<2Gy/h

Question 6

What is the main equipment for LDR source measurement?

Answer 6

The well chamber

Question 7

What implant source is used for prostate brachytherapy?

Answer 7

Iodine-125

Question 8

LDR dose calibrator well chamber is________________ to increase ______________.

Answer 8

Pressurised ; sensitivity

Question 9

Dose distribution of a point source is not a a perfect spherical, is a bit oval. This phenomenon is called ___________________.

Answer 9

Anisotropic

Question 10

What model is used to calculate unsealed source radiation therapy?

Answer 10

MIRD model

Question 11

Selective internal radiation therapy make use of which radionuclide?

Answer 11

Yttrium-90

Question 12

What is the half-life and emission of Y-90?

Answer 12

64 hr ; pure beta emission

Question 13

What are the limiting factors / consideration factors when perform SIRT?

Answer 13

Lung shunting and normal tumour size

Question 14

What is the mechanism of SIRT?

Answer 14

To block nutrient supply and induce radioactive reaction

Question 15

What is the another usage of Y-90 besides HCC?

Answer 15

Chronic synovitis and effusion of joints
Pheumatoid arthritis

Due to its good retention in the joint and less traumatic than open surgery

Question 16

For Iodine-131, what is its physical half-life and effective half-life

Answer 16

Physical half life = 8 days
Effective half-life = 5.5days

Question 17

At what dose rate and radioactivity that a patient receiving Iodine-131 treatment needed to be hospitalised?

Answer 17

Dose rate >25 uSv/h at 1m
Radioactivity >400MBq

Question 18

What are normal prescription for I-131 thyroid cancer treatment?

Answer 18

30-150mCi, mainstream 80 mCi after thyroidectomy

Question 19

More residual thyroid, __________ dose prescribed.

Answer 19

Less

Question 20

What is the use of I-131 in paediatric?

Answer 20

MIBG scan
To diagnose neuroblastoma

Question 21

For pain relief to bone metastasis, what radionuclide can be used for treatment?

Answer 21

Sr-89 and Ra-223

They have similar chemical properties as calcium
Selective uptake by bones.

Question 22

What radionuclide can be used to treat non-Hodgkin lymphoma?

Answer 22

Y-90 labelled lymphoma specific antibody

Question 23

What is the mechanism behind radioimmunotherapy?

Answer 23

• injected antibody carrying the Y-90 isotope
• the antibody target the antigen CD20 which express in lymphoma
• attachment of the antibody to the antigen induce radiation crossfiring effect and cause radiolysis
• dose delivered to kill lymphoma cells selectively

Question 24

What is the importance of brachytherapy dosimetry systems?

Answer 24

Dosimetric systems are rules to guide the source arrangement and dose levels according to the geometry of the lesions in a brachytherapy case.

Question 25

Name some common brachytherapy dosimetry systems

Answer 25

- Manchester system - Paris system - Quimby system - memorial system

Question 26

What is the principle of early Manchester system?

Answer 26

Half strength needles placed at the middle Full strength needles placed at the peripheral

Question 27

What are the 4 basic principles of the PARIS system ?

Answer 27

1. The radioactive sources must be rectilinear and parallel, and locate on the same principle plane. 2. The reference linear air kerma strength must be uniform and identical for all sources 3. Adjacent sources must be equi-distant from each other. 4. Applicable from LDR multi-sources to HDR single source brachytherapy plans.

Question 28

What is RD and BD, and what is their relation?

Answer 28

RD = reference dose ( I.e. prescription dose) BD = basal dose = local minimum RD = 0.85BD BD = 1.18RD

Question 29

What is Bystander effect?

Answer 29

Directly irradiated cells transmit damaging signal to non-irradiated cells thereby inducing a response similar to that of irradiated cells.

Question 30

Which artery are the Y-90 microspheres injected through?

Answer 30

Hepatic artery

Question 31

What is the assumption of the partition model?

Answer 31

Assume all administered activity is deposited in normal liver, tumour and the lungs

Question 32

In 3D dose-volume analyses in IGBT, what is the definition of D90, D98, D100, V90, V100, V200, D2cc, D1cc, D0.1cc and Vd

Answer 32

D90 = the minimum dose delivered to 90% of the target volume D98 = the minimum dose delivered to 98% of the target volume D100 = the minimum dose delivered to the target volume V90 = the percentage coverage of the target volume by the 90% isodose level V100 = the percentage coverage of the target volume by the prescription dose level V200 = the percentage coverage of the target volume by the high dose zone D2cc = minimum dose to the most irradiated 2cm3 of the OAR (to assess morbidity) D1cc = minimum dose to the most irradiated 1cm3 of the OAR D0.1cc = minimum dose to the most irradiated 0.1x3 of the OAR (to access morbidity of very small OARs eg urethra) Vd = volume of a certain dose D or above in a OAR

Question 33

What do MIRD stands for

Answer 33

Medical Internal radiation dose

Question 34

What are the dose limits and dose requirement for SIRT (Y-90) therapy to HCC patients?

Answer 34

Dose limit to lung = 30Gy single therapy and 50Gy cumulative Dose limit to normal liver = 80Gy to normal liver and 70Gy to cirrhotic liver Dose requirement to tumour is 120 Gy

Question 35

What are the assumptions of Y-90 SIRT dose calculations?

Answer 35

1. No biological degradation is expected 2. Assume a homogeneous distribution of micro-particles in the tumour and liver parenchyma 3. Assume Tc99m particles will approximate Y-90 microsphere distribution in partition modelling 4. Body surface area is an estimation of patient liver volumes