Wk10 TBI Flashcards
TBI is part of the conditioning regimens for __________________________.
Hematopoietic stem cell / bone marrow transplantation
What diseases require TBI?
Leukaemia, aplastic anaemia, lymphoma
What are the aims of TBI
- Residual tumor cell eradication, providing therapy to sanctuary sites not easily reached by chemo drugs eg. Brain
- Induction of immunosuppressed to minimise rejection of donor transplant
- Eradicate the patients’s hematopoetic system to allow repopulation
What are the advantages of TBI?
- No sanctuary sites for irradiation, where chemo drugs does not penetrate well
- All area of the body receive nearly the same dose of irradiation, unlike chemotherapy which depends on the metabolism
What are the disadvantages of TBI?
- Radiation-related pneumonitis, a leading cause of death after stem cell transplant
- Potential late side effects
Eg. Sterility, cataracts, growth retardation, neurological toxicity
What are acute side effects of TBI?
- nausea and vomiting
- alopecia
- diarrhoea
- low grade fever
- mucositis
- pancytopenia
What are intermediate side effects of TBI?
- interstitial pneumonitis
- nephrotosicity
- veno-occlusive disease
What are late side effects of TBI?
- restrictive lung disease
- growth retardation
- endocrine abnormalities
- sterility
- cataract
- chronic renal failure
- neurological damag
- secondary tumours
What are the treatment room requirement for TBI?
- Large treatment room
- SSD 3-4 meters - Clean treatment room
- lower risks of infections
The higher the beam energy, the _______________ is the dose uniformity for patients of any thickness.
Greater
What is the maximum thickness of the patient for using 6MV parallel opposing beam in TBI?
35cm
Why 10MV is recommended even patient thickness <35 cm?
Minimise maximum dose and better dose uniformity
What are the plan parameters of a TBI plan?
(Energy / FS / colli / dose rate)
Energy: 6MV or 10 MV
Field size: 40 x 40 cm (largest)
Collimator angle: 0 or 45 degree
Dose rate 5-50 cGy/min
Where do TBI plan prescribed at?
It is prescribed at midplane of hip or pelvis.
To deliver a uniform one to the entire body within 10% of the prescribed dose
What is the common fractionation of TBI?
2Gy x 6fr (twice daily), within 3 days
Each fraction is separated by at least 6 hrs
What are the pros and cons to treat TBI patients in semi-recumbent position (lateral opposing field arrangement)?
Pro:
- more comfortable
- smaller treatment room is required
Cons:
- greater variation in body thickness
- difficult to use lung shield
What are the pros and cons of anteropsterior TBI?
Pros: better dose uniformity along the longitudinal body axis
Cons: more difficult in patient positioning (Less comfortable and stable )
What is the use of acrylic beam spoiler?
TBI protocols do not require skin sparing
Therefore, a 1-2cm thick beam spoiler is placed between the patient and the beam (close to patient surface) to produce a high dose on the patient skin surface. Or else, dose delivered to superficial regions might be inadequate.
What materials are used to make compensators?
Aluminium or brass
(Not too bulky or of too high density )
What is the use of compensating filters?
they are used to produce a uniform dose through all body regions to within 10% of the dose specified at the prescription point.
What body regions are required to be measured in order to calculate compensator thickness?
Head / neck / shoulders / mid-mediastinum / hip / pelvis / knees / ankles
Why compensators are located at short distance from the source?
The device can be handled easier and keep small in size.
How much dose can be reduced using lung blocks?
~2Gy
12Gy—>10Gy
What is tissue deficit (TD)?
The difference in tissue-equivalent thickness between the prescription point (umbilicus or hip) and the other locations.
How to calculate tissue deficit (TD)?
For region without lung:
TD = Lref-L
Lref = half separation at the umbilicus or hip
L = half separation at that particular anatomical location
For region where lung tissue is present
TD = Lref-L+(1-P_lung)L_lung
L_lung = half separation of the lung determined form the anterior chest radiograph
P_lung = density of lung (0.25)
How o calculate compensator thickness?
TD / Density
Why do we need in volume patient dosimetry?
Measured dose is converted to midline dose, then compared with the expected dose to adjust the MU and compensator to ensure the dose received is +/- 10% of prescribed dose.
