Wk 11 - Brachytherapy Flashcards
low dose rate (LDR)
0.4-2Gy/h
medium dose rate (MDR)
2-12Gy/h
high dose rate (HDR) gy/h
> 12Gy/h
half life of 226 radium
1600 years
half life of 60 Cobalt
5.26 years
half life of 137 Caesium
30 years
half life of 192 iridium
74 days
half life of 125 iodine
59.5 days
advantages of HDR
- outpatient treatment
- dose optimisation - adpative B/T planning
- reduced radiation exposure for staff under normal situations
- more stable positioning
- smaller applicators
- high dose rate = shorter treatment times
disadvantages of HDR
- more complex treatment and planning techniques
- compressed time frame for planning
- greater potential for error due to much higher dose being delivered in a short timeframe
- potential for high radiation dose to staff and patient with source failure
brachy workflow
- implantation
- image acquisition
- catheters
- targets and OAR delineation
- treatment planning and optimisation
- quality control
- treatment delivery
high risk CTV
major risk of local recurrence - residual macroscopic tumour at time of BT (smaller than at the time of diagnosis)
intermediate risk CTV
major risk of local recurrence - initial macroscopic tumour at the time of diagnosis
low risk CTV
potential microscopic tumour spread - treated with surgery and/or EBRT but not brachytherapy
interdigitated scheduling + EBRT and HDR fractionation for cervix
give EBRT, then EBRT + HDR, then just HDR
EBRT = 45Gy in 25# to the pelvis
HDR = 30Gy in 5#, 2# per week OR 28Gy in 4#, 2# per week
pre-treatment imaging
PET, CT, MRI
- evaluate tumour
- determine treatment modality
- determine optimum treatment volume and dose
brachy imaging for each insertion
MRI, CT, US
- evaluate tumour response
- verification of applicator position
- define HRCTV, IRCTV and OAR
- adaptive RT
post-treatment imaging
- evaluate tumour response and toxicity
What is the clinical use of ultrasound in brachytherapy?
- non invasive, portable and inexpensive
- patient can be imaged during application insertion
- able to confirm applicator position and check intrauterine tandem is centres in uterine cavity
- image real time without moving the patient
Advantages of MR images
- excellent soft tissue differentiation - CTV can be located
- use to define target structures and OAR
inter and intrafraction variation
- may be significant
- may cause deviation from prescribed dose
- more significant for HDR as the number of fractions is higher than LDR
prostate HDR advantages
- image guided needle placement
- optimised dose distribution
- organ motion minimised
- radiobiological advantage
- remote afterloading
- single reusable source
what are some acute clinical issues for prostate HDR
- template/catheter movement
- haematrurial/clot retention
- perineal discomfort and back discomfort
- infection risks
- DVT prophylacxis
- defaecation
management for template/catheter movement
minimise movement of patient/bed rest
management of haematrurial/clot retention
continuous bladder irrigation
management of perineal discomfort and back discomfort
analgesics
management of infection risk
prophylactic antibiotics
management of DVT prophylaxis
stockings/heparin
management of defaecation
low residue diet prior to and during admission
How and why is brachytherapy employed in the treatment of cervical cancer?
EBRT is not effective enough in some cases to deliver the required amount of radiation without causing high toxicity to provide a tumoricidal dose. Therefore, BT is used to increase the delivered dose.
disadvantages of MRI images
- smaller aperture size, which may not accomodate all patient sizes
- small FOV - external body of the patient is not visualised
Define what a tumoricidal dose is
The amount of radiation given in order to control the cancer
How and why is brachytherapy employed in the treatment of cervical cancer?
EBRT is not effective enough in some cases to deliver the required amount of radiation without causing high toxicity to provide a tumoricidal dose. Therefore, BT is used to increase the delivered dose
limitations of MR images in brachy
- smaller aperture size, which may not accomodate all patient sizes
- small FOV - external body of the patient is not visualised
interfraction and intrafraction motion
- may be significant
- may cause deviation from prescribed dose
- more significant for HDR as the number of fractions is higher than for LDR
- high dose per fraction
- steep dose gradient around the applicators
- tumour shrinkage and normal tissue fibrosis may occur over the total length of time that HDR is delivered
What is the use of a vaginal dilator and why should patients use one?
From EBRT patients are at risk of vaginal adhesions, using a vaginal dilator can help reduce the occurrence as they can possibly prevent applicator insertion for BT
