Brachy Flashcards
Doserates
LDR 0.4-2Gy/hr
MDR 2-12Gy/hr
HDR >12Gy/hr (in reality its much higher than this 90-100gy)
Radioactive isotopes 1/2 lives
Radium 226 - 1600 years
Caesium 137 - 30 years
Cobalt 60 - 5.26 years
Iridium 192 - 74 days
Iodine 125 - 59.5 days
Clinical advantages of HDR
Outpatient treatment
• Dose optimisation –adaptive B/T planning
• Reduced radiation exposure for staff under normal situations
• More stable positioning
• Smaller applicators
• High dose rate= short treatment time
Clinical disadvantages of HDR
• More complex treatment and planning techniques
• Compressed time frame for planning
• Greater potential for error due to a much higher dose being delivered over a short timeframe
• Potential for high radiation dose to staff and patient with source failure
Brachytherapy workflow
- Implantation procedure: depends on tumour topography, size, OARs and pre planning
- Image acquisition (US, CT, MRI)
- CATHETERS 3D Digitization
- Targets and OAR delineation
- Treatment planning and optimisation
- Quality control
- Treatment delivery
What is an after loader
This controls the flow of the radioactive source
High risk CTV
major risk of local recurrence - residual macroscopic tumour at time of BT (smaller than at time of diagnosis)
Residual macroscopic tumour (Defined from MR)
intermediate risk CTV
major risk of local recurrence - initial macroscopic tumour at time of diagnosis
Significant microscopic tumour
Low risk CTV
potential microscopic tumour spread - treated with surgery and/or external beam radiotherapy but not brachytherapy
Types of imaging
Pre treatment: PET/MRI/CT
Brachytherapy imaging for each insertion: MRI/CT/US
Post treatment imaging
Pre treatment imaging
Evaluate tumour
• Determine treatment modality
• Determine optimum treatment volume & dose
Imaging for each insertion
Evaluate tumour response,
• Verification of applicator position • Define HRCTV, IRCTV and OAR • Adaptive radiation therapy
Post treatment imaging
Evaluate tumour response & toxicity
MR images
Excellent soft tissue differentiation – CTV can be located
• Use to define target structures and OAR
• Smaller aperture size, which may not accomodate all patient sizes • Small FOV- external body of the patient is not visualised
Ultrasound
Non invasive portable and cheap
Used during applicator insertion
Able to confirm applicator position and
check intrauterine tandem is centred in
uterine cavity
• Measurements can be made of uterine
dimensions for comparison with MRI
when planning
• Not able to reconstruct the applicators,
although some software has enabled
this
• Advantage of being able to image real
- time without moving the patient
Intrafraction and interfraction variation
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
Prostate HDR advantages
Image guided needle placement
Optimised dose distribution
Organ motion minimised
Radiobiological advantage
Remote after loading
Single reusable source
Needle placement
No standard arrangement
Specific to each implant due to the shape and position of the prostate, urethra and rectum contours
Often need more needles along post edge to allow sufficient modulation of dose around the rectum
Have to account for possible changes in contours after needle insertion
Acute clinical issues for prostate HDR
Template/ Catheter movement
• Minimize movement of patient/ bed rest
Haematuria/ clot retention
• Continuous bladder irrigation
Perineal discomfort and back discomfort
• Analgesics
Infection risk
• Prophylactic antibiotics
DVT prophylaxis
• Stockings/Heparin
Defaecation
• Low residue diet prior to and during admission
Clinical sites treated
Cervical cancer
Breast
Prostate
Cervical treatment
Antiverted and retroverted uterus requires different OAR priority
Antiverted - uterus tilts towards abdomen
retroverted - uterus tilts towards the rectum
- HDR Brachytherapy is necessary for the adequate treatment of cervical cancer to ensure a tumouricidal dose is delivered whilst sparing OAR
EBRT cannot escalate dose due to proximity of OARs and limitation in visualisation.
Can utilise tandem or ring
Ultrasound images estimate length of uterus
What are standard loading patterns
Tandem
Ring
What structures are dose limiting and which are not
Rectum - not dose limiting due to use of an applicator with rectal retractor
Bladder - is dose limiting, due to acutely angled tandems
Why does ICRU dose parameters not correlate
It underestimates dose to bladder
Dose to structures may only be to certain parts near to the GTV due to lack of low dose wash
