Brachytherapy and Radionuclide Therapy
What to know
Brachytherapy types (LDR, HDR, PDR), dose rate definitions, common applications (cervix, prostate, breast, skin, eye), and radionuclide therapy (I-131, Lu-177 DOTATATE, Ra-223, Y-90 microspheres).
Concepts to master
- Dose rate: LDR <2 Gy/h, HDR >12 Gy/h.
- Prescription systems: Manchester/Paris; modern image-based planning with D90, V100 for prostate.
- Cervix: HDR with tandem/ovoid; combine with EBRT; dose to point A vs MRI-based HR-CTV.
- Radiation safety: ALARA, source handling, patient release criteria for I-131.
Traps
- Confusing point A (historical) with MRI-based target volumes.
- Forgetting EBRT + brachy boost sequencing in cervix cancer.
- Mixing activity (Bq) with dose (Gy) in radionuclide therapy.
Rule of thumb
Brachytherapy dose falls as 1/r² from each source - so implant geometry and source loading dominate the plan. For cervix, EBRT first (~45 Gy) then brachy boost to reach curative dose while sparing bladder/rectum.
Worked example
A cervix patient receives 45 Gy EBRT with concurrent cisplatin. The question asks how to reach curative dose. Add HDR brachytherapy boost (image-guided, ~28 Gy in fractions) to the HR-CTV - not additional EBRT alone.