Principles of radiation therapy Flashcards
How does ionising radiation cause damage?
Indirect damage (most important) Direct damage (least important)
What is the unit for radiation therapy (RT)?
Grey(s) [Gy]
Outline basic effects of radiobiology
Mitosis delayed - 1 Grey
Cells cannot divide - 10-100 Gy)
Radiation damaged cell usually die after 1 or 2 attempts at mitosis
Which cells in the body are the most radiation sensitive?
BM
Intestinal crypt cells
Germinal layer of epidermis
Tumours
Why is fractionation important?
Allows the 4 Rs of radiation therapy to be obeyed: Repair Repopulation Reoxygenation Redistribution
Define REPAIR in the context of the 4 Rs
Tumour cells are less able to repair DNA daage as they are often oxygen and nutrient deficient
Define REOXYGENATION in the context of the 4 Rs
Tymours have a necrotic, poor oxygenated centre. A single dose of RT would tend to kill off the healthy, oxygenated tumour cells but may not affect the hypoxic cells. The death of the vascularised cells will make more room for the surviving hypoxic cells.
Define REDISTRIBUTION in the context of the 4 Rs
Cells are more sensitive to RT in some phases: M>G2 > G1> ES > LS
The remaning cells willbe ‘synchronised and eventually will move into a more sensitive phase. This is the time to deliver the next RT fraction.
What type of radiation is used for RT in animals?
X-rays
Gamma rays
Particles - beta particles (electrons) in radioisotopes
Describe a Cobalt-60 therapy unit
Radioactive source
Photons
Fixed energy (1.24 MV)
Low technical requirements
Outline a linear accelerator unit
Variable energies Electrons/photons No radioactivity High technical maintenance High accuracy
Describe an electron beam
Various energies possible (5-15 MeV)
Rapid dose reduction (depending on energy)
Therapeutically useful depth (1.5-5.6cm)
Single fields, simple dose calculations
Outline a photon beam
High penetration Slow dose reduction Field arrangements necessary Penetration of normal tissue CT-based treatment planning Sedation sufficient, no GA required
What is the best Tx option for nasal tumours?
Radiation therapy
Define gross tumour outline
what may be visible on a CT scan
Define microscopic disease outline
the peripheries of the disease that may not be clearly visible on a CT beyond the gross tumour outline
What are the 2 broad goals of RT?
Curative/definitive (cure or long term control)
Palliative (palliation, stabilisation, pain reduction/relief)
How many greys are generally used for curative/definitive RT?
Generally 40-60 but these are fractionated into small amounts
Why perform curative RT?
Absolute indications: RT has better results than other therapies
Relative indications: RT shows same tumour control, but other advantages (functional, cosmetic)
Combination therapy: RT+surgery +/-chemo (e.g. ISS for best MST)
For what tumours is RT the primary Tx modality?
Brain
Head and neck (oral, nasal)
MCT
Epulis
What is an epulis tumour?
Benign,
Require aggressive surgery (e.g. removal of underlying bone)
For which tumours is post surgery adjuvant RT indicated?
MCT
STS
FISS
What is en bloc resection (EBR)?
used in certain cancers to remove a primary lesion, the contiguous draining LNs and everything in between, as in a modified radical mastectomy
Define negative margin
For tumour removal, no cancer cells are seen at the outer edge of tissue that was removed. AKA clean or clear. Sometimes the pathologist will tell you how wide this margin is but there is no uniform definition.
