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.
Define positive margin.
Cancer cells extend to the edge of the margin. More treatment is indicated.
Indications - RT
Local therapy (local Dx, not systemic) Incompletely resected umours - then this is the Tx of choice: non-resectable tumours (results depend on tumour type) and pain control (bone cancer or metastases)
Describe nasal tumours
Describe MST with different Tx
2/3 are carcinomas
1/3 are sarcomas
MST without Tx = 3 months
Surgery alone = 3-6 months (NEVER DO THIS!)
RT alone = 8-20 months
RT + Sx = 47 months (BEST OPTION!!!)
Outline pituitary tuours
85% animals show rapid improvement in clinical signs (with RT?)
CS, localisation and size DON’T have prognostic significance
Very few side effects
MST = 24 months
Outline common canine oral tumours and their progression free intervals with RT.
ACANTOMATOUS EPULIDES = 90% tumour control. 86% 3 year PFI 4cm.
ORAL SCC: 45Gy 1 year PFI 75%
ORSAL FSA: 33-67% 1 year PFI
Define PFI
Progression Free Interval
Describe SCC - canine
Middle-aged dogs Rostral mandible Often cauliflower-like Local invasive Metastases (non-tonsillar 20%, tonsillar 70%)
Prognosis: site dependent:
Rostral: local control –> cure
Tonsillar: <10% survive 1 year (usually 3-6 months)
Tx - feline SCC
This is a very nasty tumour - 2 fractions RT/day
EPULIS:
metastases?
treatment?
No metastases
90% cure rate with curative RT
Treatment - canine STS
Surgery (+/- RT, use RT if incomplete margins)
+chemo if grade 3
Outline FISS (VAS)
Relationship between VAS and vaccination
Incidence 1/1000 - 1/10,000 cats
Tumour volume on contrast -enhanced CT - twice the vlume measured using calipers on PE
Metastasis in 12-24%
Only 10% cure rate with surgery alone (high probability of recurrence even with clear margins)
Survival after FISS treatment
Surgery, conservative: recurrence after 2 months (median)
Surgery, radical: recurrence after 9 months (median)
Surgery + RT: Recurrnce (600 days), 40% cure rate, chemotherapy>
Prognostic factors: number of surgeries, margins
Describe oral fibrosarcomas
Histologically low grade but biologically high grade Golden retriever predisposed Maxilla > mandible Very invasive locally Often intact epithelium Metastasis in 20% (LNs, lungs)
List some radiosensitive tumours with a high metastasis rate that are ideal candidates for palliative RT
Histiocytic sarcoma
Oral melanoma
HSA
MCT grade 3
List some tumours with a mass effect that are ideal candidates for palliative RT
Large head and neck tumours
Brain tumours
Large sublumbar LNs
Prostatic tumour
List some radiosensitive tumours with high local pain that are ideal candidates for palliative RT
OSA
Metastatic bone tumours
When is palliative RT indicated?
Age
Organ-related disease
Metastases
Describe malignant melanoma
Most common oral tumour in dogs
Mainly older dogs (mean age is 11.4 year)
Highly metastatic potential
1/3 amelanotic
Outline malignant melanoma response to RT
Overall response: 83-94%
Complete response: 53-69%
What are negative prognostic factors for malignant melanoma? 3
Macroscopic disease
Bone lysis
Caudal location
What are the adverse effects of radiation Tx?
ACUTE EFFECT: will resolve
rapidly dividing tissue
tumour, skin, mucosa, GI epithelium
LATE EFFECT: permanent
slowly dividing/non-dividing tissue
Bone, muscle, brain, CNS, lens, retina etc.
Describe the acute effects of RT?
After 3rd week of radiaion 7-10 days post RT --> maximum effect Normal tissue reactions: MM (mucositis), skin (alopecia, dermatitis), eyes (keratitis, conjunctivitis), CNS (transient demyelination) Self-limiting Only symptomatic Tx
Outline the skins/dermis acute side effects to RT
Target cells: stratum basale –> erythema, scaly or moist dermatitis, alopecia
Tx options: protect from mechanical traum analgesia black/green tea NO CREAMS/OINTMENTS/GELS
Outline the acute side effects at the mucous membranes of RT
Hypersalivation, nasal discharge, mucositis (fibrinous plaques)
Pain –> anorexia (rare in dogs, frequent in cats)
Tx options: Pain medication ABs Feeding tube (PEG) or oesophageal tube Metamucil/lactulose for colitis and proctitis
Define proctitis
inflammation of the anus and lining of the rectum
What is metamucil?
bulk-producing laxative and fiber supplement
What are acute side effects of RT on the eyes?
Decreased tear production, conjunctivitis, blepharitis, cornea ulceration
Tx options:
Optimmune/Vit E eye ointment
Check tear production
What are acute side effects of RT on the brain/spinal cord?
Edema (8-12 weeks post RT)
Transient demyelination
Transient recurrence of neurological symptoms
Tx options: corticosteroids
List examples of late side effects
Damage in stroma and vasculature
Earliest onset = 6 months after RT
Fibroses, contractions, strictures Non-healing ulcers Necrosis (bone, skin, CNS) Cataract, KCS Infarctions, haemorrhagias
Therapeutic intervention - difficult, to be avoided
What are late side effects of RT on the skin/dermis?
damage of vasculature and fibroblasts
fibrosis
alopecia, pigment changes
What are late side effects of RT on the eyes?
Cataract (clearing of fibres of lens not possible)
Chronic keratoconjunctivitis sicca (KCS)
What are late side effects of RT on the brain/spinal cord?
Necrosis of white matter (6-18 months)
Vasculopathy (1-4 years)
Differentiation with diagnostic imaging is difficult - side effect or recurrent disease?
Try treatment with corticosteroids
