History
Available since 1900s
Can treat at a max depth of 5mm
50-150kV
Who we treat
Older (60-80 years old)
Comorbidities - more flexible positioning
What diseases we treat
SCC
BCC
Mets
Bowens disease
Mycosis fungoides
Lesions within 5mm of skin
Curved surfaces
Basal cell carcinoma
- 93% of skin cancer treated with RT
- Not as metastatic
- Starts with small bump or flat red patch
- Develops slowly over months and years
- Non-healing sore
- 3 types - modular, infiltrating, superficial
Face, scalp, forearms
Squamous cell carcinoma
- Looks like a scab
- Ulcer or reddish skin plaque
- Raised scaly lump on sun exposed sites
- Can metastasise
Slow growing
Mycosis Fungoides
- Most common form of lymphoma
- Class of non-Hodgkins- cancer of the immune system
- Electrons can only be used if they are all flat
- TBI can be used
- If not flat, superficial done first to flatten the cancer ous spots
Most common treatment for SCC/BCC
Surgery + Superficial
Indications for superficial RT
- Patients medically unfit or refuse surgery
- Cosmetic reasons - lesions on the nose, lips, ear, corners of mouth
- Functional reasons - avoids nerve or function damage
- Patients at risk of microscopic disease or where small volume has recurred
- Older patients - less issues of telangiectasia, hypopigmentation and scarring
- Multiple lesions
- Patients prone to keloid scar formation
Superficial dose fractions
Elderly/significant comorbidities - 500cGy/ 7#
Older but healthy - 300cGy/15#
Younger patients and/or lesions <2cm - 250cGy/20#
Younger patients and/or larger lesions - 200cGy/30#
Electrons advantages
- Sharp dose fall off below the surface
- Less absorption in bone and cartilage
- Can treat 7cm below skin
- Good cosmetic results
Electrons disadvantages
- More expensive - linac needed
- Greater radiation protection measures required
- % depth doses less accurate under 4cm, field size limitation - doses are inaccurate
- Dose inhomogeneity on curved surfaces
Eyes: shields can cause scatter and bowing
Superficial Advantages
- Can treat sizes less than 4cm
- Defined beam edge: good for soft tissue sparing
- Good cosmetic results with fewer fractions: not as good as electrons
- Effective shielding around eyes
- Suitable for curved surfaces
Superficial Disadvantages
- Young patients - side effects can be disfiguring
invasion into bone. 1mm AL HVL bone absorption is almost 4 times greater compared to MV
Warm up process
Slow process
Heat dissipation in the target is very high
As it is tungsten, if the machine warm up is too fast, cracking occurs
Duty of filters
Hardens beam - reduces rapid attenuation
Smooths the beams - prevents excessive energies contributing to dose
Makes the beam more homogenous therefore able to penetrate more
Half value layer
Half Value Layer (HVL): thickness of material at which the intensity of radiation is reduced by half
Beam quality is expressed in terms of kV and HVL
Units of distance (mm)
Filter selection is done by checking material and depth. Usually 1-5mm HVL filters are used
Beam monitoring methods
Timer and backup timer
Dose monitor
Stand in
Convex surfaces
Stand off
Concave surfaces
