Mould Room

Question 1

What is the role of a thermoplastic mask?

Answer 1

holds the patient still during treatment
enables support for best positioning e.g. extended neck
enables reference marks to be drawn on the shell

Question 2

How is a thermoplastic mask made?

Answer 2

thermoplastic heated in 60-70 degree water bath for 4-5 mins until it turns translucent (from its original opaque colour)
shell is draped over patient
to accurately conform to the patient, need to press either side of the nasion (near medial canthus of the eyes) and into the patient’s inferior tragal notch
Thermoplastic masks then sets at room temperature, with the aid of frozen towels

Question 3

What are some things that need to be considered when using thermoplastic masks?

Answer 3

must inform patient of the procedure and ascertain, especially if the patient is claustrophobic
patients should remove clothing for neck and shoulder
if the patient has dentures the doctor should advise if they will be removed from treatment
need to consider wax/bolus, mouthpiece, packing as well as patient shoulder position (knee bolster, rope to hold onto foot support)

Question 4

What is the issue with dentures for treatment?

Answer 4

can cause discomfort, inflammation, ulceration due to the radiation
also, if the patient forgets to put in their dentures one day, their treatment will be compromised

Question 5

What is the role of a jelly bolus?

Answer 5

aids in creating a favourable isodose distribution
either used for
1) compensating for missing tissue or irregular tissue shape (i.e. to achieve a flat incident surface)
2) Modifying doses at the skin surface to minimise dose to critical structures or reduce tissue/ skin sparing effect

Question 6

What are the ideal characteristics for a bolus?

Answer 6

homogenous
durable
opaque
known depth
need to be able to calculate the attenuation
tissue density equivalent

Question 7

What is internal bolus packing?

Answer 7

fill any space within the field to reduce the impact of the air/tissue interface
ensure confidence in beam dosimetry

Question 8

What materials are used for internal bolus packing and what areas are filled?

Answer 8

Vas gauze, wax
nose, ears
can use to compensate for tissue deficiency e.g. scars

Question 9

What is the role of an external bolus?

Answer 9

placed directly onto skin surface to reduce skin sparing effect

Question 10

What are the types of boluses?

Answer 10

rigid bolus- over a non-altering surface e.g. head

pliable bolus- conforms to the skin surface e.g. breast (mastectomy)

Question 11

What are some important considerations when using a bolus?

Answer 11

with electron, bolus edges within the field must be avoided as it generates significant hotspots in the shadow of the edge
set-up difficulties as the opaque bolus occludes set-up marks
may lead to telangiectasia (poor cosmetic outcome)

Question 12

What are the basic steps to creating a wax block?

Answer 12

use alginate to create a mould that the plaster can be poured into then pour wax on plaster to create wax block
OR
use plaster strips to create cast, pour plaster into this cast to create a positive then pour wax onto plaster positive

Question 13

What are the properties of alginate?

Answer 13

lacks internal strength and therefore needs additional support when used for radiation purposes

Question 14

Why do we make plaster positives?

Answer 14

sometimes patient equipment is manufactured using products like hot wax or hot perspex sheeting which cannot be directly moulded onto the patient

Question 15

What are some considerations when using alginate

Answer 15

toxic powder therefore must avoid inhalation (wear masks)
once water has been added, must be applied within a matter of minutes
need to put plaster strips on top of the alginate to reinforce it
when placing alginate directly on the skin, make sure to apply aqueous cream to the area before hand

Question 16

What are the benefits of using alginate over plaster of paris?

Answer 16

provides better details and doesn’t break apart

plaster is exothermic (heats up on patient’s skin)

Question 17

Once the alginate is set and removed from the patient, what happens?

Answer 17

a backing is added to the alginate to create a ‘boat’ for the plaster to be poured into (make sure no air pockets or gaps)
aqueous cream must also be placed over the alginate before hand

Question 18

How do you apply the plaster strips?

Answer 18

criss cross and overlap to ensure a tight seal

Question 19

What was used to make the nose cast?

Answer 19

plaster strips were placed onto ‘patient’, in an overlapping orientation
once set, a ‘bowl’ was made using more plaster sheets, which was then filled with plaster to make the plaster positive

Question 20

What is the purpose of the wax nose block?

Answer 20

to raise the Dmax

Question 21

What were the dimensions of the wax nose block?

Answer 21

created a 6x6cm field for a 4cm circle field, with a 3.5cm depth at 90% dose to the surface, so that 90% dose reached the tumour

Question 22

How was the wax block constructed?

Answer 22

the plaster positive was cut so perspex sheet could form a box around the 6x6 cm field (to fit a 4cm circle), the box was seal with plasticine, then wax was pour in

Question 23

What needs to be considered when making the wax block?

Answer 23

wax will shrink once cooled therefore must fill the mould higher than required

Question 24

What are the set up instruction for treatment of the nose lesion?

Answer 24

Mask, headboard for shell
supine-patient
knee bolster
slip sheet
gel/wax bolus
nose insert
nose packing (vas gauze)

Question 25

Why are mouthpieces used?

Answer 25

to position the tongue, upper/lower lips in or out of the field
helps with stabilisation of the jaw

Question 26

How are the mouthpieces constructed?

Answer 26

softened dental wax is wrapped around a syringe plunger

while soft, mouthpiece is inserted into patient’s mouth and they bite down on it

Question 27

What consideration need to be taken into account for a mouthpiece?

Answer 27

are the tongue and lips in or out of the field
dentures: need to decide whether dentures are to be removed (having teeth makes it easier to bite down)
must make sure to not block air-hole

Question 28

What are shields used for?

Answer 28

treatment of superficial skin lesions using low energy photons

Question 29

What is used to cover eye and nose shields, and why?

Answer 29

wax is used to cover all internal shields because it stops secondary electrons as wax is a tissue equivalent material, also
lead is toxic and therefore protects patient from direct lead-contact

Question 30

Why are lead cut outs serrated?

Answer 30

for cosmetic purpose: so change in pigmentation between normal tissue and treated region is not as noticeable