Skip to main content

Head and neck immobilisation: the mask fitting workflow from CT to first fraction

A mask fitted badly is a systematic error carried through every fraction. The sequence at the water bath, the choices of mask type and thickness, what to record, and the signs that a re-scan is due.

The twenty minutes a radiographer spends moulding a mask at the CT simulator sets the reproducibility of every fraction that follows. A mask that is a little loose at the shoulders shows up as a rotation on cone beam imaging in week three. A mask moulded over a headrest that is not the one used at treatment produces a systematic offset that nobody can explain. Neither of those is a machine problem, and neither is fixed by a larger margin without also irradiating more normal tissue.

What follows is the workflow that avoids both, and the equipment decisions that sit behind it. Radiotherapy quality assurance more broadly, including film dosimetry, is covered separately on this site.

Get the indexing chain right before you touch a patient

Immobilisation is a chain, and every link has to be reproducible: treatment couch, baseplate, headrest, mask, patient. A break anywhere makes the rest irrelevant.

  1. The baseplate has to index to both couches. The CT simulator couch and the treatment couch must accept the same baseplate at the same indexed position. If they take different index bars, that adapter is a purchase, not an improvisation.
  2. The mask type has to match the baseplate. Thermoplastic masks come in frame families and they are not interchangeable. Meicen's L-type masks are made to fit standard O-type baseplates, S-type masks to fit S-type baseplates and P-type masks to fit P-type baseplates. Check the family against the frames you already own before ordering, because buying a mask family that does not match them is an expensive and completely avoidable mistake.
  3. The headrest has to be a catalogued, numbered item. Not a foam block. The headrest number goes in the record and the same numbered headrest is used at every fraction.
  4. The shoulder retraction system, where used, has to be indexed too. Pulling the shoulders down by hand to a different tension each day is not immobilisation.

Carbon fibre baseplates are the standard because they are stiff and attenuate little. Meicen's A-series all in one baseplate, for example, covers head, neck, shoulder, thorax, pelvic and whole body positions on one plate, is stated as compatible with L-type thermoplastic masks, and works with the shoulder retraction system and leg positioner cushions. That reduces the number of separate plates a department has to store, index and get wrong.

Choosing the mask itself

Three variables, and each has a real trade off.

Coverage

A head only mask immobilises the skull and does nothing below it. A head, neck and shoulder mask controls the lower neck and supraclavicular region, which is exactly where head and neck plans need control and exactly where a head only mask lets the patient drift. If the field extends into the lower neck, use the longer mask.

Thickness

Meicen supplies standard thicknesses of 2.4 mm and 3.0 mm, with 2.0 mm and 3.2 mm also available in several families. Thicker material is more rigid and more restrictive, and it takes longer to cool. Thinner material is faster and kinder and gives up some rigidity. For a routine radical head and neck course the thicker standard is the usual choice; for a palliative patient who will not tolerate it, the thinner one used well beats the thicker one abandoned.

Perforation and open face

Perforation percentage changes both the stretch behaviour and how the patient experiences the mask. Meicen's U-type masks are offered in 22 per cent and 38 per cent perforation options, and in reinforced, open face and open eyes and mouth variants. Open face designs, which Meicen states are intended to reduce claustrophobic symptoms, remove material over the face while retaining the frame contact that provides immobilisation. They are the answer for the patient who will otherwise not complete a course, and they are worth stocking even in small numbers.

For stereotactic work, the requirement changes. Meicen's X-Knife violet masks are made for X-Knife type frames from a material the manufacturer describes as combining rigidity with very low shrinkage, with a non stick coating so the mask does not adhere to hair or beard. Low shrinkage is the property that matters in stereotactic immobilisation, because a mask that continues to contract after moulding is a mask that changes the patient's position between simulation and treatment.

The fitting sequence

Run the same sequence every time and it becomes reproducible. Improvise and it will not.

  1. Set the bath first. Water at the temperature the mask manufacturer states, checked with a thermometer rather than assumed from a dial, and stable before the patient arrives. Use clean water and descale the bath, because scale deposits transfer to the mask and to the patient's skin.
  2. Position the patient before heating anything. Headrest chosen and recorded, arms and shoulders set, chin position set, alignment to the lasers confirmed. Adjusting position while a hot mask is stiffening is how a mask is fitted to the wrong posture.
  3. Immerse for the stated time. Under heating leaves the sheet stiff and cold spots unmoulded. Over heating makes it floppy, stretches it thin over the nose and increases shrinkage later.
  4. Drain and blot before it touches the patient. Hot water carried on the mesh is what burns skin, not the thermoplastic itself. Blot the mask and check its temperature on your own forearm.
  5. Apply from the centre outwards. Secure the frame at its index points first, then mould the forehead and nasal bridge, then the orbital ridges, then the chin, then the mastoid region, then down onto the shoulders if the mask extends there.
  6. Mould with flat hands, not fingertips. Fingertip pressure creates local thin spots and a raised rim, both of which change the fit.
  7. Let it cool fully on the patient and in position. Removing a mask while it is still warm allows it to relax, and every subsequent fraction is set up against a shape the patient no longer matches.
  8. Mark on the mask, not on the skin. Reference marks on the mask survive the course. Skin marks fade, wash off and move with weight change.
  9. Record everything that can vary. Headrest number, baseplate index position, which pin holes were used, shoulder retraction setting, arm position, and any accessory such as a bite block or a mouthpiece.

What goes wrong, and what it looks like on imaging

Set explicit re-scan triggers in the department protocol rather than leaving it to judgement: visible looseness, weight loss beyond a stated threshold, repeated setup corrections exceeding the department's action level, tumour response that changes external contour, or a mask that has been damaged. Write the thresholds down so the decision does not depend on who is on the machine that morning.

Stock and storage in a Bangladeshi department

Masks are single patient consumables, and a department that runs out stops simulating. Hold stock by family and thickness, not as a single line item, and include open face variants in the standing order rather than treating them as special orders, because the patient who needs one needs one on the day they present.

Store unmoulded sheets flat, dry and away from heat. In a humid climate, keep them in their packaging until use and keep the store away from an outside wall that gets afternoon sun. Vacuum cushions, if the department uses them for body sites, need their own check: a cushion that has lost vacuum overnight has lost its shape, and it should be tested before a patient is positioned on it rather than after.

Back to all Insights