The document discusses the use of thermoplastic masks in radiotherapy, emphasizing the importance of minimizing patient discomfort and treatment time while ensuring proper immobilization. It covers various types of plastics and their properties, including shrinkage and stiffness, and presents findings from studies on mask comfort and effectiveness. Additionally, strategies to mitigate shrinkage effects during mask fabrication are highlighted, alongside comparisons of different mask designs for patient tolerability.

1. Anil Kumar Talluri, Ph.D.
Medical Physicist,
Basavatarakam Indo American Cancer Hospital

2.  Radiotherapy- using ionizing radiation to
treat a disease
 Aim- to deliver high radiation dose to
tumor while minimizing dose to
surrounding healthy tissue
1. Minimize amount of time patient spends
positioned and fixed on the treatment couch.
2. Minimize beam-on time (time in which radiation
is applied to patient)

4.  Plastic is subset of polymers
 Plastic
• Thermoplastic
• Thermosets
• Thermoplastic elastomers
• Polymer composites

5.  Thermosofting plastics
 Consists of simple molecular structure
 All macro or monomers are chemical
independent
 Intermolecular forces keeps them
together

9.  Made from matrix of either themoplastics
or thermosets
 And non- miscible reinforcement closely
linked with matrix

10.  Acrylic, a polymer called poly(methyl
methacrylate) (PMMA), is also known by trade
names such as Lucite, Perspex and Plexiglas.
 Acrylonitrile butadiene styrene (ABS) is
a terpolymer synthesized
from styrene and acrylonitrile in the presence
of polybutadiene.
 Nylon
 PLA - Polylactic acid
 Polycarbonate
 Polybenzimidazole
 Polyethylene
 Polyvinyl chloride
 Teflon- is a brand name of DuPont

12.  Polycaprolactone reinforced with a
discontinuous short length fiber and/or
fines
 aramid, carbon fiber, ultra high molecular
weight (UHMW) polyethylene, cellulose, Nylon,
polyester, fiberglass, polybenzoxazole (PBO),
liquid crystal polymer fiber, polypropylene,
polyamide, polybutyleneterepthalate, man
made fiber, cotton, wood pulp and natural fiber
 fiber comprises from 2% to 60% of the total
material by weight.

13.  radiotherapy patient immobilization,
orthopedic splinting or casting, plastic
and reconstructive surgery splinting, and
orthotic or prosthetic socket cone
production or reproduction

15. Type attenuation Skin build up
6 MV 15 MV mm H2O equiv
2.4 mm maxi 0.5% 0.35% 2.3
3.2 mm maxi 0.7% 0.45% 2.9
 Physical density is about 1.13 g/cc
 Density of water is 1 g/cc

16.  Patient comfort
 Shrinkage forces
 Strength of the mask
 Thickness of the mask

17.  Thermoplastic mask-related anxiety or
(claustrophobia) is low and possibly lies in the
positive representation patients have about the
mask.The explanations provided by health
professionals on the radiotherapy mask possibly
have a very positive effect on this perception.

18.  There is a proportional relationship
between amount of post-fabrication
shrinkage and the perforation density.
 Also there is an inverse relationship
between the strength of a mask and the
perforation density (Q-fix, 2013).

19.  Moreover, the Q-fix manufacturer claims
that the material composition of a mask
can affect its rigidity and shrinkage.
 Aquaplast masks shrink more and are
less rigid than Fiberplast masks which
incorporate Kevlar fiber to increase mask
stiffness and decrease shrinkage for
improved patient comfort

20.  A study was performed to measure the
shrinkage force of three thermoplastic
masks (UONTH from Orfit Industries, Uni-
frameTH from Med-Tec, and U-frameTH
from WFR).
 The internal shrinkage force for the
thermoplastic mask material sample
increased for 24 hours, then held
relatively constant until 170 hours as
shown in

21.  Assessment of Comfort and Shrinkage in Four
Styles of Thermoplastic Masks by V Dovell et al
• A total of 24 masks were made. Shrinkage was assessed
by tracking radiopaque fiducials affixed to the masks, and
their movement over three weekly CT scans.
• Comfort was assessed weekly from the time masks were
fabricated to the end of three weeks via questionnaires
given to participants
• Standard perforation and thermoplastic composition is
the optimal mask style based on minimal shrinkage and
lowest discomfort rating. Masks with variable perforation
had the highest mean shrinkage in all directions.
Assessment of Comfort and Shrinkage in Four Styles of Thermoplastic Masks, Valerie Dovella, Shahzad Bhuttoa et al..

23.  One method to deal with the consequences of
shrinkage is to place a spacer of thickness 3mm
under the head rest for patients.
 This spacer is used during thermoplastic mask
fabrication at Juravinski cancer center.
 The main purpose of the spacer is to
accommodate shrinkage.The spacer is inserted
under the patient’s head rest flange to shift the
patient’s head in the anterior direction.
 After the mask shrinks, the spacer is removed.

24. Courtesy: Assessment of Various Thermoplastic Masks Used for Radiation
Therapy Immobilization in Terms of Shrinkage and Pressure by Ayman Sameer Tunkr

25.  The thickness of the thermoplastic mask
also influences stiffness and the amount
of shrinkage; whereby a thicker mask is
more rigid, but shrinks more.

26.  two types of Posicast thermoplastic face masks, in
terms of reproducibility, patient comfort, tolerability,
and skin damage.
 The patients were randomly assigned to use a head
mask (HM) or a head-and-shoulder mask (HSM).
 The smaller HM reduced feelings of claustrophobia,
as well as skin reactions, for patients receiving ≥60
Gy.The smaller mask did not compromise the
reproducibility of the setup.

27.  The success of conformal radiotherapy
depends greatly on rigid immobilization
of patient
 The thermoplastic mask which offers
comfort yet rigid enough is preferable.