Thickness and contour variations in head and neck regions introduce non-uniformity of radiation dose delivery leading to
severe hot spots. Tissue compensation is achieved sometimes using square non-divergent aluminum metal pieces in ‘Ellis type’
compensators. We developed Divergent Wax Tissue Compensator (DWTC) and tested their efficacy in tele-cobalt treatments.
For a photon field of size 20x20cm in a telecobalt machine with ‘Source Diaphragm Distance (SDD)’ of 45 cm, DWTC is designed
to provide compensation at ‘source skin distance (SSD)’ of 80 cm. A jig stand with 3 acrylic perforated sheets kept together
at specified gaps allow required radial divergence of the metal spokes on equally spaced grid. Dose compensator could
be fabricated by a tray mounted on the top side of the jig, using molten dental wax by measuring patient anatomy created by
a positive of ‘Plaster of Paris’ (POP).
Accuracy of fabricated DWTC is tested using 3-dimensional semi-spherical and wedge shaped contours, and measuring
transmitted radiation flux using EDR-2 verification film and evaluation by Vidar scanner. The uniformity of the dose in the
treatment region with compensator is well within 3%, 3mm dose evaluation criteria.
These wax compensators are simple, accurate and they could be adopted in linear accelerator treatments also retaining the
dose build up advantage.
Key words: Megavoltage radiotherapy; Tissue compensation; Dose uniformity; Head and neck regions
External Beam Radiotherapy (EBRT) remains the mainstay
for radical treatments, for malignancies in head and neck regions.
Inter-field thickness variations and irregular contour
variations are encountered in these patients affecting uniformity
in radiation dose delivery. Adverse reactions are encountered
because of skewed isodose curves seriously affecting
the volume dose variations in treatment planning. This
complex situation results in dose inhomogeneity resulting in
painful reactions in the head and neck region often discouraging
the patient from taking full course of treatment. There
are issues relating to adjuvant surgical management when the
texture integrity of skin in neck region is not preserved. Nonuniformity
of dose in the planned treatment volumes also adversely
influence normal tissue reactions, affecting effectiveness
of the treatment and therefore deter the Quality Of Life
(QOL) in the follow up period.
Ever since deep x-ray era, various compensation methods
have been reported in literature for such treatments[1-11].
Tissue Compensators (TC) in the shadow tray level of megavoltage
beams are implemented to preserve the shape of the
isodose curves accounting for tissue beam obliquity, preserve
dose build up advantages, and to improve dose delivery.
Ellis type compensator[1,2,5] use aluminum (Al) square
pellets to construct compensator plates for individual radiotherapy
portals. Tissue deficiencies are addressed by fixing
small thickness Al rods taking density factor. Commercially
available jigs for fabrication of aluminum tissue compensators
do not account for beam divergence at the collimator
level[12,13]. Effect of tissue compensation has been studied
for sites other than head and neck regions and also use of
other metals for such work[14-16]. Presence of metal during transit of x-ray photons induce low energy scattered x-rays,
add recoil electrons increasing surface dose.
In India, around 40% of head and neck cancers detected are
oral malignancies. In addition, there are patients with cancer of
nasal cavity, nasopharynx, paranasal sinuses and the oropharynx,
where treatment involves the oral cavity as well as neck
area. Radiotherapy (with linear accelerators and telecobalt
machines) is increasingly being used as an adjunct form of
treatment in the management of head and neck cancers. Because
of small inter-field separations, there are no specific advantages
of linacs over cobalt machines in these treatments. To
overcome treatment related morbidity in these treatments, we
investigated efficacy of wax tissue compensators accounting
for beam divergence, mounted away from the patient on
the path of the beam. The dosimetric adequacy in the application
of these compensators in clinical situations is highlighted.
Materials and Methods
Fabrication of jig
An iso-centric Theratron tele-cobalt machine (Theratronix,
Phoenix) with isocentre at 80 cms has been used for delivering
head and neck treatments with parallel opposing lateral fields.
The shadow tray distance (Source-Diaphragm Distance ‘SDD’)
is 45 cms. To simulate radial diverging beam with a field size
of 20 x 20 cm at isocentre, a special jig was prepared using
three perforated Perspex sheets mounted at fixed intervals.
Metal spokes pass through all the 3 Perspex sheets through
equally distributed holes drilled, in square matrix form. The
divergence of pencil beams, with calculated decrement factor
at different planes with respect to 80 cm SSD ensures correct
reproducibility in compensator fabrication. This system can
estimate contour variations when placed on a 3 dimensional
object. Figure 1 and Figure 2 shows the design of the developed jig, the
method of taking the impression of semi-spherical and wedge
object and the diminished size of fabricated tissue compensator.
Dental Modeling wax (a tissue equivalent material of density,
≈1.00 gm/cm3) commercially available in standard sizes 9 x
16 cm plates, having thickness ranging from 0.2 cm to 10.0
cm was used as compensator material. Water equivalence is
confirmed by measurements carried out in a water phantom
(Model WP30) of size 30 x 30 x 30 cm, using 0.6 cm3 ionization
chamber connected to an electrometer (CD-SSD-92).
Verification of the obtained water equivalent wax compensator thickness was carried out with Kodak therapy verification
film (Extended Dose Range: EDR-2) with and without TC, to
confirm improvement in flatness of horizontal radiation beam
profiles. (Figure 3 and Figure 4). The films are exposed under these
phantoms to a dose of 2 Gy on Phoenix telecobalt unit using
SAD technique keeping in a sagittal plane at a depth of 2
cm with and without TC. Intensity Modulated Radiotherapy
Omni Pro film dosimetry software (Wellhoffer Scanditronix,
Uppsala, Sweden) was used to evaluate film response with
VXR-16 vidar scanner. Flatness, symmetry, uniformity in radiation
intensity were checked with the available software.
Clinical use of DWTC
For patients planned for bilateral curative radiotherapy for
head and neck tumors were taken for tissue compensation.
These patients were immobilized with appropriate mould
sheets. A positive Plaster of Paris (POP) mould is prepared for
the head and neck together, in the mould room. This was
kept under the jig and the metal spokes were run to obtain the
shape at the tray level accounting for beam divergence. Diminished
size wax impression mould is obtained incorporating the
‘decrement factor 45cm/80cm = 0.563. Separate divergent TCs
were prepared for left and right sides of treatments. The fabricated
TCs were fixed on a Perspex tray maintaining correct
positioning with respect to central axis of radiation treatment
portal. The two compensators showing their orientations and
treatment execution in the presence of compensator in shadow
tray of cobalt machine is seen Figure 5 and Figure 6.
Water phantom measurements of beam transmission gave
broad beam attenuation coefficients for water and wax showing
good equivalence within 2 – 3 % (μwater = 0.048 cm-1 and
μdental wax 0.049 cm-1) Flatness of horizontal profiles as obtained
with TC is found to be within ±1% without any spikes showing
homogeneity in the fabricated wax impressions. The wedge
and semi-spherical wax impressions gave the required shapes
(Figure 7 and Figure 8) as confirmed from the profile plots with the
scanner. The uniformity of the dose in the region with compensation
is well within 3%, 3mm dose evaluation criteria
Figure 10 and Figure 11 the AP and Lateral views of the irradiated
patient, shows the uniformity in radiation reaction indicating
the efficacy of designed DWTC for clinical applications. Figure 12 illustrates sequential clinical photographs of a treated
patient indicating minimum radiation sequelae at the end of
treatment and in the follow up period.
This study has systematically presented the clinical implementation
of a wax-compensator based radiotherapy treatments
at our center. It is brought out clearly that any shape of the
contour could be corrected for tissue obliquity. For the quality
assurance aspect we have shown regular objects like wedges
and semispherical shapes checked with water phantom and
film dosimetry. These wax based divergent tissue compensator
is superior in reducing hot spots to improve overall dose homogeneity,
without introducing additional corpuscular stray
radiations like metal filters. The safety of their applications and
optimal dose delivery were well demonstrated.
Enormous irregularities in patient geometry in the superiorinferior
planes lead to hot and cold spots, giving complex dosimetric
situations. Beam modulation methods like IMRT either
adds or blocks radiation intensity with increase in monitor
units and also bringing in un-necessary beam orientations to
obtain optimization. The presence of wax compensator in the path of the beam corrects for absence of tissue, and brings in
dose delivery to ‘near perfect’ situation reducing additional uncertainties.
2-D compensators fabricated by milling machines
use computerized tomography CT images through DICOM
transfer depends on electronic technology adding to cost of
treatment, in addition making introduction of metal filters
which may give rise to stray radiations, thereby increasing the
risks of second malignancies in peripheral zones. In Figure 8
it appears that it is difficult to say that semi-spherical wax impressions
give the required shapes. This is because, though the
semi spherical wax impression did not give the required shape
as clearly as that for the wedge phantom, this is due to the fact
that the profile was taken along a small area on the top of the
semi-sphere around the central axis of the phantom. Figure 8 exhibited more of a method to check a fabricated compensator
with an external dosimetry system if required. Regarding
phantom selection we have resorted to closely mimic the
clinical situations in the head and neck regions. Neck is usually
semi cylinder in shape with variations along the medial
to lateral borders. A semi spherical hollow object (a plastic
ball of diameter 16 cm cut in half and closed along the base
of the hemi sphere by Perspex sheet) filled with water is selected
for dosimetry. A perspex sheet is used in order to verify
the fluence pattern under ‘IMatrix’ with and without the compensator.
A semi cylindrical shape of the neck has only 2 dimensional
contour variation. However a semi spherical water
phantom will have much higher degree of contour variation
(3 dimensional). Hence a semi spherical water phantom was
selected for dosimetric purposes to validate the uniformity of
the fluence with the compensator. It is clarified that this wax
compensator can address irregular shapes of outer contour but
will not account for inhomogeneities inside the neck in terms
of air space and cartilage.
Employing this cost effective type of tissue compensators, the
dose inhomogeneity can be reduced to within 5% which has
significant applications in the developing world. Good homogenous
agreement is observed with film dosimetry thus
instilling confidence in more optimal treatment delivery. The
dosimetric custom compensation method is highly accurate,
and reproducible. In a simple mould room setup, the custom
made divergent wax based tissue compensator could be fabricated
with insignificant cost, without much of sophisticated
inputs. Our study with cobalt beams could also be extended
in linacs because the divergence of the beam is less with more
SSD with respect to tray level. Also tissue equivalence could
be confirmed with the beam quality before implementation.
Beam hardening effects also will be minimal with wax material.
This technique will allow the radiation oncologists to treat
patients with head and neck radiation fields in a most efficient
manner. This concept could be generalized for tissue obliquities
in the scalp and breast regions and suitable compensation
could be achieved with careful prostheses.
We thank the Dean, Kasturba Medical College Hospital, Mangalore
for the kind permission to pursue this scientific work.
Technical help from radiation technologists Mr.Balakrishna,
Mr.Harish Rao, Mr.Shine K, Mr. Akhil Kumar is gratefully acknowledged.