M A Behrooz

M J Tahmasebi Birgani

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Introduction
The problem of adjacent fields has been
extensively studied [1,17]. A number of
techniques have been devised to achieve
dose uniformity in the field junction re-
gion. Some of the more commonly used
techniques are: 1) Angling technique, in
which the two beams are away from each
other (Fig. 1), so that the two beams are
aligned vertically [1]. If the width of the
treatment fields is defined according to the
50% decrement lines of adjacent fields, it
could introduce dose uniformity in the
treatment volume [18]. However, in prac-
192
Original Research 
Medical Journal of the Islamic Republic of Iran.Vol. 20, No.4, 2007, pp.192-197
A new method for ideal dose distribution of two adjacent
fields for external beam radiation therapy
M.J.Tahmasebi Birgani,PhD.1, M. Ansari, PhD2, and, M.A. Behrooz, PhD.3
Department of Medical Physics and Radiation Therapy, Jundishapur University of Medical Sciences, Ahwaz, Iran.
Abstract
Background: Adjacent treatment fields are commonly used in external beam
radiation therapy, such as mantle and inverted-Y fields for the treatment of
Hodgkin’s disease and craniospinal fields used in the treatment of medulloblas-
toma and head and neck tumors when the lateral neck fields are placed adjacent
to the anterior supraclavicular field. In each of these situations, there is a possi-
bility of introducing very large dosage errors across the junction. Consequently,
this region is at risk for severe complications if it is overdosed, or tumor recur-
rence if it is underdosed.
Methods: For prevention of adjacent field overlapping a new method is intro-
duced. In this method the patient’s couch of  the treatment machine is rotated for
90º clockwise and counterclockwise. Then the gantry is rotated for α and β that
are measured by geometrical methods in opposite direction for each field. The
adjacent fields have a common edge and then the overlap region in treatment
volume is eliminated.
Results: By phantom dosimetry, the maximum dose in the junctional volume
of the two adjacent treatment fields is measured to be 102%. This technique pro-
vides an inhomogeneity of about 2%.
Conclusion:In some cases, the measurements have shown that the dose inho-
mogeneity is as large as 45%. Compared with the dynamic intensity-modulated
radiotherapy (IMRT), this technique also provides a superior dose homogeneity
such that inhomogeneity becomes about 2%. 
Keywords: Adjacent fields, External radiation therapy, Dosimetry
1. Corresponding author, Associate professor, Medical Physics and Radiation Therapy Departments, Jundishapur University of Med-
ical Sciences, Ahwaz, Iran
2. Assistant professor, Radiation Therapy Department, Jundishapur University of Medical Sciences, Ahwaz, Iran
3. Professor, Medical Physics Department, Jundishapur University of Medical Sciences, Ahwaz, Iran.
tice, introducing such an angle is too diffi-
cult, therefore hot and cold spots will be
created.
2) Fields separation technique, in which
the fields are separated at the skin surface
(Fig. 2). The junction point is at depth
where as the dose is uniform across the
junction. The separation or gap between the
fields is calculated on the basis of geomet-
ric divergence [13] or isodose curve match-
ing [2,3,14]. Although this technique is
usually more acceptable and practical for
depths of more than 5 centimeters, cold and
hot spots are usually created at the upper
and/or lower parts of the region [15].
3) Isocentric split technique, in which
the beam is splitted along the plane con-
taining the central axis by using a half
beam block or a beam-splitter, thus remov-
ing the geometric divergence of the beams
at the split line [9,13] (Fig. 3). This tech-
nique is usually applied for orthogonal
treatment fields. Although a desired homo-
geneity is created at the abutting region, but
hot or cold spots may also be created
[13,16,19].
M. J. Tahmasebi Birgani et al.
MJIRI.Vol. 20, No.4, 2007. pp.192-197
193
Fig.1. Angling the beams away from each other so
that the two beams abut and are aligned vertically.
Fig.3. Isocentric split beam technique for head
and neck tumors.
Fig. 2. Fields separated at the skin surface. The
junction point is at depth where as the dose is 
uniform across the junction.
Fig. 4. Craniospinal irradiation using penumbra
generators.
Spoiler technique, in which two lead
wedges are used to provide  satisfactory
dose distribution across the field junction
[8]. These lead wedges are custom de-
signed (Fig. 4).
This technique is also applied for or-
thogonal treatment fields. Although the
thickness of the shields is specified by a
time consuming dosimetry, however, it is
not able to obtain a perfectly homogeneous
physical dose distribution in the junctional
area [6,7].
In clinical practice, the fields are usually
abutted at the surface if the tumor is super-
ficial at the junction point. Care is however
taken that the hot spot created due to the
overlap of the beams at depth is clinically
acceptable, considering the magnitude of
the overdosage and the volume of the hot
spot. In addition, the dosage received by a
sensitive structure such as the spinal cord
must not exceed its tolerance dose.
For the treatment of deep-seated tumors
such as in the thorax, abdomen, and pelvis,
the fields can be separated on the surface. It
is assumed in this case that the cold spots
created by the field separation are located
superficially, where there is no tumor.
To improve this treatment and eliminate
hot and cold spots, it is necessary to intro-
duce a new method for treating adjacent
fields.
Methods
We will consider the lengths of two adja-
cent fields as AB= a, and AB’ = b and the
position of radiation sources as S1 and S2 re-
spectively (Fig. 5). When one treats the pa-
tient with these two fields, hot spots will be
created as mentioned above. To prevent
such an effect the following steps can be
taken:
Rotating the treatment couch for 90°
horizontally.
A new model for ideal dose distribution...
194
MJIRI.Vol. 20, No.4, 2007. pp.192-197
Fig. 5. Rotation of treatment couches 90º, Rotation of gantry for angles α and β towards the common edge
of the treatment volume for positions S1 and S2 respectively while moving the treatment couch for h1 and h2
towards the source for two parts and treatment field, provide an inhomogeneity of about 2%.
At the position S1, for treatment of the
first field the gantry rotates for the angle a
towards the common edge of the treatment
volume (i.e. A in Fig. 5).
When the treatment machine adjusts at
position S2, the gantry rotates for angle b
towards the common edge (A) of the treat-
ment volume.
These two fields have a party ray perpen-
dicular to the surface of the treatment
couch at point A.
The result of this treatment technique is
as follows:
The central axis of the treatment fields is
not perpendicular to the surface, which is
not important in the treatment planning.
However, it is convenient to correct it by
isodose shift method [14]. 
The treatment fields increase for a length
of X1 and X2 at the surface of the body,
which is very small compared with the
length of the fields, and can be omitted.
To omit the X1 and X2, the treatment
couch moves for h1 and h2 centimeters to-
ward the source. Where:
Sin α= a/2d, Sinβ = b/2d, d= S1O=S2O’      (1)
According to Fig. 5 one can write:
h1=X1cotan2β  ,   h2=X2cotan2β (2)
and,
X1=d’1tan2α-a   ,   X2= d’2tan2β-b,               (3)
where d’1= S’1A and d’2= S’2A, are vertical
distances of the source S’ from the body  for
two fields.
Substituting eq.(3) in eq.(2) with some
calculations one can obtain:
h1=(a/2) tanα and           h 2=(b/2) tanβ,
where
d’1=   d2 – a2/4 and            d’2=    d2 – b2/4
For example if a=b=25cm and, d= 80cm one
can obtain:
α=β= 9°     and,   h1=h2= 2cm
but, if a =25cm , b=15cm and  d=100cm,
then:
α=7°,    β= 4.3°,    h1=1.6 cm and    h2= 0.6cm
Therefore, when SSD increases, α, β, h1
and h2 decrease.
Results
In clinical practice, a completely homo-
geneous physical dose distribution in the
junctional area of two adjacent fields is vir-
tually impossible to obtain. Even if a rea-
sonably homogeneous physical dose distri-
bution is obtained, because of time-dose re-
lationship, the biological effective dose
may be heterogeneous [20]. The matching
between the mantle and the para-aortic
fields in the treatment of Hodgkin’s disease
is of particular concern because of the need
to avoid an overlap of a portion of the
spinal cord. If this overlap occurs, the dose
that is received can cause irreversible neu-
rological damage [21].
The most commonly used method for
matching adjacent fields in Hodgkin’s dis-
ease is the geometric matching technique
[22]. The distance between the fields at the
patient’s surface is calculated so that the
adjacent field edges join at the chosen
depth, usually at the midline. However, the
field lengths and the depths to the central
axis plane are commonly different between
the mantle and the para-aortic field leading
M. J. Tahmasebi Birgani et al.
MJIRI.Vol. 20, No.4, 2007. pp.192-197
195
to inhomogeneity due to different beam di-
vergences. Additionally, the formula for
gap calculation assumes the patient’s sur-
face is flat, not taking into account the ef-
fect of the sloping body surface. 
Lutz and Larsen [23] described a tech-
nique to match the mantle and para-aortic
fields with the patient treated alternatively
in supine and prone position. Film dosime-
try in a solid plastic phantom with same
setup showed about 15-20% higher dose
distribution at the level of the spinal cord.
This was attributed to beam divergence dif-
ferences between the larger mantle and
shorter para-aortic fields.
Keys and Grigsby [24] proposed a modi-
fication of the existing formula for calcu-
lating the gap for the fields on a sloping
surface. The effect of sloping surface can
cause about 1 centimeter overlapping at the
spinal cord because of erroneously posi-
tioning the para-aortic field at a distance
calculated by a standard formula from the
edge of the mantle light field on the skin
surface. Film dosimetry using the skin gap
calculated using the standard formula
showed maximum dose in the junctional
area to be 145% and spinal cord dose
125%. Isodose curves using the modified
formula to calculate the gap demonstrated
much more uniformity at the junction, with
the region near the spinal cord receiving
115% of the central axis dose. 
An isocenter shift method was intro-
duced by Tae et al [15] and was more accu-
rate and easier than the others were. Al-
though after a film dosimetry they demon-
strated no hot spots, but a cold spot will be
created as low as 50 %.
A dynamic supraclavicular field-match-
ing technique for head and neck cancer pa-
tients treated with IMRT was introduced by
Duan et al [25]. 
The results of their activity show an av-
erage inhomogeneity range of 6%. They
compared their method with the conven-
tional single-isocenter and half-beam tech-
nique and they mentioned an inhomogene-
ity for the later technique as high as 22.8%. 
This new technique has been applied for
the treatment of spinal cord with two adja-
cent fields. Dosimetry showed a maximum
dose in the juctional area as much as 102%.
The same results could be obtained by us-
ing the isodose curves for the adjacent
fields.
Conclusion
A comparison between the new tech-
nique and all the previous methods, men-
tioned above, provides superior dose ho-
mogeneity not only in the abutment region
but in the upper and lower regions as well. 
In some cases, the measurements have
shown that the dose inhomogeneity is as
large as 45% [25]. Compared with the dy-
namic intensity-modulated radiotherapy
(IMRT), this technique also provides supe-
rior dose homogeneity [25] such that inho-
mogeneity becomes about 2%. 
From a practical point of view clinical
radiation therapy applying this technique
for treatment planning is very simple and
there is no delay time in treatment of pa-
tients.
The physicist can calculate α, β, h1 and h2
from the mentioned relations and apply the
steps 1, 2 and 3 for treatment of patients
(Fig.5).
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A new method for ideal dose distribution of two adjacent fields for external beam radiation therapy

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A new method for ideal dose distribution of two adjacent fields for external beam radiation therapy

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