IntroductionThe problem of adjacent fields has been

extensively studied [1,17]. A number oftechniques have been devised to achievedose uniformity in the field junction re-gion. Some of the more commonly used

techniques are: 1) Angling technique, inwhich the two beams are away from eachother (Fig. 1), so that the two beams arealigned vertically [1]. If the width of thetreatment fields is defined according to the50% decrement lines of adjacent fields, itcould introduce dose uniformity in thetreatment volume [18]. However, in prac-

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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 adjacentfields 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 ofHodgkin’s disease and craniospinal fields used in the treatment of medulloblas-toma and head and neck tumors when the lateral neck fields are placed adjacentto 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 for90º clockwise and counterclockwise. Then the gantry is rotated for α and β thatare measured by geometrical methods in opposite direction for each field. Theadjacent fields have a common edge and then the overlap region in treatmentvolume is eliminated.

Results: By phantom dosimetry, the maximum dose in the junctional volumeof 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-modulatedradiotherapy (IMRT), this technique also provides a superior dose homogeneitysuch 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, Iran2. Assistant professor, Radiation Therapy Department, Jundishapur University of Medical Sciences, Ahwaz, Iran3. Professor, Medical Physics Department, Jundishapur University of Medical Sciences, Ahwaz, Iran.

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tice, introducing such an angle is too diffi-cult, therefore hot and cold spots will becreated.

2) Fields separation technique, in whichthe fields are separated at the skin surface(Fig. 2). The junction point is at depthwhere as the dose is uniform across thejunction. The separation or gap between thefields is calculated on the basis of geomet-ric divergence [13] or isodose curve match-ing [2,3,14]. Although this technique isusually more acceptable and practical fordepths of more than 5 centimeters, cold and

hot spots are usually created at the upperand/or lower parts of the region [15].

3) Isocentric split technique, in whichthe beam is splitted along the plane con-taining the central axis by using a halfbeam block or a beam-splitter, thus remov-ing the geometric divergence of the beamsat the split line [9,13] (Fig. 3). This tech-nique is usually applied for orthogonaltreatment fields. Although a desired homo-geneity is created at the abutting region, buthot or cold spots may also be created[13,16,19].

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Fig.1. Angling the beams away from each other sothat the two beams abut and are aligned vertically.

Fig.3. Isocentric split beam technique for headand neck tumors.

Fig. 2. Fields separated at the skin surface. Thejunction point is at depth where as the dose is

uniform across the junction.

Fig. 4. Craniospinal irradiation using penumbragenerators.

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Spoiler technique, in which two leadwedges are used to provide satisfactorydose 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 thethickness of the shields is specified by atime consuming dosimetry, however, it isnot able to obtain a perfectly homogeneousphysical dose distribution in the junctionalarea [6,7].

In clinical practice, the fields are usuallyabutted at the surface if the tumor is super-ficial at the junction point. Care is howevertaken that the hot spot created due to theoverlap of the beams at depth is clinicallyacceptable, considering the magnitude ofthe overdosage and the volume of the hotspot. In addition, the dosage received by asensitive structure such as the spinal cordmust 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. Itis assumed in this case that the cold spotscreated by the field separation are locatedsuperficially, where there is no tumor.

To improve this treatment and eliminatehot and cold spots, it is necessary to intro-duce a new method for treating adjacentfields.

Methods We will consider the lengths of two adja-

cent fields as AB= a, and AB’ = b and theposition of radiation sources as S1 and S2 re-spectively (Fig. 5). When one treats the pa-tient with these two fields, hot spots will becreated as mentioned above. To preventsuch an effect the following steps can betaken:

Rotating the treatment couch for 90°horizontally.

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Fig. 5. Rotation of treatment couches 90º, Rotation of gantry for angles α and β towards the common edgeof 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%.

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At the position S1, for treatment of thefirst field the gantry rotates for the angle atowards the common edge of the treatmentvolume (i.e. A in Fig. 5).

When the treatment machine adjusts atposition S2, the gantry rotates for angle btowards the common edge (A) of the treat-ment volume.

These two fields have a party ray perpen-dicular to the surface of the treatmentcouch at point A.

The result of this treatment technique isas follows:

The central axis of the treatment fields isnot perpendicular to the surface, which isnot important in the treatment planning.However, it is convenient to correct it byisodose shift method [14].

The treatment fields increase for a lengthof X1 and X2 at the surface of the body,which is very small compared with thelength of the fields, and can be omitted.

To omit the X1 and X2, the treatmentcouch 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 verticaldistances of the source S’ from the body fortwo fields.

Substituting eq.(3) in eq.(2) with somecalculations 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 onecan 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.

ResultsIn clinical practice, a completely homo-

geneous physical dose distribution in thejunctional 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 dosemay be heterogeneous [20]. The matchingbetween the mantle and the para-aorticfields in the treatment of Hodgkin’s diseaseis of particular concern because of the needto avoid an overlap of a portion of thespinal cord. If this overlap occurs, the dosethat is received can cause irreversible neu-rological damage [21].

The most commonly used method formatching adjacent fields in Hodgkin’s dis-ease is the geometric matching technique[22]. The distance between the fields at thepatient’s surface is calculated so that theadjacent field edges join at the chosendepth, usually at the midline. However, thefield lengths and the depths to the centralaxis plane are commonly different betweenthe mantle and the para-aortic field leading

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to inhomogeneity due to different beam di-vergences. Additionally, the formula forgap 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-aorticfields with the patient treated alternativelyin supine and prone position. Film dosime-try in a solid plastic phantom with samesetup showed about 15-20% higher dosedistribution at the level of the spinal cord.This was attributed to beam divergence dif-ferences between the larger mantle andshorter 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 slopingsurface. The effect of sloping surface cancause about 1 centimeter overlapping at thespinal cord because of erroneously posi-tioning the para-aortic field at a distancecalculated by a standard formula from theedge of the mantle light field on the skinsurface. Film dosimetry using the skin gapcalculated using the standard formulashowed maximum dose in the junctionalarea to be 145% and spinal cord dose125%. Isodose curves using the modifiedformula to calculate the gap demonstratedmuch more uniformity at the junction, withthe region near the spinal cord receiving115% 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 becreated as low as 50 %.

A dynamic supraclavicular field-match-ing technique for head and neck cancer pa-tients treated with IMRT was introduced byDuan et al [25].

The results of their activity show an av-

erage inhomogeneity range of 6%. Theycompared 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 forthe treatment of spinal cord with two adja-cent fields. Dosimetry showed a maximumdose in the juctional area as much as 102%.The same results could be obtained by us-ing the isodose curves for the adjacentfields.

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 regionbut in the upper and lower regions as well.

In some cases, the measurements haveshown that the dose inhomogeneity is aslarge 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 clinicalradiation therapy applying this techniquefor treatment planning is very simple andthere is no delay time in treatment of pa-tients.

The physicist can calculate α, β, h1 and h2

from the mentioned relations and apply thesteps 1, 2 and 3 for treatment of patients(Fig.5).

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