In:. 1. Radiarion Oncology Biol. Phys.. Vol. 6. pp. 233-237 0 Pcrpmon Press Ltd.. 1980. Printed in the U.S.A.

0 Technical Innovations and Notes

FIELD SEPARATION BETWEEN LATERAL AND ANTERIOR FIELDS ON A 6 MV LINEAR ACCELERATOR

MICHAEL T. GILLIN, PhD., and ROBERT W. KLINE, Ph.D. Radiation Therapy, Medical College of Wisconsin, Milwaukee County Medical Complex, 8700 West Wisconsin

Avenue, Milwaukee, WI 53226

Field separations of 0, 2.5, 5, and 10 mm between lateral facial-cervical fields and anterior supraclavicular fields have been studied using thermoluminescent dosimeters in the Rando phantom. Measurement points were taken at three different depths in the midsagittal plane and at two locations out of that plane. The resultant radiation distribution for tbese points was measured at intervals of less than 1 mm. The mured dose distribution was compared to computer generated distributions. The field separation which resulted in the most ideal diitribution for routine cases was determined. This agreed well with the field separation determined by the standard cakxlational appn#ch, which may be used by modifying certain conditions. The application of the results of this data to the cliical situation is discussed.

Field separation, Radiation dosimetry, Isodose curves.

INTRODUCTION Patients with tumors of the upper respiratory and diges- tive systems who are treated by radiation therapy gener- ally receive lateral treatment fields in the head and neck region and an anterior field in the supraclavicular region. The dose at depth from the lateral. fields depends on the nature and extent of the disease, but usually is greater than 5000 rad. The typical dose to the anterior supracla- vicular field to control microscopic disease is 5000 rad delivered to a superficial depth. The field separation between the lateral and the anterior fields poses a chal- lenging clinical and technical problem. The clinical prob- lem involves the desire to adequately treat the entire volume without overdosing the spinal cord or causing severe subcutaneous scarring. The technical problems include understanding the three dimensional geometry of the patient and the radiation fields in the patient. Figure la shows the typical arrangement of the treatment fields on a patient, while Figure 1 b illustrates the volume of overlap which is caused by the divergence of the radiation fields when the anterior field is joined on the skin at the point where the lateral fields project to the anterior surface.

This problem has been addressed by previous authors.“’ This work presents a mapping of the radiation distribution with measurement intervals of less than 1 mm using thermoluminescent dosimeters (TLD) for vari- ous field separations on a linear accelerator. The points of

measurement include locations in the sagittal midplane and lateral neck.

METHODS AND MATERIAIS In the standard treatment technique at our institution

for patients with upper respiratory and digestive system tumors, the patients are treated while in a supine posi- tion. A 6 MV, linear accelerator is used with a 100 cm target-to-axis distance. The facial-cervical region is treated in an isocentric fashion with lateral fields which are defined by cerrobend blocks, 7.8 cm. thick. The supraclavicular region is treated with an anterior field only which is also defined using cerrobend blocks.

An anthropomorphic phantom* has been used to mimic the treatment situation. TLD 100 chips, which are 3.2 mm by 3.2 mm by 0.89 mm thick, were placed in Slice 8 at various depths. The chips were placed one on top of the other, such that there were 28 chips per location. The TLD chips were individually calibrated and the recommended annealing procedures were followed.

The first part of the study examined the dose distribu- tion in the sagittal midplane for gaps of 0, 2.5, 5, and 10 mm between the lateral and anterior fields. The gap was measured by projecting the light field from the lateral treatment position onto the anterior surface of the phan- tom and measuring from that point. Cerrobend blocks were used to define all treatment fields with the collima- tors opened well beyond the treatment volume. The

Reprint requests to Michael T. Gillin, Ph.D. *The Rando phantom, manufactured by the Alderson Accepted for publication 18 October 1979. Research Laboratory.

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234 Radiation Oncology 0 Biology 0 Physics February 1980, Volume 6. Number 2

ANlERlGR SU~VICULAR

Anterior field

Fig. 1 A. Schematic representation of left and right lateral neck fields and anterior supraclavicular field. Fig. 1 B. Line drawing illustrating the volume of overlap when the orthogonal fields are abutted on anterior surface.

TLD’s were placed at three separate locations of Slice 8. The first location was 17 mm from the anterior surface, as measured from the bottom of the slice. which approxi- mates the location of the thyroid cartilage. The next location was 56 mm from the anterior surface in the post cricoid region. The final position was 79 mm from the anterior surface in the spinal cord. The caudad border of the lateral field was located in the middle of Slice 8. The cephalad border of the anterior field was adjusted rela- tive to this position. The phantom was set up in the standard treatment position. Two hundred rad was deliv- ered to the midsagittal plane from the lateral fields and another two hundred rad was delivered to 3 cm depth from the anterior field.

The second part of the study measured the dose with a gap of 0.5 cm at points not located in the midsagittal plane. One point was located 37 mm from midline at a depth of 14 mm from the anterior surface and approxi- mates an anterior cervical node. The other point was located 51 mm from midline at a depth of 25 mm from the anterior surface and approximates a midcervical node. Figure 2 is a photograph of Slice 8 of the phantom with the various points of measurement indicated.

L- Thyroid cartilage

-Anterior cervical

L Midcervical no

-Post cricoid

-Spinal cord

node

de

Fig. 2. Rando phantom slice 8 (viewed from bottom).

A theoretical study of the various field separations was also performed. ‘** The purpose of this study was to display graphically the dose distribution in a plane, as opposed to just individual points in a plane. A midsagittal plane was chosen so that the lateral fields were viewed in a plane perpendicular to the central ray at isocenter and the anterior field was viewed in a plane containing the central ray parallel to the long axis of the body. The resultant distribution was generated for gaps of 0, 5, and IO mm.

RESULTS

The midsagittal plane dose distribution for the three different depths for field separations of 0, 0.25, 0.5 and 1.0 cm. is shown in Figure 3. The doses have been normalized to the dose at midplane of the lateral field. With a gap of 0 cm, there is a wedge of tissue with a diameter greater than 0.5 cm which receives a dose greater than 120% of the midplane dose, with maximum doses on the order of 140%. This wedge of tissue extends from the anterior surface to a depth of at least 8 cm. For a gap of 0.25 cm, the anterior dose decreases slightly relative to doses at greater depth. The volume which is overdosed is still obviously present, but there is a smaller maximum dose. For a gap of 0.5 cm. there is a region close to the anterior surface which receives a significantly smaller amount of radiation than regions posterior to it. With these conditions the dose in the region of the spinal cord is approximately 75% of the midplane lateral dose. With a gap of 1.0 cm, the volume which is underdosed extends to a depth of greater than 8 cm from the anterior surface and is greater than 0.7 cm in width even at the most posterior depth measured. The measurements at the caudad end of the curve reflect the presence of only a single anterior field with decreasing values with increas- ing depth from the anterior surface.

The dose distribution at the two points which were not located in the midplane, together with the distribution at the thyroid cartilage, is shown in Figure 4 for a gap of 0.5

**The Atomic Energy of Canada, Limited, treatment plan- ning system with programs CBEAIM and IBEAM was used.

A.

80 -

60-

C.

80

60

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GAP: 0.5cm

I+- Icm4 *\ I

Fig. 3. (A,B,C,D) Dose distribution in midsagittal plane for gaps of A) 0 mm, B) 2.5 mm, C) 5 mm, and D) 10 mm. ??- Thyroid cartilage. o - Post cricoid. A - Spinal cord. Caudad - Cephalad.

GAP : 0.5 cm 120

r

B. 14or GAP : 0.25cm

120 -

80-

60L D.

IOO-

GAP: l.Ocm

80-

60-

40-

20-

80

60 - w Icm4

Fig. 4. Dose distribution in lateral neck for gap of 5 mm. ??- Thyroid cartilage. x = Anterior cervical node. + - Midcervical node. Caudad - Cephalad.

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A. GAP: OCM

C. GAP: ICM

Radiation Oncology ??Biology ??Physics

i

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cm. A slight reduction in the dose to the nonmidplane points can be observed, but it is less than the reduction in the dose in the region of the thyroid cartilage.

The computer generated isodose distributions for gaps of 0, 0.5, and 1 .O cm is shown in Figure 5. The computer generated distributions agree with the TLD data at least qualitatively showing the region of overdose, relative uniformity, and underdose as the gap between the two fields is widened.

DISCUSSION

Proper separation between adjacent treatment fields is critical in order to avoid over or under treated volumes

February 1980, Volume 6. Number 2

B. GAP: 0.

Fig. 5. (A,B,C) Computer Generated Isodose Distributions for Gaps of A) 0 mm, B) 5mm, and C) 10 mm.

within the region irradiated. A standard calculational approach has been developed to determine the proper field separation in the case when the treatment fields are in the same plane. 2.3 When the treatment fields are in perpendicular planes, however, the approach to deter- mine the proper field separation becomes less clear. The standard calculation can be performed by rotating one of the planes 90”. This assumption simplifies the problem greatly, but may not be valid. Such a calculation has been performed for the situation studied, namely that of later- ial fields and an anterior field. For a depth of 7 cm, the depth to the cord from the anterior surface, the calcu- lated gap is 0.6 cm for the field sizes and distances used.

Field separation on a 6MV linear accelerator0 M. T. GILLIN and R. W. KLINE 237

This agrees well with the TLD results which showed that a field separation of 0.5 cm produces a relatively uniform dose distribution.

It is important that the three dimensional dose distri- bution in the region where the fields abut be understood for the normal treatment conditions. The TLD measure- ments display the changes in this distribution as the field separation is varied. Of course, the measurements are limited to the locations in the phantom at which the TLD’s were placed. The computer generated curves display the dose distribution in the midsagittal plane. It is felt that this aids in understanding the effects of the divergence of the anterior field into the area covered by the lateral fields.

Proper correspondence between the light and the radiation field is crucial to actually determine the field separation on a patient. The use of cerrobend blocks has helped with this problem. If the treatment head collima- tors are opened several centimeters beyond the field size required, it is the edge of the block which then defines the light and the radiation field border. Any discrepancies between the light and the radiation field can thus be minimized.

Proper patient set-up on a daily basis will be the

critical factor in determining the dose distribution in the volume of interest. The distributions presented here are undoubtedly smeared with variations in the daily patient set-up. There is a limit to the accuracy with which a patient can be repositioned. Field separations should reflect these variations in a realistic manner. It has become our policy to use a gap of 0.5 cm on all patients being treated with lateral head and neck and an anterior supraclavicular field. The length of these fields does not change a great deal for most patients. The lateral fields are treated first. The gap is measured on a daily basis by marking the caudad border of the lateral field projected onto the anterior surface of the patient and then measur- ing 0.5 cm caudad from that point. In our experience, this has proven to be an effective solution to this problem.

In some patients the clinical situation permits the placing of a spinal cord block of several centimeters in length in the upper part of the anterior supraclavicular field. When such a block is properly placed, it reduces the possibility of overdosing the spinal cord. It is not possible to do this in all cases, however. Thus there is a need to understand the resultant dose distributions as a function of various field separations to insure that the treatment plan is adequate for the clinical situation.

REFERENCES 1. Bukovitz, A.G., Deutsch, M., Slayton. R.: Orthogonal fields:

Variations in dose vs. gap size for treatment of the central nervous system. Radiol. 126: 795-798, 1978.

separation in multiple port radiation therapy. Am. J. Roent- genol. 102: 199-206. 1968.

2. Faw, F.L., Glenn, D.W.: Further investigations of the physi- 4. Williamson, T.J.: A technique for matching orthogonal

megavoltage fields. Int. J. Radiat. Oncol. Biol. Phys. 5: cal aspects of multiple field radiation therapy. Am. J. 111-116, 1979. Roentgenol. 108: 184-l 92, 1970.

3. Glenn, D.W.. Faw, F.L.. Kagan, A.R., Johnson, R.E.: Field 5. Wood, R.G.: Computers in Radiotherapy-Physical

Aspects. London, Buttersworths. 1974, pp. 39-44.