Inl. J. Rodiotion Oncdogy Bid. Phys.. Vol. 14. pp. 1299-I 305 0360-3016/88 $3.00 + .I0 Printed in the U.S.A. All rights reserved. Copyright 0 1988 Pergamon Press plc

??Technical Innovations and Notes

THREE-FIELD ISOCENTRIC TECHNIQUE FOR BREAST IRRADIATION USING INDIVIDUALIZED SHIELDING BLOCKS

GIANNI CONTE, M.D.,* OTTORINO NASCIMBEN, M.D.,* GIACOMO TURCATO, M.D.,* ROLANDO POLICO, M.D.,* MICHAEL BERNARD IDI, M.D.,* LAURA MARIA BELLERI, DR.,?

FRANCA BERGOGLIO, DR.,? FRANCA SIMONATO, DR.,? LANFRANCO STEA, DR.,? FLORINDO BUGIN, R.T.T.? AND NORMA BORTOT, R.T.T.*

Ospedale Umberto I”, Me&e, Italy

The three-field technique is the most common method used for breast and regional node treatment after conservative surgery. Several variants of this technique, which are characterized by complex geometrical problems, have been described. A possible simplification of this technique and the use of individualized shielding blocks both for anterior and for tangential fields is proposed, thus allowing for the simultaneous shielding of the half beam and the critical areas. Advantages of isocentrical techniques are thereby maintained, but the number of mechanical movements required is minimized and collimators and couch rotations are not needed. Patient set-up time is also greatly shortened. The accuracy of this technique has been verified u&g both photographic methods and thermoluminescent dosimetry.

Breast irradiation, Field matching, Three-field technique.

INTRODUCTION

Conservation surgery in early breast cancer has gained increasing acceptance in recent years.2~4,8,‘0.‘5~16 It is an approach which involves subsequent irradiation of the breast and often of the regional nodal areas in an attempt to achieve maximum tumor control.

Breast and peripheral lymphatics irradiation is a com- plex treatment, deployed on an irregular target volume that lies close to critical organs. The technique commonly used consists of a combination of three fields; two op- posing tangential fields are used to irradiate breast, chest wall, and ipsilateral internal mammary lymphnodes. A third anterior field is employed to irradiate axilla and su- praclavicular area.

Such a complex treatment requires a proper matching, between tangential and anterior fields, both on the body surface, and in depth. Owing to beam divergence, there is often the risk of a non-uniform dose distribution in the match region, which can result in overdosed areas, with subsequent fibrosis and cosmetic impairment.3,559 On the other hand, underdosed areas can lead to decreasing local tumor control. A further complication of the tangential field treatment is the location of internal mammary nodes

over an oblique plane close to the sternum. When irra- diating these lymphnodes a low dose should be maintained to underlying lung tissue.

Several ways have been proposed in recent years to solve these above mentioned problems.6*7,1’-14 To avoid disadvantages arising from beam divergence and to prop erly match adjacent fields, some authors suggest the use of half beam shielding blocks.” A suitable application of these blocks is illustrated in Figure 1.

The technique proposed here is a further evolution of the half beam shielding technique; it involves the use of individualized blocks both for tangential and for anterior fields, allowing for an easier set-up of patients.

METHODS AND MATERIALS

Tangentialjields The individualized blocks used for the two tangential

fields (Fig. 2a) consist of a rectangular part, to shield the upper half of the field, and a triangular one, shaped in conformity with the location of the internal mammary chain, to shield lung tissue as far as possible.

The dimensions of the shields are based on the treat-

* Divisione di Radioterapia Oncologica. t Servizio di Fisica Sanitaria. Reprint requests to: Gianni Conte, Divisione di Radioterapia,

Oncologica Ospedale Umberto I’, 30174 Mestre (VE), Italy.

Accepted for publication 5 January 1988.

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1300 I. J. Radiation Oncology 0 Biology 0 Physics June 1988, Volume 14, Number 6

(4 (b)

Fig. 1. Illustration of the three-field isocentric technique using half beam shielding blocks: (a) projection onto coronal plane; (b) projection onto sag&al plane.

ment plan (Fig. 2b). The slope of the oblique side of the triangular shield is obtained from that of the sternum.

To comply with the treatment plan, the triangular shield must cross the inferior side of the two tangential fields in the plane where patient transversal contour was taken.

A 1: 1 drawing of fields and of their shields is necessary to build blocks. This drawing (Fig. 2b) is placed on the table of the Styrofoam cutting device, at a distance from the fulcrum equal to the focus-axis-distance (FAD). When the shielded area is cut from the stryrofoam blocks, it is then filled with low melting point alloy.

Every patient needs two identical symmetrical shields, one for the medial and one for the lateral field (Fig. 3).

Supraclavicular and axillaryjeld In the irradiation of supraclavicular-axillary area the

caudad half of the beam is shielded to eliminate diver- gence. The contour of the target volume is directly out- lined on the simulator radiogram (Fig. 4). The Styrofoam block is cut following this contour, which is placed as usual at a distance from the fulcrum equal to the focus film distance (FFD).

Treatment planning: Simulation and treatment The following steps include treatment preparation and

execution with the three individualized fields isocentric technique:

1. A CT-scan of the breast is performed. The patient lies on her back, on a rigid and flat bed, with arms over

the head. The same position must be maintained dur- ing the simulation and the treatment. The length of the breast volume to be irradiated is then measured and a single scan is performed on the slice which passes through the center of this length. A radiopaque mark is positioned on the mediostemal line of the patient, and another one on the axillary medial line. The two points corresponding to the intersection between these lines and the plane of the CT-scan are tattoed on the patient’s skin, to be used as reference points during simulation. A treatment plan is studied, using a computation pro- gram that takes into account tissue heterogeneity. The algorithm used is based in the equivalent TAR method.’ A drawing in 1: 1 scale is plotted obtaining the distance (A’-B’) from the isocenter and the vertical plane across the mediostemal line and then the depth of the point A’ (Fig. 5a). A simulation of the treatment (Fig. 5b) is performed, centering on point A (skin in mediostemal region is not subject to displacements) at a source skin distance (SSD) equal to the FAD minus the A’-A distance. The simulation couch is then translated laterally for a dis- tance A-B (=A’-B’), and longitudinally for a distance B-C corresponding to a half-length of tangential field; skin is then tattoed at point C and the corresponding SSD is taken. Gantry is then rotated right and left side as planned for the two tangential beams, checking the correspondence between beam entering point previ-

__

(4 (W

Fig. 2. (a) Projection onto a sagittal plane of the irradiated and shielded areas in tangential individualized fields. CT = level of CT scan for treatment planning. (b) Drawing used to build blocks: AB = length and CD width of tangential fields according to treatment plan. DE X 2 = enlargement of the beam width. AF = final width of the tangential fields. 0 = sternal angle.

Three-field isocentric technique 0 G. CONTE et al. 1301

Fig. 3. Illustration showing the two individualized shielding blocks, one for the medial and one for the lateral tangential field.

Fig. 4. Simulator radiogram of the anterior supraclavicular field showing the contour of the areas to be shielded.

1302 1988, Volume 14, Number 6

4.

5.

\ (a) (b)

Fig. 5. Simulation phase. (a) AA’ and A’B’ distances are measured in the 1:l plot of the treatment plan. A = mediosternal point, B = isocenter of tangential fields. (b) Patient is centered on point A at a source-skin distance equal to the focus-axis distance minus AA’ distance: couch is then translated laterally to center B and longitudinally to center C. BC distance corresponds to half-length of tangential fields.

ously tattooed on the skin and on the drawing of the treatment plan. We observed that possible discrepan- cies between the CT image and the simulator’s images can be only due to an incorrect position of the patient. The slope of the sternum is measured in this stage. The collimator is rotated until its edges are parallel to the sternum and rotation degrees are recorded. Finally, the supraclavicular-axillary field is simulated setting simulator gantry to 0” and centering point C. Using data from treatment plan and simulation (field sizes, sternal angle) it is now possible to draw and then build individualized shielding blocks, as described above. The treatment stage is the most rapid one: the patient is directly centered on point C, at the proper SSD. The blocks are set and gantry is rotated clockwise and counterclockwise, to irradiate the two tangential fields. After changing block and setting gantry to O”, anterior field is irradiated. Treatment is thus performed without ever moving the treatment couch or rotating colli- mators.

Dosimetry A first control of absorbed dose homogeneity in the

match plane was performed using a radiographic film,* imbedded in two tissue-equivalent slabs, 2.5 cm thick, and placed on the sagittal plane through the isocenter (Fig. 6).

The verification of the absorbed dose distribution in the irradiated volume and in the critical organs (lungs) was performed using thermoluminescent dosimeters (LiF 100 rods) inserted in a phantom.? Verification was ob- tained following the procedure as if for a normal patient (CT, treatment planning, simulation, construction of shielding blocks), and then irradiating the phantom with

100 + 100 cGy to the two tangential fields, 200 cGy to the supraclavear axillary field.

With our technique, the central plane of the dose dis- tribution is an off-axis plane: an off-axis computation is required to obtain a correct distribution.

To evaluate the difference between the two methods we performed both off-axis and on-axis computation. The results showed that the approximation of the off-axis dose distribution with an on-axis computation leads to an un- derestimate of less than 5% of the target dose. The recon- struction of the absorbed dose distribution in the central sag&al plane is shown in Figure 7. Figure 8 shows the overlap of computed &doses and experimental dosimetry in the central transversal plane of the breast.

Note, that the figures are referred to a treatment per- formed on the phantom; the anatomical configuration of the phantom’s breast is quite different from the typical configuration of our patients. Therefore, the effect of the lung density correction, that in most of our treatment planes produces small areas of increased dose medially and laterally to the lung, here is not manifest. Nevertheless, experimental dosimetry showed a good distribution over target volumes; it confirmed, moreover, the absence of gross over or under dosed areas in the match plane be- tween anterior and tangential fields, and assured that doses absorbed from lung tissues and from the whole shielded volume were kept within an acceptable range.

DISCUSSION

In treatments of breast and regional nodal areas, half beam blocks were mainly used only for the anterior field. Cephalad edges of tangential fields were made coplanar by suitable rotations of the treatment couch.

Recently, to avoid couch movements, the use of half

* Kodak K-Omat V, Rochester, NY. t Alderson-rando, Alderson Res. Lab., Stamford, CT 06904.

Three-field isocentric technique 0 G. CONTE et a/ 1303

Fig. 6. Film taken in the sagittal plane containing the isocenter showing the matching between anterior tangential fields.

and

beam blocks was proposed also for tangential fields, thus rendering isocentric the three field technique.” The sug- gested procedure requires a collimator angulation to

properly irradiate the internal mammary chain and a ro- tation of the shield to make vertical the caudal edge. It is possible that the collimator rotation, by appreciably raising

Fig. 7. Reconstruction of the absorbed dose distribution in the central sagittal plane: the internal mammary nodes do not receive less than 90% of dose. The contribution of the anterior field is also manifest.

1304 I. J. Radiation Oncology 0 Biology ??Physics June 1988, Volume 14, Number 6

Fig. 8. Comparison between data from TLD dosimetry (point values) and computed isodoses in the transversal plane containing the isocenter. Both are expressed as percentage.

the irradiated half beam, would also require a vertical movement of the bed to maintain the entering point of the beams in the previously planned position. The method here suggested can be a solution of the geometrical prob- lems and above all a simplification of the three fields tech- nique, The use of individualized blocks allows a significant reduction in patient set-up time. In comparison with standard blocks they do not require a complex set-up and do not partially overlap with the collimated field. Useless weight excess is therefore avoided: the typical physical weight of the tangential blocks is 10 to 15 kg.

Furthermore, a single individualized shield for tangen- tial fields avoids couch displacements or collimator ro- tations which would otherwise be needed to comply with the treatment plan. Our method also maintains those ad- vantages arising from isocentrical techniques which, without additional movement, ensure the maximum re- peatability; the typical length of the tangential fields is 30-32 cm and is compatible with the most present linear accelerators.

There are, however, some disadvantages: first, of all large volumes (especially lung and contralateral breast)

receive through the blocks an unwanted dose which varies according to the shield’s thickness (with our shields such dose is roughly 5% of the delivered dose to the target vol- ume); furthermore, with our technique, filters cannot be used.

We suggest that experimental dosimetry should always be performed before carrying out this procedure as the results depend on the characteristics of the beam and the whole apparatus.

CONCLUSION

The treatment of the breast using the three fields tech- nique is a complex one. Whatever the procedure adopted, however, we believe that good results depend mostly on the experience of the whole staff and on the good coop- eration between radiotherapist, technician, and physicist. We recommend that our method is relatively simple, ac- curate, and time-efficient. Using this technique we treated over fifty patients during the last 3 years, without recording any cosmetic impairment nor, to date, any recurrences over irradiated areas.

Three-field isocentric technique 0 G. CON-E et al.

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