PII S0958-3947(97)00116-7

TANGENTIAL IRRADIATION OF THE INTACT BREAST: THE ABC OFTHE TECHNIQUE

AMR AREF, M.D.,1 GARY EZZELL, PH.D.,1 PAUL CHUBA, M.D., PH.D.,1DALE THORNTON, R.T.T., C.M.D.,1 and IBRAHIM AREF, M.D.2

1Department of Radiation Oncology, Wayne State University School of Medicine, and the Barbara Ann KarmanosCancer Institute, Detroit, MI; 2Department of Radiation Oncology, Ottawa Regional Cancer Center, Ottawa, Canada

AbstractThis paper describes in detail the setup of breast irradiation using an isocentric tangential technique.This setup method does not require any special devices or calculations beyond simple arithmetic. We will discusssome of the practical problems and pitfalls that result from the oblique incidence of the radiation beams and theslope of the chest wall and provide possible solutions. 1998 American Association of Medical Dosimetrists.

Key Words: Tangential irradiation, Radiotherapy Technique, Breast irradiation.

INTRODUCTION

Early breast cancer is commonly treated by lumpectomyand whole breast irradiation using an opposed tangentialbeam arrangement. As tangential breast irradiation is oneof the most commonly used techniques in radiation ther-apy departments, it deserves a description in full detail.We describe the technique used at Wayne State Univer-sity with special emphasis on practical problems oftenencountered. This technique can be easily installed clin-ically without the need for special devices or mathemat-ical calculations beyond simple subtraction. Patients aretreated using an isocentric technique wherein the centralaxis of the beam lies in the plane of the posterior bordersof the tangential fields. One-half of the used field isblocked at the primary collimator and only one-half ofthe field is actually used for treatment.

METHODS

Accurate body immobilization is achieved using acommercially available sloping board. We prefer to ele-vate the upper part of the patients body so that theanterior chest wall becomes parallel to the horizontalplane. Practically, this cannot be achieved in many pa-tients as the degree of the required slope may be largeenough to force the patient to slide down the board, andmost patients will be simulated with the superior part ofthe anterior chest wall sloping to varying degrees poste-riorly. The ipsilateral arm is abducted and flexed. Fol-lowing axillary dissection, many patients find this posi-tion initially uncomfortable, but adherence to a regularphysiotherapy program before simulation alleviates thisproblem to a great extent.

The upper border of the radiation fields is deter-mined clinically. The exact position of the upper bor-der varies according to the degree of the board eleva-tion used, breast size and shape, and position of thelumpectomy site. Although the upper border is oftendefined by the angle of Louis, we have found thisdefinition unsatisfactory in many women with moder-ate to large sized breasts, as the plane of this anglemay actually lie only a few centimeters superior to thenipple. Similarly, we are unable to describe the upperborder in relation to palpable breast tissue, as this ishard to define in many postmenopausal patients withslight obesity. We commonly position the upper bor-der at the level of the inferior border of the clavicularhead. The inferior border is set 2 cm below the infra-mammary fold in the midclavicular line. The mid-transverse plane between these two borders is definedby setting the upper and lower borders of the collima-tor opening at the defined points. The transverse laserbeam is then placed equidistant from the upper andlower borders. A medial setup point is defined at theintersection of the transverse laser beam with thepatient sagittal midline. The sagittal midline is definedclinically, not on fluoroscopy. Because of the abduc-tion and flexion of the arm, the sagittal midline of theanterior chest wall will often not coincide with thesagittal plane passing through the middle of the dorsalvertebrae as seen on fluoroscopy. The medial setuppoint is marked with a small radiopaque marker. Alateral setup point is defined at the intersection of thetransverse laser beam with the mid axillary line. For alateral lesion, the lateral border is moved posteriorlyto avoid geographic miss at the lumpectomy site.1,2This point is also marked by another radiopaquemarker.

The isocenter of the simulator is placed at themedial setup point (Fig. 1). The gantry is rotated

Reprint requests to: Amr Aref, M.D., Gershenson RadiationOncology Center, Wayne State University School of Medicine, and theBarbara Ann Karmanos Cancer Institute, 3990 John R, Detroit, MI48201

Medical Dosimetry, Vol. 23, No. 1, pp. 1519, 1998Copyright 1998 American Association of Medical Dosimetrists

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clockwise for right breast lesions or counterclockwisefor left breast lesions until, under fluoroscopy, boththe medial and lateral markers are superimposed.Since the isocenter of the machine is placed on themedial setup point, these two points must overlapduring the gantry rotation. The gantry angle with thevertical plane, u, is recorded. For the majority ofpatients, this angle will be in the range of 50 58. Atthis point, the amount of lung tissue included betweenthe chest wall and the central axis is assessed. Wetypically aim for about 23 cm of lung tissue to beincluded in the tangential field. Moving the medialsetup point laterally or moving the lateral setup pointanteriorly and adjusting the gantry angle will decreasethe amount of lung tissue included in the tangentialfields. The reader will note that, at this position, wehave identified a central ray that passes through thesource of radiation, the isocenter of the machine, themedial setup point, and the lateral setup point. Toachieve an isocentric setup, we desire to move theisocenter of the machine along this line so that it willbe equidistant from the both the medial and lateralsetup point (Fig. 2).

To move to the true isocenter, the table is movedin upward and medial directions until a position satisfy-ing the following two conditions is met: a) the centralaxis passes through the medial setup point and, b) thedistance between the source of radiation and the medial

setup point equals 100 cm minus one-half the separationbetween the medial and lateral setup point. In otherwords, this will mean that the isocenter of the machine isplaced equidistant from the medial and lateral setuppoints. This position may be confirmed by rotating thegantry 180. The central axis will pass through the lateralsetup point as expected and can be verified under fluo-roscopy.

Next, the gantry is returned to the initial zero posi-tion. It will now be noticed that the central axis isprojected between the medial and lateral setup points.The lateral position of the table is recorded and the tableis moved laterally until the central axis passes throughthe medial setup point and the new lateral position of thetable is recorded. The difference between these two tablereadings represents the lateral shift. The distance be-tween the radiation source and the medial setup point isrecorded as the setup depth (Fig. 3).

In an actual treatment setup, with the gantry in udegree position, the isocenter of the machine is posi-tioned over the medial setup point and the table is movedvertically until the distance between the radiation sourceand the medial setup point equals the setup depth. Thefield is moved horizontally towards the ipsilateral breastto a distance equal to the lateral shift.

The gantry is rotated by u degrees and treatment isgiven through the medial tangential field (Fig. 4). Thegantry is rotated to the opposing angle for the lateraltangential field.

Fig. 1. The isocenter of the simulator is placed at the medialsetup point (M). The source-to-skin distance is 100 cm. L 5lateral setup point, Ub 5 upper border, Lb 5 lower border.

Fig. 2. A central ray is identified that passes through the sourceof radiation (S), the medial setup point (M), which also over-laps the isocenter of the machine, and the lateral set up point(L). The desired position of the isocenter is at I where IM 5 IL.

The distance SM 5 100 cm.

Medical Dosimetry Volume 23, Number 1, 199816

DISCUSSION

The tangential technique is a parallel opposed-pair technique. The plane of the radiation source andthe central axis, which in our technique also forms the

posterior border of both the medial and lateral tangen-tial fields, does not have a perpendicular incidence tothe skin surface as in other conventional parallel op-posed-pair techniques (anterior/posterior and lateralarrangements) used often in radiation therapy. Thisdifference accounts for the following observations: theprojection of the central axis on a sloping surface willnot be a straight line, as in the cases of conventionalparallel opposed- pair techniques, but will be curved.In most patients, the superior part of the chest wallslopes posteriorly and, to some extent laterally as well,and therefore, the light projection of the central axiswill not be a straight line coinciding with the sagittalplane of the patient, but rather curved and concavelaterally (Fig. 5). This may result in missing the uppermedial part of the treated breast from the radiationfield. This problem, if small in magnitude, can becorrected by rotating the collimator clockwise for theright breast and counterclockwise for the left breast;however, care should be exercised as rotating thecollimator may also result in excluding the lowermedial part of the treated breast from the radiationfield. Another approach to this problem is to mark thesagittal midline with a radiopaque wire and design ablock that matches this wire from a simulator film(Fig. 6). Such a block should not be designed directlyfrom the simulation film without the aid of skin mark-ings as it is difficult to predict the position of thisblock on the patients skin and part of the medialaspect of the contralateral breast may be unintention-ally included in the treatment field. In obese patients,the inferior part of the anterior chest wall slopesanteriorly and the inferior part of the light field defin-ing the medial border of the tangent will be projectedaway from the midline towards (and may include) the

Fig. 3. The central axis is projected between the medial andlateral setup points. MI 5 lateral shift.

Fig. 4. Actual treatment position. MS 5 100 - MI and MI 5IL, ML 5 patient separation, S 5 source of radiation, M 5medial setup point, I 5 isocenter of the machine, L 5 lateral

set point.

Fig. 5. The projection of the central axis in the treatmentposition will be curved and is not a straight line.

Tangential irradiation of the intact breast l A. AREF et al. 17

lower inner quadrant of the contralateral breast. Again,placing a radiopaque wire over the midline wouldfacilitate constructing a block that alleviates this prob-lem (Fig. 7). Adequate coverage of an extremely me-dial lesion may prove difficult to achieve withoutincluding the medial aspect of the contralateral breastin the radiation field. The contralateral breast, becauseof its thickness, may obstruct the path of the medialtangential field before it covers the lower lying skinover the manubrium. This problem can be solvedeither by taping down the contralateral breast or

changing the gantry angle and accept a more posteriorexit point with increased amount of lung tissue in theradiation field. Alternatively, a separate electron fieldmay be used to treat the medial part of the ipsilateralbreast. The posterior border of the lateral tangentialfield is commonly defined as passing through the mid-axillary line, implying that it intersects the patientslateral chest wall along a line that lies in the coronalplane of the patient. Again, this is not an accuratedescription, as the posterior border of the lateral tan-gent often projects as an oblique line with the lowerpart anterior to and the upper part posterior to themid-axillary line. The posterior border of the lateraltangent will meet the mid-axillary line only along thetransverse midline plane of the field. Attempts at col-limator rotation, beyond a few degrees, during thesetup of the medial tangential field will cause the beamto exit at an even more posterior level superiorly andmore anterior level inferiorly. This may result in treat-ing a large area of the upper posterior chest wall anddistal axilla, which may cause excessive dry or moistdesquamation especially in obese patients, and under-dosing the lower outer quadrant of the ipsilateralbreast. One should always appreciate that the medialand lateral tangential fields are mirror images of eachother and any change in one field will also result inchanges in the other field if all points in the radiationvolume are to be treated by both fields (Fig. 8). Thisinteraction between both fields is best examined clin-ically as it is difficult to appreciate it during fluoros-copy or from review of the simulation films.

Fig. 6. A wire is placed over the patient sagittal midline and acerobend block is designed from the simulator film along this

wire.

Fig. 7. Projection of the central axis in the treatment positionwhen treating an obese patient. The upper part of the centralaxis is deviated towards the ipsilateral breast and the lower partis deviated away from the midline towards the contralateralbreast. W 5 radiopaque wire placed along the patient sagittalmidline, M 5 projection of the posterior border of the medial

tangential field.

Fig. 8. The interaction between the medial tangential and lateraltangential fields. M 5 the projection of the posterior border ofthe medial tangential field, CM 5 the projection of the medialtangential field after collimator rotation, L 5 the exit of theposterior border of the medial tangential field, CL 5 the exit ofthe posterior border of the medial tangential field after colli-

mator rotation.

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CONCLUSION

We have described in detail an easy way to clin-ically set-up a tangential isocentric technique and dis-cussed some clinically relevant observations. The tan-gential field technique is a special type of a parallelopposed pair technique and has all of its characteris-tics. Because of the oblique incidence of the radiationbeam on the skin and the sloping shape of the breastand chest wall careful clinical evaluation of the setupis important, as potential pitfalls cannot be readily

recognized using fluoroscopy, simulation, or portfilms.

REFERENCES

1. Bedwinek, J. Breast conserving surgery and irradiation: the impor-tance of demarcating the excision cavity with surgical clips. Int. J.Radiat. Oncol. Biol. Phys. 26:675679; 1993.

2. Fein, D.A.; Fowble, B.L.; Hanlon A.L.; Hoffman, J.P.; Sigurdson,E.R.; Eisenberg, B.L. Does placement of surgical clips within theexcision cavity influence local control for patients treated withbreast-conserving surgery and irradiation? Int. J. Radiat. Oncol.Biol. Phys. 34:10091017; 1996.

Tangential irradiation of the intact breast l A. AREF et al. 19