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Int.

J. Radiation

Oncology

Biol.

Phys.,

Vol.

28. (c I” the

No. 1994

3, pp. All

753-760, Science rights $6.00 +

1994 Ltd .OO reserved

Pergamon

Copyright Printed

Elsevier

USA.

0360-3016/94

0 Technical Innovations and Notes
A MONO ISOCENTRIC TECHNIQUE FOR BREAST AND REGIONAL THERAPY USING DUAL ASYMMETRIC JAWS NODAL

ERIC E. KLEIN,M.S., MARIETAYLOR,M.D., MARTHA MICHALETZ-LoRENz,C.M.D., DEBRA ZOELLER,C.M.D. AND WILLIAM UMFLEET,B.S.
Washington University School of Medicine. Mallinckrodt Institute of Radiology, 5 10 S. Kingshighway, St. Louis, MO 631 IO

Purpose: Definitive radiation therapy for breast cancer with regional nodal involvement often introduces treatment ofent abutted regions. Many methods describe techniques to achieve aneffective transverse plane match. Our facility recently adopted a matching technique using asymmetric jaws to beam-split all portals along the central axis plane. Our technique uses one isocenter to treat the opposed tangential breast fields, the supraclavicular port and the posterior axillary field. Methods and Materials: Our linear accelerator has four collimator jaws capable of being setindependently. The longitudinal (Y) iaws beam-snht all the mortals at the match nlane, namelv the upper border of the tangential beams fields, andYthe bottom border of the nodal fields. The transverse (X) jaws define the-lateral borders of thenodal and in a near beam-split fashion in conjunction with customized Cerrobend, block the lung for the tangential beams. The unique isocenter is chosen along themid-bridge through the tangential match plane. Dosimetric qualities and calculational techniques of the asymmetric beams were analyzed with ionimetric water scans, ion chamber studies, and film. The match-line is clinically confirmed with composite port films. Results: Our dosimetric studies show asymmetric jaws provide nearly equivalent field edge definition and superior absorption in comparison withCerrobend blocks. The use of one isocenter results in a reduction of in-room treatment time by a factor of two. The burden of lifting heavy Cerrobend blocks has been removed. A composite port film, which includes the medial tangential and supraclavicular ports, shows a perfect match-line in all cases. Similar composite port films taken with our previous technique of geometric matching withcollimator and table angulation exhibit slight overlap or underdose regions in many cases. Conclusion: Our treatment technique takes full advantage of dual asymmetric jaws to achieve a perfect match-line, necessitates only one isocenter and set-up point, and supplies more absorption in reference to lung and contralateral breast dose. The pure match-line is accompanied by the fact that the patient doesnot have to move in any direction. Breast, Field matching, Asymmetric jaws, Treatment techniques.

INTRODUCTION

External beam radiation therapy for breast cancer with nodal involvement introduces noncoplanarity. Previously we employed a conventional three-field source-to-skin distance (SSD) technique using collimator and table angulations to achieve a match-line. We have adopted a new techniquefor breast and local nodal irradiation which takes full advantage of dual asymmetric jaws. By having abutted beam-split fields via the Y-jaws (longitudinal), all three portals use one isocenter and hence one set-up point. The transverse (X) jaws, in combination with Cerrobend, are used to beam-split the medial and lateral tangential portals at the chest wall level and also define the lateral andmedial edge of the supraclavicular port. In addition

the posterior axillary field will use the unique isocenter and be collimated as needed using the independent jaws and Cerrobend. Many methods have been published to achieve an ideal match of the anterior oblique supraclavicular (SCV) field’ s caudal edge and the cephalad edge of the tangential fields. A nondivergent SCV field edge is always...
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