doi:10.1016/j.ijrobp.2004.10.033 PHYSICS CONTRIBUTION HYBRID IMRT PLANS���CONCURRENTLY TREATING CONVENTIONAL AND IMRT BEAMS FOR IMPROVED BREAST IRRADIATION AND REDUCED PLANNING TIME CHARLES S. MAYO, PH.D., MARCIA M. URIE, PH.D., AND THOMAS J. FITZGERALD, M.D. Department of Radiation Oncology, University of Massachusetts Medical School, Worcester, MA Purpose: To evaluate a hybrid intensity modulated radiation therapy (IMRT) technique as a class solution for treatment of the intact breast. Methods and Materials: The following five plan techniques were compared for 10 breast patients using dose���volume histogram analysis: conventional wedged-field tangents (Tangents), forward-planned field-within- a-field tangents (FIF), IMRT-only tangents (IMRT tangents), conventional open plus IMRT tangents (4-field hybrid), and conventional open plus IMRT tangents with 2 anterior oblique IMRT beams (6-field hybrid). Results: The 4-field hybrid and FIF achieved dose distributions better than Tangents and IMRT tangents. The volume of tissue outside the planning target volume receiving 110% of prescribed dose was largest for IMRT tangents (average 158 cc) and least for 6-field hybrid (average 1 cc) the FIF and 4-field hybrid were comparable (average 15 cc). Heart volume 30 Gy averaged 13 cc for all techniques, except Tangents, for which it was 32 cc. Average total lung volume 20 Gy was 7% for all. Contralateral breast doses were 3% for all. Planning time for hybrid techniques was significantly less than for conventional FIF technique. Conclusions: The 4-field hybrid technique is a viable class solution. The 6-field hybrid technique creates the most conformal dose distribution at the expense of more normal tissue receiving low dose. �� 2005 Elsevier Inc. IMRT breast techniques, Breast irradiation, Hybrid IMRT. INTRODUCTION Radiation therapy is an established component in the care of patients afflicted with breast cancer. As an adjunct to both surgery and chemotherapy, radiation therapy provides a survival advantage for lymph node���positive breast cancer patients (1) and a benefit in improving local control for all patients (2). In selected patients of limited constitutional status, radiation therapy can be the sole locoregional mo- dality of care. Anatomically, the breast presents a very challenging ge- ometry for radiation therapy. For locoregional disease con- trol, a minimal dose to all breast tissue is required. For good cosmetic results, dose homogeneity within the breast must be maximized and ���hot spots��� outside the target tissue minimized. Because most patients have a long life expect- ancy, doses to the lung and heart must be kept low to avoid long-term complications. Another restriction is dose to the contralateral breast, out of concern for possible induced second malignancies. Seminal studies (1, 2) demonstrating a survival benefit to breast cancer patients treated with radiation therapy used two-dimensional planning. Single isodose distributions through the isocenter provided the infrastructure for evalu- ating the role of radiation therapy in this disease. However, the full extent of the dose heterogeneity on the breast and the location and magnitude of the hot spots with conven- tional wedged-field tangents have become appreciated only as 3D treatment planning with CT scans obtained in the treatment position have become common for breast patients. With this conventional treatment strategy, areas of maxi- mum dose often are located in tissues outside of the in- tended target. Advances in multileaf collimator (MLC) use have helped compensate for these effects with the use of forward- or inverse-planned intensity modulated radiation therapy (IMRT) fields. These provide the radiation oncolo- gist an opportunity to optimize treatment to the target and to develop conformal avoidance of normal tissue. Several groups have reported on the improvement in dose homogeneity that may be achieved by using several MLC- formed subfields (3���9). Commonly referred to as forward- planned IMRT, this technique improves the dose homoge- neity throughout the breast and reduces the magnitude of the hot regions outside the target region. It also reduces the maximum dose to the ipsilateral lung. The primary disad- Reprint requests to: Charles Mayo, Ph.D., Radiation Oncology, UMASS/Memorial Medical Center, 55 Lake Avenue North, Worcester, MA 01655. Tel: (508) 856-5551 Fax: (508) 856-5006 E-mail: mayoc@ummhc.org This work was supported in part by a grant from Varian Medical Systems. Received Feb 23, 2004, and in revised form Oct 14, 2004. Accepted for publication Oct 18, 2004. Int. J. Radiation Oncology Biol. Phys., Vol. 61, No. 3, pp. 922���932, 2005 Copyright �� 2005 Elsevier Inc. Printed in the USA. All rights reserved 0360-3016/05/$���see front matter 922

vantage of these techniques is the increased treatment plan- ning time, because defining the shape and number of sub- fields is an iterative process. Inverse-planned IMRT offers the potential of extremely conformal dose distributions for many disease sites. The conformal and avoidance dose distributions are achievable by rapidly varying the fluence intensities of multiple beams from multiple angles. However, for breast treatments, the optimal beams for minimizing dose to the nearby normal tissues (lung and heart) are the nearly opposed tangent fields that geometrically avoid them. Success with IMRT may be limited by this geometry. Several groups have investigated the use of IMRT for intact breast (10���14). We too have investigated IMRT for breast irradiation and have developed a class solution that applies to both sides and all sizes and shapes of intact breast. By combining open conventional tangent beams with IMRT beams from the same medial and lateral angles, dose distributions that are superior to those of conventional tangents and IMRT-only tangents can be readily achieved. Even more conformal dose distributions can be achieved by adding anterior oblique IMRT beams, but at the expense of greater volumes of normal tissues receiving low doses. This approach meets our goal of using inverse planning to reduce treatment planning time, compared to our conventional techniques, and minimizing the set of optimization constraints. This method produces consistent results with less dependence on the advanced skills of the treatment planner and is a class solution for very common treatments. METHODS AND MATERIALS Five patients from each of the left- and right-sided treatment site groups were selected. The range of breast volumes for patients in each group was typical of those usually encountered. The patients were immobilized in a custom -cradle device and had CT scans performed in the treatment position. Scans were transferred to the treatment planning computer (Varian Eclipse version 7.1.35 Var- ian Medical Systems, Palo Alto, CA), and the breast tissue (clinical target volume) [CTV]) was defined by a radiation oncologist. The contralateral breast, right lung, left lung, and heart tissues were delineated on the CT scans. Breast volumes ranged from 370 to 1600 cc, separations from 17 to 27 cm. The following considerations were used by the radiation oncol- ogist in delineating the breast CTV. With the use of anatomic references, the CTV is generally defined superiorly by the inferior aspect of the clavicular head and inferiorly by the inframammary fold as identified on skin reconstruction and physical examination. Medially, the CTV is limited by the sternum and is generally delineated 2 cm medial to the edge of the sternum. Laterally, the breast tissue is identified in the midaxillary line. Exceptions to these anatomic references are made by the physician on a case- by-case basis based on the physical examination and image as- sessment. A breast planning target volume (PTV) was defined as the breast CTV plus a margin of 0.5 cm. It was modified to include only intersection with the body contour to facilitate evaluation of dose��� volume histograms. Treatment beams maintained ���flash��� neces- sary to assure coverage of the treatment area as defined by the unmodified PTV. A planning volume, the IMRT PTV, was defined to facilitate inverse planning optimization. It was created with a margin of 0.5 cm on the breast PTV and then modified to exclude 0.5 cm of the buildup region near the skin (See Fig. 1). This additional margin pushes the high-dose gradient produced by IMRT plans farther away from the edge of the PTV to reduce the effect of day-to-day variability in patient setup on actual PTV coverage. Excluding the region near the skin guides the optimization algorithm away from attempting to achieve full dose in the buildup region. Once opti- mization on the IMRT PTV was complete, normalization of the plan was based on coverage of the breast PTV. The body was delineated on the CT scans, and Boolean opera- tions were used to construct a modified body volume that excluded breast tissue with a 0.5-cm margin. Dose���volume histograms of this tissue outside the breast (VOB) were used to characterize doses to nontarget tissue within the radiation fields. For each patient, 5 treatment plans were developed to treat the breast to 45 Gy using 6 MV photon beams. All 5 plans for each patient used the same isocenter and tangential beam angles. Typ- ical beam���s-eye���views for medial beams are illustrated in Fig. 2. The first plan (Tangents) was a conventional breast treatment using medial and lateral tangential rectangular beams with wedges. In this article, this plan is considered to be the standard to which others are compared. All other plans were normalized to achieve isodose coverage of the breast tissue at least as good as the tangent plan. The second plan (field-within-a-field [FIF] tangents) was a manually developed field-within-a-field treatment (or forward- planned IMRT). This is the technique routinely used in our clinic to reduce hot spots and to increase the dose homogeneity to the breast (3). Wedges are used, and the radiation areas of medial and Fig. 1. For IMRT planning, the breast PTV contour (dark gray) is modified to create an IMRT PTV (contour line) that facilitates achieving good coverage of the breast PTV using the optimization algorithm. IMRT intensity modulated radiation therapy PTV planning target volume CTV clinical target volume. 923 Hybrid IMRT for breast ��� C. S. MAYO et al.