Brachytherapy in the Treatment of Breast Cancer

Introduction

Breast brachytherapy has been in use for most of the 20th century. In the 1920s, Keynes used interstitial radium needles to implant the entire breast, and frequently, the peripheral lymph nodes, to treat cancer of the breast.[1] With the advent of megavoltage radiation, external-beam radiation therapy (EBRT) was used to treat the whole breast, with brachytherapy being utilized as a boost for unresected tumors. The high total doses resulted in poor cosmetic results, and therefore, the trend was to perform lumpectomy followed by EBRT and lower doses of brachytherapy.[2,3]

Published reports, however, showed that lumpectomy (with biopsy-negative margins) followed by EBRT without an intersitial boost was as effective as mastectomy.[4] This raised the question of whether boosts were necessary. In addition to these reports, the widespread availability of the less invasive electron-beam boost led to a decline in the role of brachytherapy boost in treating primary breast cancers.

Whole-breast EBRT involves a 6-week course of fractionated treatments. In contrast, brachytherapy can be completed in a 4- to 5-day treatment course.[5-7] This feature has renewed interest in breast brachytherapy as a sole treatment modality after lumpectomy. The American Brachytherapy Society (ABS) recognizes that EBRT is the standard radiation modality used in the treatment of breast cancer. Brachytherapy is an alternative that is being investigated for specific circumstances.

The ABS Guidelines for Brachytherapy

Breast brachytherapy techniques vary widely, and few radiation oncologists have extensive experience in this realm. Because limited guidelines exist for the clinical use of this therapy,[8] the ABS formed a panel to issue guidelines specifically for the use of brachytherapy in breast carcinoma.

The ABS panel consisted of selected ABS members with expertise in breast brachytherapy. The panel performed a literature review, and supplemented with their clinical experience and biomathematical modeling, formulated specific recommendations and directions for future investigations in breast brachytherapy. Technical details were also included to benefit readers who may not be able to find such details in standard textbooks.

Recommendations were made by consensus and supported by evidence in the literature whenever possible. The ABS’s categories of consensus are similar to those used by the National Comprehensive Cancer Network.[9] Unless specifically noted, these recommendations generally reflect a category 2 consensus. Definitions of categories 1 through 3 are as follows:

Category 1: There is uniform panel consensus based on high-level evidence that the recommendation is appropriate.

Category 2: There is panel consensus based on lower-level evidence, including nonpublished clinical experience, that the recommendation is appropriate. There is no major disagreement among panel members.

Category 3: There is major disagreement among panel members that the recommendation is appropriate.

The initial report was revised due to additional comments of external experts who were not members of the panel. These reviewers are listed in the acknowledgments below. The Board of Directors of the ABS approved the final document.

Please note: These guidelines represent a consensus of the authors regarding a currently accepted treatment. These guidelines will be updated as significant new outcome data are available. Any clinician following these guidelines is expected to use clinical judgment to determine an individual patient’s treatment. The American Brachytherapy Society makes no warranties of any kind regarding the use of these guidelines and disclaims any responsibility for their application in any way.

General Considerations Regarding Technique

Clinical Target Volume

The ABS recommends precise definition and meticulous delineation of the clinical target volume. There are various methods of identifying this parameter, including the use of surgical clips, computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, nonionic radio-opaque contrast material, and/or direct visualization at the time of surgery.[8,10-12] Patients should be seen by both a surgeon and radiation oncologist before treatment.

The optimum volume to be treated by breast brachytherapy is controversial; treating too small a volume would result in high recurrence rates.[13,14] The ABS currently recommends a 2-cm margin around the lumpectomy cavity. This results in the irradiation of a large volume, but reduces local recurrence. Only controlled clinical trials will be able to demonstrate whether it is possible to treat a smaller volume and still obtain good local control.

The clinical target volume is defined as the volume encompassed by an irregularly shaped surface approximately 2 cm outside of the excision cavity, unless extending the margin would go beyond the breast tissue.[8] If the skin restricts the superficial margin, then the superficial target margin is redefined as 0.5 cm below the skin surface. Similarly, if the chest wall restricts the deep margin, the new deep-target margin boundary would be redefined as the surface of the chest wall.

The margin can be modified under special circumstances, depending on the tumor size and extent of surgical resection (consensus category 3). For brachytherapy, the clinical target volume is the same as the planning-target volume. In order to guarantee adequate coverage, the treated volume is ideally larger than the target volume.[8]

Treatment Planes

The ABS recommends using a minimum of two planes, unless the amount of breast tissue limits the target volume to less than a 2-cm thickness. For a larger target volume (> 2.5- to 3-cm thickness), additional planes should be considered.

Optimized values of interplanar spacing for a range of biplanar implant sizes are available.[15-17] Within a given plane, the separation between catheters is 1.0 to 1.5 cm. Ideally, the catheters should extend 1 to 2 cm beyond the edge of the clinical target volume. Coverage of the target volume along the length of the catheters is determined by the length of the active sources or dwell positions along each catheter. They should also extend 1 to 2 cm beyond the target volume.

To allow for this, skin entry and exit points should extend 2 to 4 cm beyond the edge of the target volume. This will allow a "dead space" of 1 to 2 cm from the last radioactive sources to the skin, which minimizes the risk of skin hyperpigmentation and telangiectasia.[8] To minimize the risk of a high dose to the ribs, a minimum distance of 0.5 cm between catheters in the deep plane and the ribs is suggested.

If a wide separation between catheters or planes is noted, or if a peripheral edge at the clinical target volume is not covered by the prescription isodose curve, it is preferable to insert additional catheters to avoid underdosage. Similarly, if the target volume is thicker on one side, additional crossing catheters may be required to adequately cover the thicker side. A less desirable alternative would be a higher weighting (increased dwell times) on the thicker side. However, this may lead to problematic local high-dose regions surrounding these catheters.

Catheter Concerns

The ABS deems the use of rigid templates as optional. The advantage of templates is that they effect precise geometric source distributions. However, this strategy may produce coverage problems in cases of curved, irregularly shaped target volumes. The freehand technique, especially with an open wound, permits catheter placement that conforms to the shape of the lumpectomy cavity. Nonetheless, this may result in less uniform isodose distributions.

The ABS recommends the appropriate use of dose optimization in high-dose-rate therapy. However, optimization should not be used to compensate for inadequate or improper catheter placement. It is preferable to have too many catheters implanted rather than too few. Since the catheters are commonly left in place for 1 to 2 weeks when brachytherapy is used as a sole modality, the ABS recommends appropriate catheter care to reduce the risk of infection and minimize discomfort.[8]