Partial Breast Irradiation: Accelerated and Intraoperative

Background and Rationale

Randomized studies with greater than 20 years of long-term follow-up data have established equivalent outcomes in patients who undergo mastectomy or breast-conserving therapy. Studies have further shown increased rates of local recurrence with the omission of radiation therapy following breast-conserving surgery, and a large meta-analysis has demonstrated an improvement in breast cancer mortality with the addition of radiation therapy to breast-conserving surgery. Historically radiation therapy given via whole breast irradiation (WBI) in earlier clinical trials used bony landmarks and two-dimensional planning techniques. Over the past several decades, WBI has evolved to utilize computed tomography (CT)-based three-dimensional (3D) techniques, which allow for better coverage of targets while sparing adjacent normal organs at risk, including the heart and lungs.

Whole breast radiation was commonly given in multiple daily fractions over a 5- to 6-week period, frequently incorporating a tumor bed boost. Due to the length of treatment, many patients find it difficult to comply with the entire treatment course, with studies showing noncompliance associated with older age, low socioeconomic status, distance from a radiation facility, and receiving treatment during winter months. In fact, the length of treatment impacts whether some patients decide to have breast conservation or a mastectomy. Studies have reported substantial subsets of patients who choose mastectomy over breast-conserving therapy due to an inability or unwillingness to receive adjuvant radiation therapy after breast-conserving surgery. Multiple studies have documented that up to 15% to 20% of patients forgo adjuvant radiation therapy, potentially compromising their breast cancer outcomes and survival. Hypofractionated whole breast radiation, which reduces the overall treatment course to 3 to 4 weeks, has become a well-established standard of care with level 1 evidence for clinical efficacy and acceptable toxicity, yet many patients continue to seek alternatives that further shorten the duration of radiation therapy. The increasing use of accelerated partial breast irradiation (APBI), which includes a variety of techniques that deliver adjuvant radiation therapy in 1 week or less, has been studied in part to allow greater access to breast-conserving treatment to more patients ( Fig. 48.1 ).

Whole breast radiation is given to eradicate any residual microscopic disease in the breast after surgical removal of the gross disease. Observational studies of patterns of failure in patients who underwent breast conservation have demonstrated that the majority of ipsilateral in-breast tumor recurrences occur within close proximity to the lumpectomy cavity, with lower rates of failures occurring elsewhere in the breast. Multiple studies have confirmed that the development of metachronous new primaries in the breast is not modified by the delivery of whole breast radiation. Pathologic specimen evaluation has quantified residual disease most commonly located within 1 to 2 cm of the surgical cavity after breast-conserving surgery. The concept of partial breast radiation, which involves treating only the breast tissue in the periphery of the lumpectomy cavity rather than the whole breast, is based on these scientific data.

The purpose of this chapter is to describe the evidence-based treatments used in partial breast irradiation, specifically APBI including interstitial brachytherapy, applicator-based brachytherapy, external beam techniques, and intraoperative radiation therapy (IORT).

Accelerated Partial Breast Irradiation

APBI can be delivered using several different techniques and fractionation schemes, with the most common techniques being interstitial brachytherapy, applicator-based brachytherapy, and external beam techniques ( Fig. 48.2 ). Interstitial brachytherapy represents the original technique with the longest follow-up to date, followed by the development of single-lumen applicators and subsequently multilumen applicators and external beam techniques.