Breast conserving therapy (BCT), consisting of surgical lumpectomy followed by whole breast radiation therapy has become the standard of care for treating early-stage breast cancers. In comparison with mastectomy, BCT demonstrated similar outcomes with superior cosmesis and reduced psychological and emotional trauma based on multiple randomized trials. At the time of the lumpectomy, the surgeon removes the tumor and a surrounding rim of normal tissue (margin), typically leaving surgical clips to help designate the resection cavity or tumor bed (TB) for the radiation oncologist. The current standard of radiation therapy for breast cancer is to deliver treatment to the whole breast to 45-50.4Gy in 25 to 28 treatments Monday through Friday. Following whole breast radiation, a 'boost' is delivered to the TB in order to deliver 60 - 66Gy to the tumor bed. Two prospective trials have demonstrated a statistically significant reduction in local failures with the addition of a boost of 10Gy(in 4 fractions @ 2.5 Gy per fraction) or 16 Gy in 8 fractions @ 2 Gy per fraction), respectively.

Boost treatments can be delivered through a variety of techniques including a single electron field (used for superficial tumor beds) or multiple photon fields (2 or 3 fields typically) for tumors that are deep to the skin (usually \> 3 cm). With the use of CT simulation to guide the delivery of the boost, the need for deep TB coverage has become more apparent and now most patients receive photons for the boost portion of their therapy because the use of electrons often misses part of the tumor bed. However, when photon beams are used, in comparison to electrons, more generous margins posterior to the surgical cavity are required to account for daily set up error and respiratory motion which is not necessary for a single en face electron field. Furthermore, there are only limited directions along which the radiation can be directed to the TB, and as a result, large volumes of normal breast tissue receive a substantial fraction of the prescription dose which can lead to internal scarring (fibrosis) and poor cosmesis. The largest clinical series evaluating this issue demonstrated increased fibrosis and worse cosmetic outcome using photons. The clinical target volume for the boost is the TB, while an additional 1-1.5 cm margin of normal breast tissue is added isocentrically to account for daily set-up error and respiratory motion to define a planning target volume. Typically the boost is delivered after the whole breast portion of treatment, however, in various cases this sequence can be changed. For example, if significant skin breakdown occurs during the whole breast radiation phase, investigators can stop the whole breast radiation therapy and change to deliver dose only to the TB while allowing time for the rest of the breast to heal. This allows a continuous course of therapy to the highest risk of subclinical disease (i.e. the tumor bed).

Hypofractionation, or delivery of greater than standard 1.8 - 2 Gy fraction sizes per day, is a method of shortening overall treatment time in early stage breast cancer. Historically, standard fraction sizes of 1.8-2.0 Gy for whole breast irradiation (WBI) were based primarily on studies examining squamous cell cancers from cervix and head and neck regions. The smaller fraction sizes exploited a biological differential in squamous cell cancer fractionation sensitivity versus normal tissue fractionation sensitivity. This allowed relative sparing of surrounding normal tissue from low dose per fraction. However, investigators from the United Kingdom hypothesized that the fractionation sensitivity for adenocarcinoma of the breast is close to that of the normal breast tissue. Therefore, with increasing fraction size a sufficiently large reduction of total dose could be implemented to keep late toxicity constant without reducing the probability of tumor control.