Partial breast irradiation (PBI) is becoming a new paradigm for breast cancer radiation.
No type I or II evidence is currently available to demonstrate equivalence to standard
whole breast radiotherapy, and a prospective randomized trial jointly sponsored by NSABP
and Radiation therapy Oncology Group (RTOG) (NSABP B-39 and RTOG 0413) has completed
accruing patients, comparing whole breast radiotherapy to PBI, either by brachytherapy or
external beam techniques (EB): results of this trial are pending. Until results of this
or similar trials are available, PBI remains a research domain, and it should be offered
to patients only in the context of a clinical experimental protocol. Despite the fact
that less extensive experience than that of brachytherapy is available, PBI delivery
through an external-beam has many advantages. First of all, it is likely to be more
acceptable to the patient since it is non-invasive and it does not require a surgical
procedure or anesthesia. Moreover, since it is delivered after surgery, the pathological
analysis of the segmental mastectomy specimen is available to inform the selection of the
best candidates. In addition, EB-PBI is likely to become more widely reproducible, since
it does not rely on the experience and skills of the radiation oncologist performing the
brachytherapy implant. Besides, once the technique is established, it can be widely
applied at any facility provided with a linear accelerator, without the risk presented by
some brachytherapy approaches that cannot be completed because of the unfavorable
interplay of patient's anatomy with the technical limitations of the applicator. Finally,
in terms of health care economics, an external beam approach spares the costs of an extra
surgical procedure and several days as inpatient (in the case of Low dose Rate (LDR)
brachytherapy).
A prone approach for partial breast radiation has been tested at NYU in a clinical trial
sponsored by an IDEA grant of the Department of Defense (NYU 00-23). Results of the first
47 patient accrued originally demonstrated feasibility. Eligibility to this study was
limited to post-menopausal women with non-palpable, mammographically detected tumors. In
addition, the protocol required patients to have first refused to undergo standard
six-week radiotherapy. Five fractions of 6 Gy were delivered to the Planning Target
Volume (PTV) over ten days (Monday-Wednesday-Friday, Monday-Wednesday). The dose and
fractionation was based on radiobiological modeling, aimed at determining a dose to
deliver in five fractions that would achieve equivalence to the tumor control estimates
of 50 Gy in 25 fractions, while maintaining a risk of fibrosis at the tumor bed
comparable to that of a standard regimen of 60 Gy in 30 fractions. An α/β = 4 for tumor
control was used, and its validity has been recently confirmed by the results of a
prospective randomized trial comparing accelerated to standard whole breast radiotherapy.
