Breast Radiotherapy. Part 2

Partial breast irradiation (PBI)

The theory

It is recognised that most local recurrences are close to the tumour bed. The NSABP-06 trial reported 86% of local recurrences within the reference quadrant. The Milan trial reported similar findings with 79% of recurrences at or close to the original tumour site. Several trials are testing the hypothesis that reducing the volume of breast tissue irradiated will provide a better trade-off between local tumour control, cosmetic outcome and patient convenience versus standard WBRT.

Brachytherapy

This technique entails delivering a higher dose of radiation in a reduced time to a defined volume of breast tissue (accelerated partial breast radiation–APBI). In the past this was achieved using interstitial implants placed in and around the tumour cavity often using low-dose-rate (LDR) tissue irradiation.

With the increased availability of high-dose-rate (HDR) brachytherapy units, these techniques are being used once more. This involves placing a balloon catheter in the tumourectomy cavity either at the time of surgery or, more commonly, afterwards under local anaesthetic. The balloon is inflated with contrast agent and connected to the HDR brachytherapy source. This then permits the delivery of 10 fractions of radiotherapy over 5 days. The main problems include increased skin toxicity due to proximity to the skin if the tumour is superficial in its location, and an increased incidence of fat necrosis. The treatment may also be problematic if subsequent WBRT is indicated. As breast cancer can recur years after treatment, and whilst current trials have short follow-up data, many believe that APBI should not be considered to be standard practice until the follow-up data is more robust.

Intra-operative techniques

This technique involves delivering PBI at the time of surgery using an electron beam radiotherapy unit. It has the logistical advantage of being a one-stop procedure for patients, thus avoiding other postoperative visits. However, it does mean that the definitive histological margins around the lumpectomy cavity are not known at the time of the procedure. Within the UK this technique is being investigated within the TARGIT (Targeted Intraoperative Radiotherapy for Early Breast Cancer) trial.

External beam radiotherapy techniques
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Computed tomography (CT) planned conformal techniques have been developed to deliver PBI. Stringent selection criteria are used, as for brachytherapy patients, and this has the advantage that the postoperative histology is known. It also eliminates a second surgical procedure with improved dose homogeneity that may in turn improve cosmesis and reduce the risk of fat necrosis (more commonly seen in brachytherapy studies). The fact that this is fractionated treatment over a number of weeks may have a radiobiological advantage over single-fraction techniques. As an external beam linear accelerator is used, extra margins need to be added to take account of internal movement and inconsistencies in patient set-up. This may mean the irradiated area is larger and in turn this may adversely affect the cosmetic result.

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