SP-0565: Target coverage and dose to organs at risk using different techniques of APBI (EBI, IORT, BT)

The evidence that most breast cancer local recurrences develop in close proximity to tumour bed paved the way to the introduction of APBI, an approach in which only the lumpectomy bed plus a 1-2 cm margin is treated, rather than the whole breast. Because of the small volume of irradiation, a higher dose can be delivered in a shorter period of time. Various approaches have been proposed for APBI: multi-catheter interstitial or balloon catheter brachytherapy (BT), intraoperative radiation therapy (IORT) and external beam irradiation (EBI). These techniques are intrinsically different in terms of both radiation delivery and obtainable dose distributions thus leading to different target coverage, dose conformality, and OARs sparing. A number of papers aims at exploring the differences between APBI planning with different techniques. Starting from these comparison studies, we'll go through the main differences between the treatment modalities, trying to identify subgroups of patients who would benefit from a specific approach and to see whether different results are due no only to the technique that was used but also the way it was implemented (e.g. use of non- coplanar beams, partial arcs, blocked regions). In BT no margin is applied around the CTV, which allows reducing the breast normal tissue irradiated with high doses, which seems to correlate with some adverse cosmetic effects. Moreover BT can provide highly conformal dose distribution and steep dose gradients, which allow optimal sparing of critical structures. Nevertheless, due to the shape of the dose distribution, the OARs sparing is strongly dependent on the location of the PTV. IORT could minimize some potential side effects since skin and the subcutaneous tissue can be displaced during radiation delivery. The dose distribution is characterized by a sharp dose fall-off but possibilities to shape the dose distribution are very limited. EBI is attractive since no specific manual skills are required. Several strategies have been used: 3D conformal radiotherapy (3D-CRT), static field intensity modulated radiation therapy (IMRT), volume modulated arc therapy (VMAT), helical Tomotherapy and proton therapy. At the current state, the chosen approach mostly depends on the technical availability. A general result is that intensity modulation techniques are characterized by a better dose conformity when compared to 3DCRT. Non- coplanar beams may be used to reduce the proportion of ipsilateral breast receiving high doses. Proton beams generally show the best performances, allowing the smallest volume of ipsilateral breast to be exposed to low dose, even though scattered beam techniques may be associated to higher skin doses. Not only the planning technique, but also the delivery could affect the dose to OARs: - image guidance could allow for margin reduction thus further reducing dose to uninvolved breast; - left sided lesion could benefit from using a deep inspiration breath-hold technique, which reduces heart dose; - real-time tracking could allow further margin reduction. Currently, we are far from being able to understand whether the dosimetric differences between the different treatment techniques are clinically relevant.