ESTRO 38 Abstract book

S252 ESTRO 38

Proton beam irradiation, due to the more favourable dose characteristics of proton beam, can provide the least dose to organs at risk, but the accessibility of the technique is limited and the available data are scarce. SP-0492 Radiobiology of APBI: aspects and limitations J. Guinot 1 , V. Gonzalez-Perez 2 1 Fundación Instituto Valenciano De Oncologia, Department Of Radiation Oncology, Valencia, Spain; 2 Fundación Instituto Valenciano De Oncologia, Department Of Radiation Physics, Valencia, Spain Abstract text Accelerated partial breast irradiation (APBI) has become a standard in early low-risk breast carcinomas. Alfa/beta around 4 is accepted for breast, what means that higher doses per fraction are recommended, and are favourable for patients due to a shorter total time of treatment in five days or less. Several techniques are being used, but with different dosimetries and different biological effects. Intracavitary procedures limit the irradiation to a certain margin from the cavity edge, but with a high gradient of dose on the surface. The volume of treatment is several times larger with external beam radiotherapy (EBRT) compared with multicatheter interstitial brachytherapy (MIBT) because a PTV must be added due to positional uncertainties. The skin and ribs cannot be easily preserved with EBRT. The accelerated schemes of EBRT with 38.5Gy in ten fractions or 30 Gy in 5 five fractions, correspond to a higher EQD2 dose than MIBT for alfa/beta 4, but the difference is greater for alfa/beta 2, chosen for late effects. We do not know if it has some impact on long term results and complications, as it happens with intracavitary devices, where rib fractures and skin necrosis are described in a small amount. On the other hand, the prescription dose with EBRT is quite homogeneous. With MIBT it corresponds with the peripheral dose, and the gradient of dose around every catheter supposes a higher biological effect inside the treated volume, but a higher risk of fibrosis. Five-year results are only the first step to know the efficacy of APBI, and longer follow-up is needed to be sure that the low total EQD2 doses are similar for EBRT and MIBT. Shorter number of fractions are under investigation, and will probably be implemented in the next future, to increase the comfort and compliance of APBI. The extremely low percentage of recurrences, under 2-3% at five years, indicates that low total doses are enough to control the remaining tumour cells after conserving surgery. Very accelerated partial breast irradiation (VAPBI) in a few or even a single fraction are promising. But the radiobiological effect is not well known and we must wait for the clinical trials to find the best option. Probably, several approaches will be useful and must be offered to each patient according to the availability, experience and preferences.

based APBI is also proven. All these facts should be a stimulus for radiation oncologists to make available brachytherapy based APBI worldwide for each suitable patient with early breast cancer. SP-0491 For which patient which technique is the best from view of the physicist? T. Major 1 1 National Institute of Oncology, Radiotherapy, Budapest, Hungary Abstract text Accelerated partial breast irradiation (APBI) can be performed with various techniques including both brachytherapy (BT) and external beam radiotherapy (EBRT) having different dosimetric characteristics. All methods are clinically feasible and there is no „one size fits all” technique. The best technique depends on patient anatomy and tumour location. However, the choice for treatment in the institutions mostly depends on the physician's preference and the technical availability. Among all techniques multicatheter interstitial BT (MIBT) has the longest experience. MIBT can provide highly conformal dose distribution, large dose gradient at target edge, low doses to organs at risk (OARs), but a possible geometric miss can result in significant under dosage of the target. Technically, the use of single-entry intracavitary applicators is easier. With balloon-type applicators no geometric miss can occur, but proper tissue conformance is not always guaranteed. With multi-lumen balloons sphere-like target volume with limited asymmetric margins can be irradiated, and the dose modulation potential is modest. In some anatomical situation the balloon can be asymmetric resulting in asymmetric target coverage. The intracavitary hybrid applicators are more flexible regarding shaping the dose distribution and reducing dose to OARs without compromising the target volume coverage. During intraoperative radiotherapy (IORT) large dose is given in one fraction without imaging and 3D treatment planning. In intraoperative electronic BT using spherical applicator the dose distribution is also spherical and a large dose inhomogeneity develops due to the sharp dose fall-off of the low energy X-ray beam. The margin is always symmetric, but the geometric accuracy is always ensured. At IORT with electron beam dose modulation possibility to shape the dose distribution is very limited. Standard dose distributions are used, the thoracic wall is protected with a physical device and the skin with retraction. Linear accelerator based EBRT techniques expose relatively large volumes of non-target breast to high dose mainly due to the extended target volume created from CTV. In three-dimensional conformal radiotherapy (3D- CRT) dose to contralateral breast, lung or heart can be reduced with proper selection of beam orientations. With intensity modulated radiotherapy (IMRT) highly conformal dose distribution can be achieved, but volumes irradiated by low doses can be larger than with 3D-CRT. Regarding the dose to OARs, with MIBT the critical structures can be better spared than with 3D-CRT/IMRT except for the heart. The dose to heart in BT is strongly dependent on the location of the PTV. With image guidance in EBRT the dose to OARs can be significantly reduced due to smaller target volume. At left sided lesion the dose to heart can be considerably decreased with deep inspiration breath- hold technique. With special EBRT devices such as Cyberknife or Tomotherapy which are equipped with image guidance smaller CTV-PTV margin can applied which reduces the dose to OARs while maintaining proper target coverage. Real-time tracking with Cyberknife can provide good target volume coverage and spares nearby critical organs, but the treatment time is too long.

Proffered Papers: Biomarkers and bioimaging in radiotherapy

OC-0493 CTCs in patients with brain metastases under radiotherapy: do they indicate treatment response? C. Petersen 1 , Y. Goy 1 , A. Krüll 1 , K. Pantel 2 , H. Wikman 2 , K. Borgmann 3 1 University Medical Center Hamburg - Eppendorf UKE, Department of Radiation Oncology, Hamburg, Germany ; 2 Institut of Tumor Biology, Center of Oncology, Hamburg, Germany; 3 Laboratory of Radiobiology & Experimental Radio-Oncology, Department of Radiotherapy, Hamburg, Germany