Abstract Book

S711

ESTRO 37

particular when treating the internal mammary nodes or in case of challenging anatomy such as a funnel chest. The aim of this study is to evaluate dosimetric differences in unintentional dose to the lower axilla between tangential fields radiotherapy and VMAT. Ultimately our goal is to estimate whether results of past clinical trials that omitted axillary clearance and treated patients with tangential fields can safely be extrapolated towards VMAT. This is the first study to evaluate the unintended dose to the lower axilla when delivering WBRT with VMAT. Material and Methods Fifty consecutive patients treated with WBRT using tangential fields with a forward IMRT technique were identified in order to populate two groups: group 1 was treated with WBRT + axillary levels III-IV (n=25) while group 2 was treated with WBRT alone (n=25). Patients in which axillary levels I-II were part of the target volume were excluded. Two additional plans with a VMAT technique were created for every patient: one VMAT plan using a single 270° arc and another VMAT plan using two small (≤90°) opposing arcs that mimic tangential fields setup. In a last step axillary levels I-II were contoured, as such blinding the VMAT planner for these structures. Results Mean dose to axillary level I was 31.1 Gy, 32.2 Gy and 33.3 Gy with forward IMRT, VMAT 270° arc and VMAT two 90°arcs (p=0.001), respectively. For axilla level II the results were 19.4 Gy, 21.2 Gy and 22.3 Gy (p<0.001), respectively. Mean DVHs are given in Figure 1 and 2.

Table 1: V90% and V50% to the axilla V 90 % level I (%) V 90 % level II (%)

V 50% level I (%)

V 50% level II (%)

WBRT + axilla level III-IV (n=25) Forward IMRT

EP-1297 RT for bone-only oligometastases in breast cancer patients: a survey of current clinical practice. M. Bonet 1 , M. Arenas 1 , N. Farré 2 , V. García 3 , M. Algara 4 , B. Farrús 5 , J. Fernández 6 , V. Reyes 7 , A. Eraso 8 , A. Alvarez 9 , M. Cambra 10 , A. Pedro 11 , J. Vayreda 12 , A. Montero 13 , P. Poortmans 14 1 Hospital Universitari Sant Joan de Reus, Radiation Oncology, Reus, Spain 2 Hospital de la Santa Creu i Sant Pau, Radiation Oncology, Barcelona, Spain 3 Hospital Universitari Arnau de Vilanova, Radiation Oncology, Lleida, Spain 4 Parc de Salut Mar, Radiation Oncology, Barcelona, Spain 5 Hospital Clínic, Radiation Oncology, Barcelona, Spain 6 Hospital Quirón, Radiation Oncology, Barcelona, Spain 7 Hospital de la Vall d'Hebron, Radiation Oncology, Barcelona, Spain 8 Institut Català d'Oncologia Hospital Duran i Reynals, Radiation Oncology, Hospitalet de Llobregat, Spain 9 Institut Català d'Oncologia Hospital Germans Trias i Pujol, Radiation Oncology, Badalona, Spain 10 Hospital General de Catalunya, Radiation Oncology, Sant Cugat del Vallès, Spain 11 Hospital Plató, Radiation Oncology, Barcelona, Spain 12 Institut Català d'Oncologia Hospital Dr. Josep Trueta, Radiation Oncology, Girona, Spain 13 Hospital Universitario HM Sanchinarro, Radiation Oncology, Madrid, Spain 14 Institut Curie, Radiation Oncology, Paris, France Purpose or Objective While it is known that patients with bone-only metastases have a better prognosis, there is still a lack of consistency in the approach to treatment of these patients, which might be explained by insufficient evidence. The aim of this survey was to evaluate current clinical practice in treating breast cancer bone lesions in an oligometastatic scenario.

63.75

58.91

87.62

94.53

VMAT

single

54.79

48.75

96.95

99.25

270°arc

VMAT

two

58.92

67.85

97.47

99.44

opposing ≤90°arcs

P

0.43

0.09

<0.001 0.01

WBRT

alone

(n=25) Forward IMRT

56.46

0.93

71.72

4.42

VMAT

single

41.85

0

81.07

8.87

270°arc

VMAT

two

opposing ≤90°arcs

50.43

0

83.9

12.35

P

0.05

0.23

0.62

0.31

Conclusion For the low doses, VMAT treats a larger volume of the axilla whereas for the high doses tangential fields treat a larger volume. The clinical implication of the small but significant differences in unintended dose to the lower axilla is unclear and should be studied by examining clinical outcome at long term in a statistic significant population. However, because of the higher dose shoulder, 2 smaller tangential arcs seem preferential over a single 270° arc.