ESTRO 37 Abstract book

S1063

ESTRO 37

Equations [1] and [2] represent the effective contributions to the systematic and random components due to the reduced number of fraction. Equation [3] includes the penumbra contribution in lung tissue and equation 4 is the total margin expression used. Results Results yielded by equations [1-4] and systematic contributions are presented in table 1.

ipsilateral lung, contralateral breast and lung. The mean dose (Dmean) and the V50Gy of the skin were also evaluated. Arcs' arrangements in both cases were kept identical and the delivery treatment time on the Elekta Agility 160 was calculated. Results The mean PTV coverage was 96.5% and 97.3% for FF and FFF respectively. By using FFF beams, the mean heart dose was reduced from a mean value of 28% (0- 75% range) for L-RT and from 16% (0-44%) for LR-RT. The Dmean at the ipsilateral lung was identical for L-RT but slightly decreased of 4% (0-15%) for LR-RT, when the V5Gy was 7 % (0-17%) reduced with FFF-beam for both L and LR RT. The V50Gy of the skin was also lower and decreased from 150% (0-900%) and from 95% (0-400%) for L and LR-RT respectively. The OARs' sparing (especially of the skin) observed with FFF was mainly due to a combination of factors: softer X spectra, sharp fall-off of the penumbra, less electrons' contamination, lower MLC transmission, lower patient's scattering, which induced less dose to the organs surrounding the PTV. The Dmean at the contralateral breast was identical for FF and FFF: 1.1±0.5Gy and 1.8±0.6Gy for L and LR RT respectively. The V5Gy at the contralateral lung was negligible in all cases (<0.1Gy). As expected, the delivery time per arc with FFF beam was also reduced from 17±1s to 12±1s for L-RT and from 18±1s to 13±1s for LR- RT. Conclusion As expected, FFF-VMAT beams reduce the delivery time for a DIBH treatment which makes it more comfortable. PTV coverage is achieved with a dose reduction to OARs, especially the heart and ipsilateral lung, while preserving the contralateral organs during breath hold. EP-1954 Use of dose painting technique in single fraction celiac plexus radiosurgery M. Ben Ayun 1 , L. Hamer 1 , D. Alezra 1 , D. Sergey 1 , T. Lev 1 , W. Ilana 1 , S. Zvi 1 , R.L. Yaakov 1 1 Chaim Sheba Medical Center Institute of Oncology, Oncology, Tel Nashomer, Israel Purpose or Objective Celiac plexus radiosurgery is a new palliative treatment for severe pain experienced in pancreatic cancer, and is the subject of an ongoing clinical trial (NCT02356406). A single-fraction of 25 Gy is targeted to the celiac plexus. Close approximation of the plexus to the duodenum (small bowel) creates a severe dosimetric challenge. Here we compare two planning techniques: 1) physician prescribed dose-painting based upon distance from the duodenum (Four PTVs ranging from 25Gy to 10Gy) and 2) prescribing a uniform dose of 25Gy to the entire PTV with appropriate cost-functions to achieve bowel constraints. Material and Methods All patients underwent both 3D and 4D CT simulations with abdominal compression. Fifteen patients were planned and treated with a single fraction to celiac plexus. Individual bowel loops were contoured. The celiac plexus was contoured from T12 to L2 (GTV), and a 1cm expansion added to create the overall PTV. Dose-painting technique: Volumes were created for the purpose of dose-painting included expand-bowel_0.5cm, expand-bowel_1cm, expand_GTV_1cm. A range of PTVs (25Gy, 20 Gy, 15Gy and 10Gy) were generated based upon Boolean operations: for instance, PTV25 = [GTV SUBTRACT expand bowel 1cm], PTV20 = [expand_GTV_1cm SUBTRACT (PTV25 OR expand- bowel_1cm)]. Uniform-dose technique: The entire PTV structure was prescribed 25Gy. Constraints for single fraction treatment (based upon Timmerman) included mean kidney dose 5.5Gy, max small bowel dose 11.2Gy. Calculations performed with AAA algorithm. Treatment plans were optimized using a

Table 1: Results obtained for random, systematic and total contributions. For hypofractionated treatments the error in estimating the average value is palpable when calculating the systematic and random standard deviations contributions, as shown in equations [1] and [2]. This effect diminishes for increasing number of fractions. The inclusion of the penumbra factor clearly reduces the random contribution to the margin, as σ P >> σ (eq.3). A more exhaustive analysis might take into account tissue heterogeneities nearby the tumour regarding penumbra contributions. Margin asymmetry in the lateral coordinate is directly correlated with lower respiratory amplitude in this direction as expected. Conclusion Margins were obtained for a cohort of 10 patients with NSCLC undergoing lung SBRT treatments. Inter-fraction and intra-fraction motion, delineation error, respiration, penumbra contribution, number of fractions and set up errors were considered. Margin calculations were carried out using a 90% confidence level (α=2.5) and 95% prescription isodose (β=1.64). Values obtained are in agreement with published results that appear in the literature. EP-1953 Dosimetric effect of using FFF-VMAT beams for left-sided breast radiotherapy in breath hold C. Tamburella 1 , G. Faessler 1 , G. Guibert 1 , L. Hirschi 1 , L. Pion 1 , P. Weber 1 , P. Tsoutsou 1 1 Hôpital Neuchâtelois, radiotherapie, la chaux de fonds, Switzerland Purpose or Objective The association of volumetric arc therapy (VMAT) and deep inspiration breath hold (DIBH) is increasingly used to reduce the dose to the heart during left-sided breast radiotherapy (RT). Using flattening-filter-free (FFF) beams allows a higher dose rate which can speed up the delivery time in DIBH. In this study, we evaluate the dosimetric influence by using FFF-VMAT beams compared to Flattening Filter (FF) VMAT. Material and Methods Treatment planning was performed with the TPS Pinnacle 14 (Philips) using FF and FFF beams (6MV) for 10 patients who received a local irradiation (L-RT) of the whole breast (50Gy) with a simultaneous integrated boost of 60Gy, and for 10 patients who received a loco-regional (LR) RT with target volumes consisting of the supraclavicular area, the upper axilla and the internal mammary chain. For an identical PTV coverage (at least 95% of PTV covered by 95% of the prescribed dose), the dosimetric comparison between FF and FFF VMAT plans was undertaken for organs at risk (OAR), such as heart,