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density classifications and the criterion could be adapted respectively. 3) Dose distributions of each patient could be compared just as rDD as shown in figure 2 with predefined thresholds for each density curve. EP-2358 Exploration of flattening filter free vs flattening filter beam for Breast irradiation N. Gajperia 1 1 Royal Free Hospital, Radiotherapy, London, United Kingdom Research Questions The primary aim of this study was to investigate the suitability of FFF beam compared to FF beam for left sided breast/chest wall patients. • Does the use of a FFF beam provide a better dose distribution to the breast compared to a conventional 3D conformal and IMRT FF beam? Purpose or Objective

assessing CI. The 3DCRT plans were deemed statistically (p<0.05) quicker to deliver when timed then both IMRT plans produced. Further consideration needs to be given to the increased number of MU to deliver FFF beam. The author feels further research should be carried out within department before considering the implementation of beam energy for DIBH patients. Future studies should be carried out to determine the long term radiobiological effect of a high dose rate and greater MU to deliver the same prescription as the present study did not seek to address this. EP-2359 Comparing lung dose parametres between rigid and non-rigid registration in advance stage lung cancer X. SUSAINATHAN SELVANATHAN 1 , J.Y.S. CHER 1 , T.Y. CHAN 1 1 National Cancer Institute- Singapore, RADIATION ONCOLOGY, SINGAPORE, Singapore Purpose or Objective In re-irradiation, the cumulative lung dose from both the previous and current radiation plans are of clinical significance. Before the advent of non-rigid registration (also known as deformable registration), radiation plans were fused using rigid registration. This may affect the accuracy of lung dose calculations. Differences in lung doses are expected when using non-rigid registration. Hence, the objective of this study is to evaluate whether there is a difference in lung doses with rigid and non-rigid registration using 5 parametres:D minimum (D min ),D maximum (D max ),D mean ,Volume receiving 20Gy(V 20Gy ) and Volume receiving 15Gy(V 15Gy ). Material and Methods 12 consecutive advance stage lung cancer patients receiving re-irradiation were selected. Eclipse™ Treatment Planning System was utilized to perform rigid registration. Velocity™ was utilized to perform non-rigid registration. In both registration techniques, lungs dose parametres values were extracted from the dose volume histogram (DVH). The paired-sample t test was performed. Results Results are significant if p<0.05.The results are right(RT) lung D min ( Rigid vs Non-Rigid: 71.2cGy vs 69.4cGy, p=0.30), RT lung D max ( Rigid vs Non-Rigid: 5668.6cGy vs 5738.6cGy, p=0.06), RT lung D mean ( Rigid vs Non-Rigid: 1811.9cGy vs 1803.1cGy, p=0.80), RT lung V 20Gy ( Rigid vs Non-Rigid: 40.2% vs 39.0%, p=0.04), RT lung V 15Gy ( Rigid vs Non-Rigid: 46.8% vs 45.7%, p=0.08), left(LT) lung D min ( Rigid vs Non-Rigid: 36.8cGy vs 38.4cGy, p=0.27), LT lung D max ( Rigid vs Non-Rigid: 4986.2cGy vs 4890.9cGy, p=0.03), LT lung D mean ( Rigid vs Non-Rigid: 863.9cGy vs 1005.8cGy, p=0.42), LT lung V 20Gy ( Rigid vs Non-Rigid: 19.1% vs 18.0%, p=0.01), LT lung V 15Gy ( Rigid vs Non- Rigid: 24.1% vs 22.8%, p=0.01), Bilateral lungs D min ( Rigid vs Non-Rigid: 29.5cGy vs 34.8cGy, p=0.11), Bilateral lungs D max ( Rigid vs Non-Rigid: 5818.5cGy vs 5841.6cGy, p=0.04), Bilateral lungs D mean ( Rigid vs Non-Rigid: 1192.3cGy vs 1290.0cGy, p=0.03), Bilateral lungs V 20Gy ( Rigid vs Non-Rigid: 25.3% vs 24.4%, p=0.03), Bilateral lungs V 15Gy (Rigid vs Non-Rigid: 31.0% vs 30.0%, p=0.03). Conclusion To date, studies have demonstrated that non-rigid registration allows greater accuracy in estimating cumulative lung doses as changes in lung volumes, body shape and treatment positions of the patient are better accounted with non-rigid registration. This study

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To what extent does a FFF beam impact cardiac and lung dose volume histograms (DVH) compared to FF beam? Does FFF beam reduce the dose to the OAR thus reducing the need for DIBH for this group of patients? What are the average treatment times required to deliver FFF vs FF planned treatments? Produce data that could be used to determine the efficacy of FFF treatments for left sided whole breast/chest wall treatments. Determine speed of treatment delivery with FFF beam, which could aide patients during a DIBH gated treatment. To use the outcome of this study to consider implementing the use of the FFF beam for left breast/chest wall treatments.

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Material and Methods This study initially selected 20 patients with a free breathing CT data set but who were ultimately treated with deep inhalation breath hold (DIBH). The 3D conformal (3DCRT) radiotherapy plan will be retrospectively re-planned with FinF IMRT with a FF and FFF beam. Only 18 patients were ultimately analysed; 2 patients were excluded due to the changes needed in the treatment borders between the original and IMRT plans as per clinician’s judgment on breast tissue coverage. All plans were in line with the ICRU 50/62 guidelines. The plan qualities were assessed in relation to PTV coverage, organs at risk (OAR) and treatment delivery times. Results The overall plan quality between both IMRT plans was comparable with a mean conformity index (CI) of 1.14 and 1.10 for the FF and FFF beam plans respectively. The IMRT plans both produced a more superior coverage of breast tissue compared to 3DCRT plans as determined by the clinician. Although, there was no statistically significant difference between the CI and HI when comparing the FFF IMRT vs 3DCRT plans. The FFF IMRT plan required significantly (p= 0.0002) more monitor unit than the FF IMRT plan. The 3DCRT plans were deemed to be statically (p <0.05) quicker at delivering treatment than both IMRT FF and FFF beams. Conclusion The FFF IMRT plans did produce a statically (p = 0.012) better plan conformity compared to the FF IMRT plan. Although when compared to the 3DCRT plans no statistical difference was found with plan quality when