ESTRO 37 Abstract book

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

Purpose or Objective 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?

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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.

Results The biggest schematic deviations were observed in bony structures over the whole dose range. The AAA overestimates on average over all dose and density regions compared to the MC algorithm. The evaluation shows exponential dose dependence towards higher deviations (~ 20 %) at lower doses and less deviation at higher dose levels (see figure 2) for all three density regions. DVH analysis shows directly, whether a deviation is located in the PTV or in an OAR. Furthermore, the DVH lines show, if there are higher deviations in air, soft or bony tissue for each structure.

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Outcomes •

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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 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

Conclusion In general, the concept of gamma evaluation should be reconsidered. Deviation levels can be defined depending on the dose level (low or high dose region).Different new evaluation scenarios are conceivable: 1) DVH analysis with automatic determination of predefined thresholds and classification of OARs as well as PTVs into rDDs. 2) Gamma analysis could be complemented by the three 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.