ESTRO 36 Abstract Book

S844 ESTRO 36 _______________________________________________________________________________________________

D98%, D2%, CI, HI, MU, number of segments and estimated delivery time. For the principal organs at risk, such as the optical nerve, contralateral eye and contralateral lens, the maximum and mean dose were reported. Results Table I summarizes the results for the PTV.

Inverse planning improves PTV coverage in comparison with forward planning. Also the near-minimum dose D98% is higher for IMRT and VMAT than 3DCRT. In addition the near-maximum dose D2% is lower for IMRT and VMAT. In terms of conformity, the three plans have similar results; meanwhile the 3DCRT technique has a worst homogeneity index. VMAT treatment plans reduce the monitor units and number of segments in comparison with the IMRT plans. For the OAR, the maximum dose (mean value) to the contralateral lens was 4.30Gy for the 3DCRT plans; 3.33Gy for IMRT and 2.86Gy for VMAT plans. For the left optical nerve the maximum dose was 9.39Gy for 3DCRT; 7.31Gy for IMRT and 6.96Gy for VMAT, and for the left eye was 7.20Gy for 3DCRT; 6.37Gy for IMRT and 5.16Gy for VMAT. Dose distribution (axial view) for one representative patient is shown in Fig I.

Conventional uniform-dose IMRT plans for three patients did not pass the clinical acceptance criteria due to the patient geometry, particularly due to violation of rectal dose-volume constraints. In all cases, biologically- optimized IMRT plans were superior to other treatment modalities on the basis of rectum and bladder NTCP. All comparison results are summarised in Table 1. In Figure 1, the DVH curves for one patient demonstrate the ability of biologically optimized IMRT planning to minimize dose to healthy tissues while maintaining a high TCP. Conclusion In conclusion, biological optimization of prostate IMRT using a TCP model has the potential to improve the clinical outcome by maximizing TCP and simultaneously minimizing dose to normal structures. This has been demonstrated in the context of population-based tumour cell density distributions. Future work will include the implementation of voxel-level patient-specific data from multiparametric imaging which will allow a personalized approach to IMRT planning with a dose distribution tailored to the specific patient/tumour biology. EP-1567 Inverse planning versus forward planning for orbital lymphoma E.M. Ambroa Rey 1 , A. Ramirez Muñoz 1 , D. Navarro Jiménez 1 , R. Gómez Pardos 1 , M. Colomer Truyols 1 1 Consorci Sanitari de Terrassa, Medical Physics Unit- Radiation Oncology Department, Terrassa, Spain Purpose or Objective Non-Hodgkin’s lymphoma (NHL) of the orbit is an unusual presentation of extranodal NHL and represents 8% of all extranodal NHL and about 1% of all NHL. The purpose of this study is to present a dosimetric analysis between inverse and forward plans for mucosa associated lymphoid tissue (MALT) lymphoma. Material and Methods Three patients diagnosed with MALT lymphoma were analyzed. Three types of plans (3DCRT, IMRT and VMAT) were created for each patient. The whole orbital volume was included in the CTV. The organs at risk (OAR), right and left eye, right and left lens, right and left optical nerve, were contoured. Prescription was 27 Gy at 1.8 Gy per fraction. The 3DCRT plans were elaborated using the treatment planning system (TPS) XiO version 5.1. Five isocentric beams were used. Inverse plans calculations (IMRT and VMAT) were carried out using the Monaco TPS version 5.10. IMRT plans were created based on a five-field arrangement and dynamic MLC delivery method. For VMAT plans, two partial arcs were used. For every plan, the following data for the PTV was recorded: V95%, D50%,

Conclusion According to the results, we can conclude that inverse planning is feasible for MALT orbital lymphoma treatment, providing excellent dose coverage of target volume and reduces dose to the organs at risk. Furthermore, we can establish that in terms of coverage and sparing OAR, IMRT and VMAT techniques are very similar, but VMAT technique achieves a faster delivery treatment, resulting in a lower probability of patient movement during the treatment. Despite the fact that the number of examined patients is low, we are sure that inverse planning is an improvement for every patient with this type of malignance, but further investigations, with more cases, between IMRT and VMAT should be done. EP-1568 Benefit of a breath hold radiotherapy technique for breast and internal mammary nodes irradiation F. Jouyaux 1 , A.L. Blin 1 , I. Lecouillard 1 , M. Benchalal 1 , C. Lafond 1,2,3 1 Centre Eugène Marquis, Radiotherapy, Rennes CEDEX, France 2 Rennes 1 University, LTSI, Rennes, France 3 INSERM, U1099, Rennes, France Purpose or Objective Loco-regional radiotherapy of left-sided breast cancer represents a treatment planning challenge when the internal mammary lymph nodes (IMN) are included in the