ESTRO 37 Abstract book

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ESTRO 37

Table 1. CTV-PTV Margins ( 2.5∑ + 0.7σ) (mm) X(L/R) Y(A/P) Z(S/I) CTV-PTV (BL) 5.1 6.1 6.4 CTV-PTV (PC) 7.1 7 5.9 CTV-PTV (FM) 4.9 7.2 5.6 CTV-PTV (PG) 5.5 7.5 5.6

Conclusion Preliminary results from this study demonstrate some evidence to support the use of PCs as an alternative to FMs for prostate IGRT. Future analysis will include location of PC according to the PIRADs schema and patient feedback in relation to FM implantation. PV-0366 Stereotactic ablative RT with focal boosting in prostate cancer: feasibility on a 1.5 T MRI linac E.N. De Groot - van Breugel 1 , J. Van der Voort van Zijp 1 , B.W. Raaymakers 1 , J. Hes 1 , L.G.W. Kerkmeijer 1 , J.C.J. De Boer 1 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective Recently we have treated intermediate and high-risk prostate cancer patients in a clinical multicentre phase II trial for hypofractionated prostate radiotherapy of 5 × 7 Gy, with local GTV boosts uptil 5 × 10 Gy (hypo-FLAME trial, NCT02853110). Although the targeting accuracy based on kV fiducial marker imaging was high, we expect that on a 1.5 T MRI linac we can fully account for rotations, intrafraction motion and deformations, thus further reducing PTV margins. We investigated if the current clinical MRI linac (Unity, Elekta AB) can achieve dose distributions of a quality similar to conventional state-of-the-art prostate treatments. Material and Methods The UMC Utrecht hypo-FLAME patients were treated on Agility linacs (5 mm leaf width) using 10 MV VMAT plans created in the Monaco TPS (Elekta AB). The GTVs were contoured on multi-parametric MRI (T2/DCE/DWI). The CTV-PTV margin was 4 mm. The most important constraints are summarized in Table I. The larger leaf width (7 mm) of the MRI linac, the fixed collimator angle, its IMRT delivery (beam energy 7 MV) and the strong magnetic field (in particular electron-return-effect near gas pockets) could potentially deteriorate plan quality. The clinically commissioned Monaco TPS for the MRI linac (v5.19) was used to create 7 beam IMRT plans for 10 hypo-FLAME patients using the hypo-FLAME constraint list. We compared these MRI linac dose distributions to those of the corresponding clinically delivered conventional plans.

Results In the conventional treatment plans as well as the MRI linac plans, all planning criteria were met, with a small (clinical acceptable) exception in the rectum V35Gy in one patient. MRI linac IMRT plans consisted on average of 118 segments, delivered in 11.5 minutes. Both visual inspection (Fig. I) as well as DVH analysis revealed no significant differences between conventional and MRI linac treatment plan quality (Fig. I). Regarding the GTV boost region, the V40Gy was 99.1% ± 1.0% (1 STD) for conventional treatment versus 99.3% ± 0.9% for the MRI linac treatment. Corresponding numbers for the GTV mean dose were 45.6 Gy ± 2.1 Gy versus 45.0 Gy ± 1.9 Gy. For the PTV coverage V95% was 99.1% ± 0.5% versus 99.7% ± 0.3%. Similarly, organs at risk (OAR) doses were almost identical. For instance, the conventional bladder V37Gy was 2.2 cc ± 1.9 cc versus 1.6 cc ± 2.1 cc for the MRI linac. The corresponding values for rectum V35Gy were 1.1 cc ± 0.8 cc versus 1.0 cc ± 0.6 cc. The conventional rectum Dmax mean dose was 37.6 Gy ± 1.1 Gy versus 37.1 Gy ± 1.1 Gy for the MRI linac. No discernible negative effects due to gas pockets were found in rectal dose nor target coverage.

Conclusion Planning of stereotactic ablative boosting in prostate cancer on a high field MRI linac is safely feasible, without deterioration of plan quality compared to conventional treatments. In addition, the continuous soft tissue imaging and online plan generation on the MRI linac will allow for PTV margin reduction, allowing for increased sparing of OAR. PV-0367 Relationship between labyrinth dose and dizziness in vestibular schwannoma patients treated with SRS A. Bambery 1 , A. Cameron 2 1 University of the West of England, Department of Allied Health Professions- Faculty of Health and Applied Sciences, Bristol, United Kingdom