Abstract Book

S187

ESTRO 37

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 2 Bristol Haematology and Oncology Centre, Radiotherapy, Bristol, United Kingdom Purpose or Objective Dizziness is a major factor affecting quality of life in patients with vestibular schwannoma (VS) post stereotactic radiosurgery (SRS). The aim of this study was to investigate if there is any indication that dizziness relates to labyrinth dosimetry in our patients treated with gamma knife SRS in order to guide the direction of local research into reducing the impact of dizziness symptoms in this patient group. Material and Methods The patient group consisted of consecutive patients treated at the Bristol Gamma Knife Centre October 2013- March 2016 with SRS for VS who had completed a Dizziness Handicap Inventory (DHI) at a minimum of treatment day and 3 months post SRS. Labyrinth and labyrinth vestibule were retrospectively contoured on all treatment plans and length of vestibular nerve treated with treatment dose measured. Dosimetric data collected from this was related to change in DHI scores utilizing Pearson correlation for statistical analysis. Results 114 patients were treated with SRS for VS in the period of whom 86 patients had intact labyrinth and DHI at 0 and 3 months, 69 at 1 year and 38 at 2 years. The VS treated was mean 1.8cc [0.07-8.9], received a mean 12.3Gy to 50.1% isodose SRS with the Perfexion/Icon Gamma Knife. Coverage mean 0.99, gradient index mean 2.83 and Paddick Conformity Index mean 0.82. The mean vestibule dose was 5Gy [1.3-8.7Gy] and maximum to 1mm 3 mean 7.3Gy [1.5-16.1Gy]; labyrinth mean 3.3Gy [1-6.2Gy] and maximum to 1mm 3 mean 5.9Gy [1.8-9.5Gy]; and length of nerve receiving treatment dose mean 18.5mm (6.9- 28.2mm). There was no statistically significant correlation between any of the dose measurement and changes in DHI at 3, 12 or 24 months post SRS.

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.

In addition, DHI score changes for all patients were mapped across the 4 measurement periods. Variability in results was considerable, and showed no discernable trend.