ESTRO 2024 - Abstract Book

S951

Clinical - CNS

ESTRO 2024

In 1995 when we initiated vestibular schwannoma (VS) treatment with Stereotactic Radiosurgery (SRS), the recommended single fraction (1#) dose was 25Gy. However, due to high toxicity rates a schedule of 30Gy in six fractions (6#) over two weeks was developed. In 2015, after a literature review, we gradually transitioned to using 13Gy in 1# for smaller lesions whilst retaining 30Gy in 6# for larger lesions. Since 2011, we have used the Brainlab treatment planning system (TPS) iPlan. A 1mm symmetric margin is added to GTV to create the PTV and the same margin applied to OARs to create PRVs. The treatment was planned and delivered with 3 or 4 dynamic conformal arcs. When we changed SRS TPS from Radionics X-knife to iPlan, we commenced a prospective audit to monitor toxicity using CTC score (hearing, balance, trigeminal and facial nerves), and the change in size of the VS by recording maximum transverse linear measurement on annual MRI scans.

This analysis was conducted to investigate the change in practice over time, acute and long-term toxicity, and two year local control rate.

Material/Methods:

The database was analysed for patients treated between September 2011 to October 2022, and to assess change in practice over time we divided the patients into four sequential cohorts (C1-4) each with 71 patients in each. As many VS transiently enlarge following SRS we assessed local control in patients with a minimum of 24 months of followup.

Results:

A total of 284 patients with median age 62 years (range 20-88) were treated. 40 had had prior surgery (all received 6#). 182 (64%) patients received 30Gy in 6# (13 to 80% and 169 to 90%) and 102 (36%) 13Gy 1# to 80%,

Table 1 sets out the change in fractionation over time and shows that 6# schedule is now used for larger lesions.

TABLE 1

1 fraction

6 fractions

GTV

GTV

COHORT

N (%)

median min

max

N=

median min

max

1

10 (14%) 15 (21%) 36 (51%) 41 (58%)

0.85

0.24

2.31

61 (86%) 1.44

0.34

10.8

2

0.88

0.3

2.5

56 (79%) 1.95

0.31

7.92

3

1.1

0.3

2.81

35 (49%) 2.45

0.7

7.95

4

0.88

0.29

2.7

30 (42%) 3.07

0.47

7.66

ALL

102

0.92

0.24

2.81

182

2.27

0.31

10.8