3160 - Dosimetric Feasibility of Hippocampal-Avoidance Whole Brain Radiotherapy with Simultaneous Integrated Boost In Patients with = 5 Brain Metastases

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 20

POSTER

Presenter(s)

Andrea Vucetic, MD, MS, BS - Schulich School of Medicine and Dentistry, Western University, London, ON

A. Vucetic¹, K. Nishimura², B. Millman¹, A. Celinski¹, G. Rodrigues¹, G. S. Bauman¹, H. Fakir^1,3, and J. Laba¹; ¹Division of Radiation Oncology, London Health Sciences Centre, London, ON, Canada, ²Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada, ³Department of Medical Biophysics, Western University, London, ON, Canada

Purpose/Objective(s):

Patients with multiple (= 5) intact brain metastases (BMs) have traditionally been treated with whole brain radiotherapy (WBRT). To reduce neurocognitive sequelae, strategies have included omitting WBRT in favour of lesion-directed therapy or incorporating hippocampal sparing techniques. This study evaluates the dosimetric feasibility of WBRT with hippocampal avoidance (HA) and simultaneous integrated boost (SIB) to individual metastases using volumetric modulated arc therapy (VMAT) in patients with = 5 lesions, with the goal of maximizing intracranial control while mitigating neurocognitive toxicity.

Materials/Methods:

Ten patients with multiple intact BMs (5-9 lesions, n = 4; = 10 lesions, n = 6) were retrospectively selected for treatment planning. All cases were planned using CT simulation and MRI fusion for target and hippocampal delineation. The brain and brainstem were contoured as CTV1, with a 3 mm isotropic expansion to generate PTV1. A dose of 30 Gy in 10 fractions was prescribed. PTV1_eval was defined as PTV1 excluding the hippocampal avoidance region. Individual BMs were contoured on contrast-enhanced T1-weighted MRI as GTV_boost; PTV_boost was defined without additional expansion and prescribed an SIB dose of 50 Gy in 10 fractions. Bilateral hippocampi were contoured according to RTOG 0933 guidelines, with a 5 mm HA planning risk volume and protocol-defined dose constraints. Standard organ-at-risk constraints were applied. Planning objectives for PTV1_eval were V30 = 90%, D98 = 25 Gy, and D2% = 40 Gy; and for PTV_boost V50 = 95% and Dmax < 107%.

Results:

The mean number of BMs was 9 (range 6-13). The mean volume of GTV_boost was 0.5 cc (range 0.01-6.9 cc). The mean dose to 99% of GTV_boost (D99) was 49 Gy (range 42.1-50.3 Gy). The mean GTV_boost D_mean was 51.2 Gy (range 49.3-52.2 Gy). The mean Brain-GTV_boost D_meanwas 31.7 Gy (range 31.2-32.2 Gy). The mean dose to 100% of the hippocampi (D100) was 9 Gy (range 8.6-9.5 Gy), while the mean D_max to the hippocampi was 17.5 Gy (range 14.3-16.8 Gy).

Conclusion:

HA-WBRT with SIB using VMAT is feasible for patients with = 5 intact BMs. GTV coverage had to be compromised for lesions within or near the brainstem. Treatment of numerous small targets can present technical challenges in achieving optimal conformity and coverage, necessitating integration of whole brain and stereotactic planning strategies and additional non-coplanar beams. Overall, HA-WBRT may be a reasonable option for carefully selected patients with multiple BMs who are not candidates for stereotactic radiation.