Rx: The patient received LINAC-based, fractionated stereotactic radiotherapy to a single post-operative brain metastasis cavity.

Simulation/Planning:

The patient was CT-simulated using 1mm image slices with immobilization using a thermoplastic mask in supine position.
Volumetric MRIs with 1-2 mm slice thickness were fused with the planning image set using rigid registration, utilizing the T1 weighted post gadolinium and T2 weighted sequences.
Treatment was delivered by two co-planar arcs and one non-planar arc at a single isocentre with a 6MV FFF photon beam using VMAT technique on a hexapod couch with a 1mm tolerance for positional error.
Daily CBCT was used for treatment setup verification.

Case contributed by Queen's University

GTV/HTV:

Gross residual disease if present.
Hypoxic tumor volume (HTV) if present.
The enhancing cavity was delineated using the T1 post gadolinium sequence as the GTV.
Finally, a 2mm isotropic expansion was applied to the GTV or CTV for intact and cavities, respectively, to generate a planning target volume (PTV).

CTV (per Soliman IJROBP 2018, Mahajan Lancet Oncol 2017, Minniti Radiation Oncology 2021)

Entire contrast-enhancing surgical cavity on T1-weighted post-gad MRI.
A subsequent 2-3mm expansion is added to the GTV to incorporate any microscopic disease as a clinical target volume.
Do not include edema or bone.
Include entire surgical tract.
Bone flap coverage:
- If tumor was in contact with dura, include 5-10 mm along bone flap beyond the initial contact.
- If not in contact with dura, include 1-5 mm along the bone flap.
- If tumor contacted venous sinus preoperatively, include 1-5 mm along the sinus
PTV: 2 mm* uniform expansion on CTV
- *Dependent on institution, technology, and setup

Post-op FSRT Prescription Alliance A071801 trial
PTV Volume	Dose / Fractionation
<30cc	27 Gy / 3 fractions
≥ 30 cc to <5cm	30 Gy / 5 fractions
Plan Specification: The dose should be prescribed to the highest isodose line encompassing the PTV (surgical cavity plus 2mm), which can range from 45-95% of the maximum dose Dose Uniformity, Conformity, and Dose Spillage: Conformity Index CI = V100%/PTV should be between 1-2, for lesions less than 5mm up to 3.0 is acceptable

Structure

3 fractions

5 fractions

Brainstem

V18 Gy ≤ 0.5cc;

Maximum point dose ≤ 23.1 Gy

V23Gy ≤

0.5 cc;

Maximum point dose < 28 Gy

Optic Pathway

V13.8 Gy < 0.2cc;

Maximum point dose < 17.4 Gy

V20 Gy < 0.2cc;

Maximum point dose < 23 Gy