Does size matter? Investigating the optimal planning target volume margin for postoperative stereotactic radiosurgery to resected brain metastases

Abstract

OBJECTIVE The optimal margin size in postoperative stereotactic radiosurgery (SRS) for brain metastases is unknown. Herein, the authors investigated the effect of SRS planning target volume (PTV) margin on local recurrence and symptomatic radiation necrosis postoperatively. METHODS Records of patients who received postoperative LINAC-based SRS for brain metastases between 2006 and 2016 were reviewed and stratified based on PTV margin size (1.0 or > 1.0 mm). Patients were treated using frameless and framed SRS techniques, and both single-fraction and hypofractionated dosing were used based on lesion size. Kaplan-Meier and cumulative incidence models were used to estimate survival and intracranial outcomes, respectively. Multivariate analyses were also performed. RESULTS A total of 133 patients with 139 cavities were identified; 36 patients (27.1%) and 35 lesions (25.2%) were in the 1.0-mm group, and 97 patients (72.9%) and 104 lesions (74.8%) were in the > 1.0-mm group. Patient characteristics were balanced, except the 1.0-mm cohort had a better Eastern Cooperative Group Performance Status (grade 0: 36.1% vs 19.6%), higher mean number of brain metastases (1.75 vs 1.31), lower prescription isodose line (80% vs 95%), and lower median single fraction-equivalent dose (15.0 vs 17.5 Gy) (all p < 0.05). The median survival and follow-up for all patients were 15.6 months and 17.7 months, respectively. No significant difference in local recurrence was noted between the cohorts. An increased 1-year rate of symptomatic radionecrosis was seen in the larger margin group (20.9% vs 6.0%, p = 0.028). On multivariate analyses, margin size > 1.0 mm was associated with an increased risk for symptomatic radionecrosis (HR 3.07, 95% CI 1.13-8.34; p = 0.028), while multifraction SRS emerged as a protective factor for symptomatic radionecrosis (HR 0.13, 95% CI 0.02-0.76; p = 0.023). CONCLUSIONS Expanding the PTV margin beyond 1.0 mm is not associated with improved local recurrence but appears to increase the risk of symptomatic radionecrosis after postoperative SRS.

Effect of setup error in the single‐isocenter technique on stereotactic radiosurgery for multiple brain metastases

“…28,29 Other researchers have reported that a 0.1 cm PTV was appropriate in SRS. 21,30 Thus, for patients with brain metastases, it would be advisable to apply SIV-MAT so that a 0.1-cm PTV margin ensures the dose coverage within the tolerance value at a given distance from the isocenter to the target. We observed that the maximum distance at which the clinical PTV margin satisfied each tolerance value for each GTV diameter decreased with a decreasing diameter and an increasing 6DoF setup error (Table 6).…”

“…In SRS, a 0.1-cm PTV margin is often used in clinical settings to concentrate the radiation on the GTV and to minimize the doses to surrounding normal tissue. 21,22 The effect of the rotational error becomes more significant as the distance between the target and the isocenter increases. It has been speculated that the clinical PTV margin of 0.1 cm cannot be secured for dose coverage in SIVMAT.…”

Effects of Multileaf Collimator Design and Function When Using an Optimized Dynamic Conformal Arc Approach for Stereotactic Radiosurgery Treatment of Multiple Brain Metastases With a Single Isocenter: A Planning Study

“…Keeping residual setup error minimized with the utilization of ExacTrac at non-coplanar couch positions, according to Table 3 tolerances, allows for a smaller margin selection. This is particularly important because recent publications have demonstrated that margins on the order of 3.0 mm increase the potential risk of radionecrosis, and expanding the margin beyond 1.0 mm is not associated with improved local control [18][19]. Treatment of postoperative cavities with fSRS and a minimum of 2.0 mm margin is shown to be effective [20].…”

Post-operative radiation therapy to the surgical cavity with standard fractionation in patients with brain metastases

“…In this setting, the neurocognitive effects of radionecrosis may deprive patients of quality of life to an extent that rivals tumor progression. A recent retrospective study found after cavity SRS a 1-year rate of symptomatic radionecrosis of 6.0% for patients treated with a margin of <1.0 mm and up to 20.9% for patients with a margin of >1.0 mm 23 . In our series, there were no observed incidents of late radionecrosis, and this is likely related to the lower biological effective dose to normal tissues.…”