Top Line: Fractionated SRS is often used to reduce the risk of radiation necrosis (RN), but that risk isn’t zero. What dosimetric factors are associated with RN risk when treating with 3-fraction SRS?
The Study: In this retrospective study, 434 patients with 2518 brain metastases were treated with 3-fraction SRS. They were all treated on LINACs typically using a single isocenter even for multiple targets. The typical treatment planning approach was to use a 2mm PTV margin around the GTV. The PTV was typically prescribed 24 Gy in 3 fractions while the GTV received 27 Gy. The median number of targets treated per course was 3. RN was infrequent with just 17% of patients and 4.9% of lesions experiencing any grade RN. Grade 3+ RN was seen in 7.8% of patients and 2.3% of brain metastases. RN occurred steadily over time as the cumulative incidence of grade 3+ RN was 3.6% at 1 year and 7.6% at 2 years. In fact, the median time to developing RN was 11.4 months. Dose-volume predictors of RN were evaluated on a per-lesion basis using a structure comprising each PTV plus a 1 cm margin. In other words, the structure they used to evaluate dose and the risk of RN included the target and surrounding brain, not just the rim of tissue surrounding the target. All dose-volume measures were significantly higher in patients who developed RN, but the two that performed the best at predicting RN were brain V20 (any grade RN) and V23 (grade 3+ RN). A V20 <4cc was associated with a <5% risk of any RN and a V20 <20 cc was associated with <10% risk. A V23 <15cc was associated with a <5% risk of grade 3 RN. A key takeaway is that the risk of RN is driven by the volume of a target and thus the irradiated volume on a per-target basis. The presence of multiple targets (and thus the cumulative irradiated volume) was not associated with RN risk. A final important takeaway was that 59.5% of patients who developed grade 3 RN received post-op SRS to a surgical cavity. The paper includes plots and tables to estimate the risk of RN for a given V20 and V23.
TBL: The irradiated volumes receiving 20 and 23 Gy in 3 fractions are the strongest predictors of radiation necrosis risk for 3-fraction SRS. | Upadhyay, Int J Radiat Oncol Biol Phys 2023
The Study: In this retrospective study, 434 patients with 2518 brain metastases were treated with 3-fraction SRS. They were all treated on LINACs typically using a single isocenter even for multiple targets. The typical treatment planning approach was to use a 2mm PTV margin around the GTV. The PTV was typically prescribed 24 Gy in 3 fractions while the GTV received 27 Gy. The median number of targets treated per course was 3. RN was infrequent with just 17% of patients and 4.9% of lesions experiencing any grade RN. Grade 3+ RN was seen in 7.8% of patients and 2.3% of brain metastases. RN occurred steadily over time as the cumulative incidence of grade 3+ RN was 3.6% at 1 year and 7.6% at 2 years. In fact, the median time to developing RN was 11.4 months. Dose-volume predictors of RN were evaluated on a per-lesion basis using a structure comprising each PTV plus a 1 cm margin. In other words, the structure they used to evaluate dose and the risk of RN included the target and surrounding brain, not just the rim of tissue surrounding the target. All dose-volume measures were significantly higher in patients who developed RN, but the two that performed the best at predicting RN were brain V20 (any grade RN) and V23 (grade 3+ RN). A V20 <4cc was associated with a <5% risk of any RN and a V20 <20 cc was associated with <10% risk. A V23 <15cc was associated with a <5% risk of grade 3 RN. A key takeaway is that the risk of RN is driven by the volume of a target and thus the irradiated volume on a per-target basis. The presence of multiple targets (and thus the cumulative irradiated volume) was not associated with RN risk. A final important takeaway was that 59.5% of patients who developed grade 3 RN received post-op SRS to a surgical cavity. The paper includes plots and tables to estimate the risk of RN for a given V20 and V23.
TBL: The irradiated volumes receiving 20 and 23 Gy in 3 fractions are the strongest predictors of radiation necrosis risk for 3-fraction SRS. | Upadhyay, Int J Radiat Oncol Biol Phys 2023