Central Nervous System
PV QA 2 - Poster Viewing Q&A 2
Purpose/Objective(s): For large vestibular schwannomas (VSs) with brainstem compression, surgery is often performed prior to fractionated stereotactic radiotherapy (fSRT) to safely debulk tumor volume and decrease potential radiation toxicity on the brainstem and cranial nerves. We have previously presented data from our institution showing excellent tumor control and toxicity profiles for treatment of large VSs with fSRT. The goal of this retrospective study was to further evaluate the dosimetric advantage of debulking surgery prior to fSRT for large VSs.
Materials/Methods: From 60 VSs patients treated with fSRT to 25 Gy in 5 fractions between 2009 and 2016, we identified 18 of whom had tumors with Koos stage 3-4 at the time of radiation. 8 patients had surgery prior to fSRT. In order to analyze tumor volume and brainstem dosimetry before and after surgery, we imported pre-surgical MRIs into our treatment planning system and generated fSRT plans for comparison. Clinical outcomes (local control rate, survival, and both cranial nerve and brainstem toxicity) were analyzed based on dosimetric parameters.
Results: For all 18 large VS patients, median follow up was 15 months (range 2 – 49 months), median PTV was 4.9 cc (range 1.4 – 22.6 cc), median brainstem Dmax was 26.1 Gy (range 21.7 – 30.4 Gy), median V23 was 0.175 cc (range 0 – 0.62 cc), and median D0.5cc was 20.5 Gy (range 9 – 23.8 Gy). Local control was 94.4%. One patient had cranial nerve VII damage as a result of surgery, but the incidence of cranial nerve and brainstem damage from fSRT were both 0%. For the 8 patients who had surgery prior to fSRT, the average tumor volume was reduced from a volume of 21.9 cc (range 4.4 – 41.4 cc) to a volume of 9.6 cc (range 3.1 – 22.6 cc) (p=0.014). Brainstem Dmax was 26.5 Gy prior to surgery vs. 26.6 Gy after surgery (p=0.98). Brainstem D0.5cc was lower in post-surgical plans (23.4 vs. 19.5 Gy, p=0.0156), and V23 was also lower in post-surgical plans (1.33 vs. 0.21 cc, p=0.0234). Even though there was no brainstem toxicity in this study, the pre-surgical plans of 3 patients did not meet the TG-101 brainstem dose constraint of V23 < 0.5 cc. However, all post-surgical plans met this brainstem criteria. The relative seriality normal tissue complication probability (NTCP) model revealed that the NTCPs for pre-surgical fSRT ranged from 0.36% to 2.70%, while the NTCPs for post-surgical fSRT ranged from 0.00% to 0.49% (p < 0.05). For pre-surgical fSRT, 1 patient had an NTCP > 1% at 2.70%, while the remaining 7 patients had NTCPs < 1%.
Conclusion: Our results show a statistically significant decrease of target volume and improvements in brainstem D0.5cc and V23 after debulking surgery. NTCP analysis revealed a very low probability of brainstem toxicity with either debulking followed by fSRT or fSRT alone to non-debulked tumors; however, there was a statistically significant difference favoring debulking surgery prior to fSRT.
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