Central Nervous System
PV QA 2 - Poster Viewing Q&A 2
Purpose/Objective(s): Recently, some studies have reported an association between hippocampal dose–volume parameters and memory decline. However, the parameters that are easily achieved and the factors that are obstacles for accomplishing the parameters remain unclear. Therefore, we retrospectively evaluated the feasibility of the hippocampal dose–volume parameters that have been associated with memory decline for volumetric modulated arc therapy using simultaneous integrated boost (SIB-VMAT).
Materials/Methods: We analyzed 18 patients who underwent SIB-VMAT with a dose of 60 Gy in 30 fractions for supratentorial tumors from June 2016 to January 2018 (high-grade gliomas, 15 patients; atypical meningiomas, 3 patients). Clinical target volume (CTV) was defined as follows: CTV1, an enhanced lesion plus 0.5–1.5 cm or an edematous lesion; CTV2, CTV1 plus 0.5 cm or an edematous lesion plus 0.5–1.5 cm. As a planning target volume (PTV) margin, 0.3–0.5 cm was added to each CTV. Doses of 60 Gy for the PTV1 and 48–51 Gy for the PTV2 were delivered in 30 fractions. The hippocampal dose constraint at our institution was as low as reasonably achievable. As the hippocampal dose–volume parameters have been associated with memory decline, based on previous studies using biologically equivalent doses in 2 Gy fractions, we calculated parameters of the bilateral hippocampi with 20% risk of memory decline for 30 fractions as follows: 1) D50 of 29.6 Gy, 2) D40 of 13.1 Gy, and 3) V55 of 5.0%. We retrospectively evaluated which parameter achieved the best results and used Fisher’s exact test to identify unfavorable clinical factors for accomplishing the 3 parameters.
Results: Median D50, D40, and V55 were 18.0 Gy (range, 3.3–62.0 Gy), 24.6 Gy (range, 3.6–62.5 Gy), and 21.3% (range, 0.0–76.6%), respectively. D50 of 29.6 Gy, D40 of 13.1 Gy, and V55 of 5.0% were achieved in 67%, 17%, and 33% of patients, respectively. We accomplished all parameters in 3 patients (100%) with atypical meningioma, while no parameters were achieved in 5 of 6 patients (83%) with PTV2 ≥ 500 cc. For D50 of 29.6 Gy, PTV2 ≥ 500 cc (p = 0.004) and tumor located in the temporal lobe, corpus callosum, or basal ganglia (p = 0.009) were significant unfavorable factors. For D40 of 13.1 Gy, non-meningioma (p = 0.001) and tumor spread to the subventricular zone (p = 0.025) were significantly unfavorable. Non-meningioma was a significant unfavorable factor for V55 of 5.0% (p = 0.025).
Conclusion: In the dose–volume parameters of the bilateral hippocampi with 20% risk of memory decline, D50 of 29.6 Gy was most achievable in SIB-VMAT with a dose of 60 Gy in 30 fractions for supratentorial tumors. However, even D50 of 29.6 Gy may be difficult to be accomplished in patients with PTV2 ≥ 500 cc or tumors located in the temporal lobe, corpus callosum, or basal ganglia.
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