Impact of clinical target volume margin reduction in glioblastoma patients treated with concurrent chemoradiation

Background. Glioblastoma (GBM) is widely treated using large radiotherapy margins, resulting in substantial irradiation of the surrounding cerebral structures. In this context, the question arises whether these margins could be safely reduced. In 2018, clinical target volume (CTV) expansion was reduced in our institution from 20 to 15 mm around the gross target volume (GTV) (ie, the contrast-enhancing tumor/cavity). We sought to retrospectively analyze the impact of this reduction. Methods. All adult patients with GBM treated between January 2015 and December 2020 with concurrent chemoradiation (60Gy/2Gy or 59.4Gy/1.8Gy) were analyzed. Patients treated using a 20 (CTV ₂₀, n = 57) or 15 mm (CTV ₁₅, n = 56) CTV margin were compared for target volumes, dose parameters to the surrounding organs, pattern of recurrence, and survival outcome. Results. Mean GTV was similar in both groups (ie, CTV ₂₀: 39.7cm ³; CTV ₁₅: 37.8cm ³; P = .71). Mean CTV and PTV were reduced from 238.9cm ³ to 176.7cm ³ (P = .001) and from 292.6cm ³ to 217.0cm ³ (P < .001), for CTV ₂₀ and CTV ₁₅, respectively. As a result, average brain mean dose (D _mean) was reduced from 25.2Gy to 21.0Gy (P = .002). Significantly lower values were also observed for left hippocampus D _mean, brainstem D _0.03cc, cochleas D _mean, and pituitary D _mean. Pattern of recurrence was similar, as well as patient outcome, ie, median progression-free survival was 8.0 and 7.0 months (P = .80), and median overall survival was 11.0 and 14.0 months (P = .61) for CTV ₂₀ and CTV ₁₅, respectively. Conclusions. In GBM patients treated with chemoradiation, reducing the CTV margin from 20 to 15 mm appears to be safe and offers the potential for less treatment toxicity.