Scalp Irradiation with 3D-Milled Bolus: Initial Dosimetric and Clinical Experience

Simple Summary Scalp irradiation for cutaneous malignancies presents complex challenges due to the high radiation dose, irregular surface anatomy, and radiosensitive organs at risk. Scalp convexity provides unique aspects to consider when attempting to deliver a homogenous dose with external beam radiation. Traditional bolus methods face several issues, including the risk of air gaps and placement difficulty due to skull shape. Here, we outline the steps involved with 3D-milled bolus creation, which has the potential to deliver a homogenous dose to the surface target with rapid dose falloff when treated with VMAT radiation. To our knowledge, this is the first study to report clinical outcomes using this approach. We showed homogenous dose target coverage and OAR doses lower than those reported in other studies utilizing alternative dose delivery and bolus techniques. We also showed that while acute skin toxicity was substantial, the treatment was well tolerated in later follow-up with only 5% late grade 3 toxicity.Abstract Background and purpose: A bolus is required when treating scalp lesions with photon radiation therapy. Traditional bolus materials face several issues, including air gaps and setup difficulty due to irregular, convex scalp geometry. A 3D-milled bolus is custom-formed to match individual patient anatomy, allowing improved dose coverage and homogeneity. Here, we describe the creation process of a 3D-milled bolus and report the outcomes for patients with scalp malignancies treated with Volumetric Modulated Arc Therapy (VMAT) utilizing a 3D-milled bolus. Materials and methods: Twenty-two patients treated from 2016 to 2022 using a 3D-milled bolus and VMAT were included. Histologies included squamous cell carcinoma (n = 14, 64%) and angiosarcoma (n = 8, 36%). A total of 7 (32%) patients were treated in the intact and 15 (68%) in the postoperative setting. The median prescription dose was 66.0 Gy (range: 60.0-69.96). Results: The target included the entire scalp for 8 (36%) patients; in the remaining 14 (64%), the median ratio of planning target volume to scalp volume was 35% (range: 25-90%). The median dose homogeneity index was 1.07 (range: 1.03-1.15). Six (27%) patients experienced acute grade 3 dermatitis and one (5%) patient experienced late grade 3 skin ulceration. With a median follow-up of 21.4 months (range: 4.0-75.4), the 18-month rates of locoregional control and overall survival were 75% and 79%, respectively. Conclusions: To our knowledge, this is the first study to report the clinical outcomes for patients with scalp malignancies treated with the combination of VMAT and a 3D-milled bolus. This technique resulted in favorable clinical outcomes and an acceptable toxicity profile in comparison with historic controls and warrants further investigation in a larger prospective study.