Acute Skin Toxicity In Patients Treated With Sbrt

The incidence of acute skin toxicity (AST) seen in patients treated with SBRT is reported from 0-38% in the literature. This is an analysis of patients treated with SBRT at a single institution to assess various factors that could be related to the incidence of toxicity to predict strategies in future planning to reduce the incidence of AST. Consecutive patients treated for extracranial sites of disease with > 3 months follow-up from 1/2016-11/2018 were included in the analysis . 116 patients were treated on the a linear accelerator with either VMAT (3-6 arcs) or 9-10 noncoplanar beams and 115 patients on the SRS with multiple noncoplanar beams. Immobilization devices used were included in body contour for dose calculation. Skin contour was defined at 0.5 cm from the external contour. Toxicity grading was performed per the NCI/CTCAE criteria. Data was collected for each patient regarding site of treatment, prescribed dose, history of previous radiation to same site, closest PTV distance from the skin, number of beams/arcs, immobilization device used, thickness of the immobilization device, beam energy and PTV volume. Sites treated were- Abdomen- 19, Liver - 44, Lung- 101, Pelvis- 26, Head and neck-8, Spine-29, Breast-4. One patient had Grade 1 AST who was treated for liver and one patient had Grade 2 AST treated for lung as primary site. Total incidence was 2/231(0.9%) patients developed Grade 2 acute skin toxicity. One patient had toxicity on the Right lower back and the second had toxicity on left lower back. Both patients were treated on a linear accelerator. There was no skin toxicity noted in patients treated on a SRS. Both patients were in an immobilization device, the distance from PTV to skin<1 cm. Both patients were treated with 3 VMAT arc plans. One patient was re-irradiated to the same site after 60 Gy to the lesion and had skin reaction with previous treatment and subsequent skin thickening. Both patients were planned with 10MV-FFF. One patient was treated with 45 Gy in 5 fractions and second 35 Gy in 5 fractions. TG 101 recommended dose constraints were met on both patients on pretreatment planning. AST was noted only on the lower chest wall posteriorly with skin to PTV distance<1 cm, on Linac with 3 partial VMAT arcs. The location of AST is not reported in literature however, all published images are from similar locations of tumor. The toxicity cannot be predicted by the current TPS which may be secondary to inaccuracy of surface dose calculation. We recommend in vivo dosimetry with first fraction for posterior lower chest /liver tumors when PTV to skin distance <1 cm, in addition to other strategies (like increasing the number of arcs to spread the skin dose, moving isocenter away from skin, increasing arc lengths and increasing thickness of immobilization device to reduce the bolus effect) which will allow to scale skin dose down on subsequent fractions. Future studies are needed for tumor sites in this specific anatomic location to reduce incidence of skin toxicity.