Feasibility of 3-Field IMPT for Large-Volume Skin Cancer Using Custom Bolus and LET-Weighted Dose Assessment
Abstract
Purpose
To evaluate the feasibility of treating a large superficial skin (scalp) cancer target with a 3-field static IMPT technique and to assess the potential impact of elevated LET in high-dose regions.
Methods
A retrospective replanning study was performed on a previously treated helical tomotherapy case (55 Gy in 20 fractions). A new 3-field IMPT robustly optimized plan was generated to cover the large target volume using a patient-specific bolus and a 4-cm WET range shifter. The same IMPT plan was evaluated in two dose reporting modes: (1) nominal constant-RBE dose (RBE=1.1), and (2) LET-weighted dose calculated using a linear model with k=0.052, where RBE (and RBE-weighted dose) increases linearly with dose-averaged LET (LETd). Plan evaluation focused on target coverage and homogeneity index, HI=(D2−D98)/D50), and brain V5Gy, D0.03cc and D1%. Additional nearby OARs (brainstem, optic apparatus, lenses, lacrimal glands, cochleae) were reviewed.
Results
The constant-RBE IMPT plan achieved clinically acceptable target coverage (CTV D95%=55.06 Gy[RBE], D99%=54.45 Gy[RBE]) with low brain dose and low-dose bath (brain V5Gy=12.49%; brain D0.03cc=41.55 Gy[RBE]; brain D1%=13.32 Gy[RBE]). LET-weighted evaluation increased biologic dose in high-dose regions, increasing CTV D95% to 57.26 Gy(RBE) and CTV D99% to 56.68 Gy(RBE), while also increasing hotspot metrics (CTV D0.03cc:58.21→64.66 Gy[RBE]). Dose homogeneity decreased under LET-weighting (HI: 0.037→0.088). In the brain, LET-weighted evaluation increased near-maximum dose metrics (D0.03cc: 41.55→46.82 Gy[RBE]; D1%: 13.32→15.45 Gy[RBE]) with essentially unchanged low-dose exposure (V5Gy: 12.49%→12.47%). Other adjacent OAR doses remained very low in both reporting modes.
Conclusion
Three-field static IMPT using a custom bolus and a 4-cm WET range shifter is feasible for large-volume skin cancer, achieving clinically acceptable target coverage with low brain dose. However, LET-weighted evaluation shows increased biologic hotspots and higher brain near-maximum dose, highlighting the need to explicitly assesses high-LET/high-dose regions in the brain before broader clinical adoption.