Monte Carlo-based optimization of CTV-to-PTV margins for image-guided VMAT prostate radiotherapy.

[BACKGROUND] The van Herk margin formula, derived for 3-D conformal radiotherapy with a uniform 3.2 mm dose penumbra and no intrinsic dose buffer, remains widely used to design CTV-to-PTV expansions in contemporary image-guided prostate radiotherapy. However, coplanar VMAT techniques often feature broader penumbrae and explicit PTV-isodose clearances, potentially violating the assumptions underlying the original formulation and leading to overly conservative margins.

[PURPOSE] We investigated whether the widely adopted van Herk margin formula overestimates clinical target volume (CTV)-to-planning target volume (PTV) expansions for contemporary coplanar volumetric modulated arc therapy (VMAT) prostate treatments.

[METHODS] Fifty consecutive intact-prostate VMAT radiotherapy plans (two coplanar arcs; clinical margins 3 mm, except for 2 mm posterior) were exported. Direction-specific 90% isodose-to-PTV gaps and penumbra widths were measured. Candidate anisotropic margins were tested by eroding the PTV to create CTV_eval. Monte-Carlo simulations combined systematic (Σ) shifts with Gaussian random (σ') blurring kernels of 0-2 mm were performed. Acceptability criteria of (i) CTV Dmin0.03 cc ≥ 90% Rx in ≥ 90% of simulated scenarios or (ii) population tumor-control probability (TCP) loss < 1 % were used.

[RESULTS] VMAT plans exhibited intrinsic 90% isodose clearances of 3-5 mm laterally/anteriorly and 1-2 mm superior-inferiorly, while axial-plane penumbras were up to fivefold broader than van Herk's assumption. With a [0, 0, 2] mm (LR/AP/SI) margin, ≥ 90% of patients maintained Dmin0.03 cc ≥ 90% Rx provided Σ lay within an ellipsoid of [2.0, 1.5, 1.8] mm and σ' ≤ [1.5, 2.0, 1.5] mm. Under TCP criteria the safe Σ ellipsoid for high-risk disease was [2.5, 1.9, 2.2] mm, while low-intermediate risk was even less sensitive.

[CONCLUSIONS] For image-guided coplanar VMAT prostate radiotherapy, an anisotropic [0, 0, 2] mm CTV-to-PTV margin is sufficient for target coverage. A modified margin expression that subtracts the measured isodose-to-PTV gap and uses reduced random-error coefficients better reflects modern practice.