Validation of the mid-position strategy for lung tumors in helical TomoTherapy

Abstract

Purpose: To compare the mid-position (MidP) strategy to the conventional internal target volume (ITV) for lung tumor management in helical TomoTherapy, using 4D Monte Carlo (MC) plan simulations.

Materials and methods: For NSCLC patients treated by SBRT (n = 8) or SIB-IMRT (n = 7), target volumes and OARs were delineated on a contrast-enhanced CT, while 4D-CT was used to generate either ITV or MidP volumes with deformable registrations. PTV margins were added. Conformity indexes, volumetric and dosimetric parameters were compared for both strategies. Dose distributions were also computed using a 4D MC model (TomoPen) to assess how intra-fraction tumor motion affects tumor coverage, with and without interplay effect.

Results: PTVs derived from MidP were on average 1.2 times smaller than those from ITV, leading to lower doses to OARs. Planned dose conformity to TVs was similar for both strategies. 4D MC computation showed that ITV ensured adequate TV coverage (D95 within 1% of clinical requirements), while MidP failed in 3 patients of the SBRT group (D95 to the TV lowered by 4.35%, 2.16% and 2.61%) due to interplay effect in one case and to breathing motion alone in the others.

Conclusions: Compared to the ITV, the MidP significantly reduced PTV and doses to OARs. MidP is safe for helical delivery except for very small tumors (<5 cc) with large-amplitude motion (>10mm) where the ITV might remain the most adequate approach.

Keywords: Geometric uncertainties; Lung cancer; Monte Carlo simulation; TomoTherapy; Tumor motion.