Comparison of Different Strategies to Use Four-Dimensional Computed Tomography in Treatment Planning for Lung Cancer Patients

Purpose

To discuss planning target volumes (PTVs) based on internal target volume (PTV_ITV), exhale-gated radiotherapy (PTV_Gating), and a new proposed midposition (PTV_MidP; time-weighted mean tumor position) and compare them with the conventional free-breathing CT scan PTV (PTV_Conv).

Methods and Materials

Respiratory motion induces systematic and random geometric uncertainties. Their contribution to the clinical target volume (CTV)-to-PTV margins differs for each PTV approach. The uncertainty margins were calculated using a dose–probability-based margin recipe (based on patient statistics). Tumor motion in four-dimensional CT scans was determined using a local rigid registration of the tumor. Geometric uncertainties for interfractional setup errors and tumor baseline variation were included. For PTV_Gating, the residual motion within a 30% gating (time) window was determined. The concepts were evaluated in terms of required CTV-to-PTV margin and PTV volume for 45 patients.

Results

Over the patient group, the PTV_ITV was on average larger (+6%) and the PTV_Gating and PTV_MidP smaller (−10%) than the PTV_Conv using an off-line (bony anatomy) setup correction protocol. With an on-line (soft tissue) protocol the differences in PTV compared with PTV_Conv were +33%, −4%, and 0, respectively.

Conclusions

The internal target volume method resulted in a significantly larger PTV than conventional CT scanning. The exhale-gated and mid-position approaches were comparable in terms of PTV. However, mid-position (or mid-ventilation) is easier to use in the clinic because it only affects the planning part of treatment and not the delivery.

Computed tomography, Lung cancer, Geometric uncertainties, Planning target volume margin