Dosimetric Implications of Number of Breathing Phases Used in the Definition of Internal Target Volume [ITV] in the Treatment of Non-Small Cell Lung Cancers Using Stereotactic Body Radiation Therapy (SBRT)

Determination of intrafraction motion in stereotactic body radiation therapy (SBRT) of non-small-cell lung cancer (NSCLC) usually involves generating an internal target volume (ITV) based on target segmentation in every one of the 10 phases of a 4-dimensional computed tomography (4DCT) dataset which increases dosimetry work load substantially. This study explores the feasibility of using a smaller number of phases to compile an ITV to get equivalent results. Twenty-five lung cancer patients treated with SBRT were retrospectively assessed. Patients were categorized by the anatomic location of the GTV within different lobes of the lungs, 5 in each lobe. Ten GTVs were contoured by the radiation oncologist in 10 different phases of one complete respiratory cycle. Five samples (Sample 1–5) were created using (0%, 20%, 40%, 60%, 80% i.e. taking every other phase), (0%, 30%, 60%, 90% i.e. skipping two successive phases), (0%, 20%, 30%, 50% i.e. essentially taking inhale, exhale & a phase in between), (0%, 30%, 60%), (0%, 50% i.e. using completely inhale and exhale phase only) phase GTVs, 0% is designated as the most inhaled phase and 50% as the most exhaled phase. Appropriate sample ITVs and PTVs were created in the same manner as the clinical plan which was then adapted to each sample set with minimal modification. Sample plans were compared for equivalent dose coverage, center of mass, and ITV/PTV volume differences against the clinical treatment plan. The average % ITV underestimation against the clinical ITV increased from a minimum of 7.3% in sample 1 (0%, 20%, 40%, 60%, 80%) to a maximum of 24.5% in sample 5 (0% & 50%) under the conditions of controlled breathing. A similar trend was seen in other samples with the underestimation in the ITV/PTV volume increasing with the decrease in the number of phases irrespective of the tumor location. The average variation in the center of mass of the ITV was minimal (0.43 ± 0.11 mm). Use of ITV/PTV combination derived from using less than all 10 phases resulted in the maximum clinical PTV under-dosage of 5.9% for sample 1 and 12.3% for sample 5, respectively. If fewer phases in the generation of ITV are used, a larger ITV-to-PTV margin might be necessary to get equivalent PTV coverage.