Is Maximum Intensity Projection an Optimal Approach for Internal Target Volume Delineation in Lung Cancer?

Respiratory-induced tumor motion is a major challenge in lung cancer (LC) radiotherapy. Four-dimensional computed tomography (4D-CT) using a maximum intensity projection (MIP) dataset is a commonly used and time-efficient method to generate internal target volume (ITV). This study compared ITV delineation using MIP or tumor delineation on all phases of the respiratory cycle.

Thirty consecutive patients of LC who underwent 4D-CT from January 2014 to March 2017 were included. ITV delineation was done using MIP (ITV_MIP) and all ten phases of the respiratory cycle (ITV_10Phases). Both volumes were analyzed using matching index (MI). It is the ratio of the intersection of two volumes to the union of two volumes. A paired sample t-test was used for statistical analysis, and P < 0.05 was considered statistically significant.

The mean ± standard deviation volume of ITV_10Phases was significantly larger compared to ITV_MIP (134 cc ± 39.1 vs. 113 cc ± 124.2, P = 0.000). The mean MI was 0.75 (range 0.57-0.88). The mean volume of ITV_10Phases not covered by ITV_MIP was 26.33 cc (23.5%) and vice versa was 5.51 cc (6.1%). The mean MI was 0.73 for tumors close to the mediastinum, chest wall, and diaphragm. MI was not different between tumors ≤5 cm and >5 cm. The average time required for delineation was 9 and 96 min, respectively. The center of mass of two ITVs differed by 0.01 cm.

ITV using MIP is significantly smaller and may miss a tumor compared to ITV delineation in 10 phases of 4D-CT. However, the time required is significantly less with MIP. Caution should be exercised in tumors proximity to the mediastinum, chest wall, and diaphragm.