The use of heterogeneity corrections and its impact on delivered dose in patients treated with stereotactic body radiotherapy of the lung

Tissue density correction algorithms are widely used in North America when planning stereotactic body radiotherapy (SBRT) treatments and influence calculations regarding delivered dose. In turn, this may have an impact on complications and local control on tumors treated in the lung. This study evaluated the impact of patient-related and tumor-related factors on the dose to the tumor with and without heterogeneity corrections.

Fifty-one consecutive patients were treated for malignant lung tumors with SBRT using the RTOG 0236 guidelines, without heterogeneity corrections. Using the same beam arrangements and monitor units, dose distributions were recalculated using heterogeneity corrections, and the percent difference in dose to 5 (hot spot), 50, and 95 % of the internal target volume (ITV; D₅, D₅₀, and D₉₅) between the two plans was computed. Correlation analysis and independent samples t tests were used to identify significant differences in these statistics based on tumor distance from the chest wall, central vs. peripheral location, lobe involved, ITV, gross target volume (GTV), and body mass index (BMI).

On average, the use of heterogeneity corrections resulted in increases in D₉₅ (3.8 %; range −7.7 to 11.2 %), D₅₀ (7.0 %; range −3.0 to 15.3 %), and D₅ (8.7 %; range −14.4 to 17.6 %). Tumors greater than 1 cm away from the chest wall were associated with a significantly larger increase in D₅ (10.1 vs. 7.4 %, p = 0.046). Central tumors were associated with a significantly larger increase in D₅₀ (8.9 vs. 6.2 %, p = 0.022). ITV was correlated to the magnitude of increase in D₉₅ (R = .277, p = 0.049), and ITV greater than the median value (13.9 cm³) resulted in a larger increase in D₅₀ (5.8 vs. 8.1 %, p = 0.035) and D₉₅ (1.8 vs. 5.8 %, p = 0.002). Tumors of the lower lobes were associated with trends toward significance for increase in D₅ (9.8 vs. 7.8 %, p = 0.079) and D₅₀ (8.3 vs. 6.3 %, p = 0.078), and left upper lobe tumors were associated with smaller increases in D₅₀ (5.0 vs. 7.5 %, p = 0.043). GTV size and BMI had no impact on these values.

The use of heterogeneity corrections increases the dose to the ITV. The magnitude of this effect is increased by a larger ITV, tumor location in the lower lobes where diaphragmatic excursion causes greater tumor motion, increased distance from chest wall to tumor, and central location.