Quantitative evaluation of the impact of heterogeneity correction on left breast cancer radiotherapy performed with respiratory gating
Purpose: Our objective was to assess the impact of a heterogeneity correction to the calculated dose for left breast cancer gated radiotherapy.
Methods: Ten patients with left breast cancer were studied. For each patient 2 treatment plans were generated. In plan 1 the dose was calculated using a Pencil Beam Convolution (PBC) algorithm. In plan 2 the dose was calculated using the Modified Batho's (MB) density correction method. To compare the two plans a dosimetric analysis was carried out including monitor units (MU), isodose curves, cumulative and differential dose volume histograms (cDVH, dDVH), coverage index, conformity index for target volume and the two dimensional (2D) gamma index (γ). Wilcoxon signed rank and Spearmen's tests were used to calculate p-values and correlation coefficients (r), respectively.
Results: MB method reduced the MU by on average 1.12 ± 5.33%. The analysis of cDVH showed that the MB method calculated significantly higher doses for target volumes, lung and heart, p < 0.05. The data demonstrated a strong correlation between the dosimetric parameters derived from plan 1 and plan 2 with r > 0.9. The 2D γ analysis showed that the difference between plan 1 and plan 2 could reach ± 10%. The γ evaluation showed a high impact of density correction for left breast cancer with gating technique.
Conclusion: This study confirms that using the MB method integrated with a PBC algorithm, the calculated dose will be increased to target volumes, lung and heart. Even more so since gating usually tends to decrease average lung density by about 39% by treating during an arrested inspiration phase. Thus, attention should be paid when changing from PBC to newer algorithms with gating techniques, since the probability of cardiac mortality and lung toxicity are correlated to absorbed dose.
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