Session Item

Deep Inspiration Breath Hold (DIBH) has emerged as a cost-effective and easy-to-implement strategy to minimize the absorbed dose at the heart for left breast cancer treatments with external beam radiotherapy (EBRT). Among the several strategies for DIBH guidance, RPM relies on an external marker block placed at the chest of the patient to describe the chest movement in real-time. This study is aimed to quantify how stable and reproducible is the internal anatomy in terms of dosimetry indices for breast cancer treatments in DIBH triggered with the RPM system.

For 33 left breast cancer patients treated with EBRT (3DCRT or IMRT) we acquired between three and four DIBH-CBCT exams during the EBRT course. Body, heart and lung volumes were segmented and treatment plans were copied onto CBCT exams that had been co-registered on-line. For lung volumes, new ipsi and contralateral volumes were segmented on the planning CT with the same length as in the CBCT. Then, the dose distribution was calculated, and the following DVH parameters were obtained: V₃₀, V₂₅ and D_mean for the heart, and V₂₀ for the lung. We analyzed differences between the values derived from the planning CT study and those calculated on CBCTs. We performed a t-test to see whether the distribution of values obtained in CBCTs were different to those obtained on the planning CT. We also analyzed intra-patient stability as the 95% confidence interval (2σ) for each parameter for each patient.

Figure 1 shows that differences between values calculated on the planning CT compared against those calculated on the various CBCTs were around 0 for each parameter. Standard deviations of the distributions were around 1%.  The distribution was less peaked for lung V₂₀ than for the heart parameters.

Figure 1. Difference between each index calculated in CBCT exams and that calculated in the planning CT.

Table I shows the statistics of the differences between each index calculated on CBCT and that calculated on the planning CT exam. Average differences in volumes were around 0.2%, well below 1%, and 0 Gy in dose. p-values were much larger than 0.05 for all indices. This means that values calculated on planning CTs were not statistically significantly different from those calculated on CBCT exams. Intrapatient stability was around 1% volume or 0.6Gy for the heart parameters, and around 2% for lung V₂₀. The larger difference for the lung parameter could stem from the process of cropping the planning CT lung volume to calculate in the same length of lung as in CBCT exams.

The RPM system guarantees the stability of the internal patient geometry on DIBH left breast cancer radiotherapy treatments. The relevant dose-volume parameters of lung and heart during the treatment therefore agreed with those calculated on the planning CT.