Session Item

To evaluate the impact of the variability of the heart position and shape in the heart dose for patients with left-sided breast cancer who received external beam radiotherapy (RT) in deep inspiration breath hold (DIBH).

A total of 150 treatment fractions for 10 patients who received RT of left-sided breast cancer (2.67 Gy x 15 fx) with the DIBH technique were included in this study. The patients were initially positioned with the Catalyst (C-RAD, Uppsala, Sweden) surface scanning system in free breathing position. A CBCT scan was subsequently acquired in treatment position (DIBH) to derive the necessary couch translations to align the patient in accordance with the planning CT (pCT).  The daily CBCT images were exported to the Monaco (Elekta AB, Stockholm, Sweden) treatment planning system (TPS). The online couch translations were thereafter used to rigidly register the CBCT images to the pCT.  The pCT was deformably registered to all the daily CBCT images (using the ‘Adapt-To-Anatomy’ functionality in Monaco), and the reference heart contour was subsequently propagated to each CBCT to generate 15 ‘heart-of-the-day’ contours. The daily heart contours were then rigidly copied to the pCT. Thus, for each patient we had 16 heart contours segmented in the pCT: 1 reference heart contour and 15 representing the day-to-day variation in heart position and shape (Figure 1). To estimate the heart dose for each fraction, we re-calculated the treatment plan 15 times.  For each re-calculation, a relative electron density (ED) of 1 (relative to water) was assigned to the heart-of-the-day contour and for the reference heart contour, the relative ED was set to 0.01. The body contour of the pCT was assumed to be representative for the geometry of each fraction. For each treatment fraction, the mean heart dose was registered and the differential DVH of the heart was exported and used as input in the calculation of the NTCP (risk for late cardiac morbidity), using the relative seriality model. 

For all patients, the mean heart dose calculated on the pCT did not exceed our planning constraint of 4.005 Gy. The mean heart dose calculated for the different treatment fractions, using the heart contour transferred from the daily CBCT, ranged from -21.1 % to 58.7 %, relative to the mean heart dose determined for the reference heart contour. A summary of the results is presented in Figure 2. The NTCP for late cardiac morbidity for the reference heart was 0 % for all patients. However, for the heart contour taken from the CBCT images, the NTCP varied between 0 % and 0.33 %. 

The variation in the heart position and shape between fractions could lead to higher doses being delivered to the heart, compared to the dose calculated on the pCT. This could lead to an underestimation of the expected heart NTCP due to RT. These variations should be considered in the plan evaluation for left-sided breast cancer patients receiving RT in DIBH.