A Retrospective Dosimetric Study of Radiotherapy Patients with Left-sided Breast Cancer; Benefits of the Deep Inspiration Breath Hold Technique

Background:
Several studies have investigated cardiac dose reduction when utilising the deep inspiration breath hold (DIBH) technique in patients undergoing radiotherapy for left-sided breast cancer. However, gaps still exist in terms of developing a selection criteria for patients who receive the most benefit.

Aims:
The aim of this honours study was to recommend a potential patient selection criteria. This was achieved through:
1. Quantifying the benefit of the DIBH technique compared to the conventional free breathing (FB) technique through comprehensive dose metrics of organs at risk including the heart, left anterior descending artery and lung volumes.
2. Determining the relationship, if any, between the magnitude of dose reduction to organs at risk in DIBH and anatomical parameters measured from computed tomography (CT) simulation scans on a group and individual basis.

Methods and Materials:
In this retrospective single institution study, data sets of twenty patients who underwent both FB and DIBH planning simulation scans, were obtained from Adelaide Radiotherapy Centre. Inter- and intra-observer reliability tests were performed to evaluate the clinical acceptability of the plans and contouring. A dosimetric comparison was performed between plans to quantify the dose reduction to cardiac structures, the left anterior descending coronary artery and the lungs. A correlation analysis was performed, using Pearson’s R coefficient, between the magnitude of dose reduction and patient anatomical factors (shape and anatomical variation of chest shape, breast tissue, chest wall separation, lung volume and others) with the aim of identifying potential selection criteria.

Results:
There was no difference in target coverage and therefore tumour control probability between the prospective FB plans compared to the pre-delivered retrospective DIBH plans. Evaluation across all metrics remained the same as per departmental protocol. On average, employing the DIBH technique significantly reduced the dose to cardiopulmonary structures. Comparing the techniques (FB vs. DIBH), the average mean dose received by the heart reduced from 2.7 Gy to 1.4 Gy and left anterior descending artery from 14.1 Gy to 6.6 Gy respectively. Paired t-tests, comparing the dose distributions between the plans, demonstrated significant cardiac dose reduction for some patients but not for others. Cardiac dosimetry for one patient was worse for their DIBH plan compared to FB plan. Linear regression analysis identified a positive correlation between the patient’s total lung volume and chest wall separation (as identified on the FB CT simulation scan) and the magnitude of dosimetric benefit received.

Conclusion:
DIBH reduces dose to critical structures without compromising target coverage. The chest wall separation and total lung volume taken in FB simulation scans are potentially good predictive factors for cardiac exposure. The outcome of this study can assist with patient section for DIBH, predicting which patients will experience the most clinical benefit from the technique. This is important in resource allocation, as DIBH may be unnecessarily recommended for some patients with little dosimetric benefit.