SU‐F‐J‐123: CT‐Based Determination of DIBH Variability and Its Dosimetric Impact On Post‐Mastectomy Plus Regional Nodal Radiation Therapy

Abstract

Purpose: Breast cancer radiotherapy delivered using voluntary deep inspiration breath‐hold (DIBH) requires reproducible breath holds, particularly when matching supraclavicular fields to tangential fields. We studied the impact of variation in DIBHs on CTV and OAR dose metrics by comparing the dose distribution computed on two DIBH CT scans taken at the time of simulation. Methods: Ten patients receiving 50Gy in 25 fractions to the left chestwall and regional lymph nodes were studied. Two simulation CT scans were taken during separate DIBHs along with a free‐breathing (FB) scan. The treatment was planned using one DIBH CT. The dose was recomputed on the other two scans using adaptive planning (Pinnacle 9.10) in which the scans are registered using a cross‐correlation algorithm. The chestwall, lymph nodes and OARs were contoured on the scans following the RTOG consensus guidelines. The overall translational and rotational variation between the DIBH scans was used to estimate positional variation between breath‐holds. Dose metrics between plans were compared using paired t‐tests (p < 0.05) and means and standard deviations were reported. Results: The registration parameters were sub‐millimeter and sub‐degree. Although DIBH significantly reduced mean heart dose by 2.4Gy compared to FB (p < 0.01), no significant changes in dose were observed for targets or OARs between the two DIBH scans. Nodal coverage as assessed by V90% was 90%±8% and 89%±8% for supraclavicular and 99%±2% and 97%±22% for IM nodes. Though a significant decrease (10.5%±12.4%) in lung volume in the second DIBH CT was observed, the lung V20Gy was unchanged (14±2% and 14±3%) between the two DIBH scans. Conclusion: While the lung volume often varied between DIBHs, the CTV and OAR dose metrics were largely unchanged. This indicates that manual DIBH has the potential to provide consistent dose delivery to the chestwall and regional nodes targets when using matched fields.

An effective deep-inspiration breath-hold radiotherapy technique for left-breast cancer: impact of post-mastectomy treatment, nodal coverage, and dose schedule on organs at risk

“…Target-field (CTV and PTV) delineation, planning, and delivery methods also appear to have substantial influence. 8 – 14 , 18 , 29 Importantly, our data confirm previous limited data that show dose schedule can contribute to increased OAR doses in DIBH treatment. 17 This accords with data previously obtained in FB.…”

“…This is likely because of individual patient anatomy and target-field morphological variations, such as chest shape, lung size, breast shape, size, position, and mobility, which influences lung volume proportion within the tangential field and proximity to high-dose regions with inflation/deflation. Target-field (CTV and PTV) delineation, planning, and delivery methods also appear to have substantial influence 8–14,18,29. Importantly, our data confirm previous limited data that show dose schedule can contribute to increased OAR doses in DIBH treatment 17.…”

“…Heart doses were among the lowest reported for DIBH IMRT 6,8,13. Lung doses (ilung MD and ilung V20Gy) were also lower overall as compared with many other studies,9–14,18,28,29,32 although substantial inter-patient variability was observed. This is likely because of individual patient anatomy and target-field morphological variations, such as chest shape, lung size, breast shape, size, position, and mobility, which influences lung volume proportion within the tangential field and proximity to high-dose regions with inflation/deflation.…”

“…Moreover, patients received OAR doses lower than other reported DIBH methods. 6 , 9 – 11 , 13 , 14 , 18 , 28 , 29 Nevertheless, the main clinical challenges of the technique included: incorporating patient selection into routine practice; the need for an additional planning CT scan at simulation for those “at risk” of cardiac toxicity; the financial and space implications for extra equipment required; the additional planning requirements to incorporate low OAR constraints and 20 second beam-on times; and the additional clinic time required for staff training, and longer appointment times to incorporate patient selection, patient coaching, DIBH radiation delivery, and daily quality assurance.…”