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What Level of Accuracy Is Required? A Motion Escalation Study of Gating Window Tolerances in Breath Hold Breast Radiotherapy

L Conroy

L Conroy1,2*, S Quirk1,2 , E Watt1,2 , G Ecclestone2 , JL Conway1,2 , IA Olivotto1,2 , T Phan1,2 , WL Smith1,2 , (1) The University of Calgary, Calgary, AB, (2) Tom Baker Cancer Centre, Calgary, AB


SU-H3-GePD-J(B)-2 (Sunday, July 30, 2017) 4:00 PM - 4:30 PM Room: Joint Imaging-Therapy ePoster Lounge - B

Purpose: There are numerous methods for clinical implementation of deep inspiration breath hold (DIBH) for cardiac sparing in left-sided breast cancer patients. Visually-monitored, manually-gated DIBH methods can be used in clinics with limited resources; however, these techniques have the potential for larger breath-hold level (BHL) variability than commercial techniques with automated beam-holds. In this motion escalation study, we investigate the dosimetric impact of increasing BHL variability.

Methods: Fifteen left breast cancer patients were treated in DIBH using the Real-time Position Management (RPM) system. Forward-planned locoregional breast plans including the internal mammary chain (IMC) nodes were created on the DIBH CT scan. Patients were treated in 16 fractions with a ±3 mm gating window centered on the simulator BHL. The residual motion within the gating window was used to create patient-specific probability density functions (PDFs). These PDFs were then scaled to simulate BHL variability of ±1, 5, and 7 mm. The positional probabilities were incorporated into the treatment plans and dose was recalculated. Dose to the breast, IMCs, heart, and lung were evaluated.

Results: Target coverage and plan quality was maintained for most patients (12/15) even at the highest BHL variability of ±7 mm. However, patients with a tendency towards shallower median BHLs during treatment compared to their simulator position were more susceptible to loss of breast and IMC nodal coverage with larger BHL variation. Change in lung V20Gy and mean heart dose due to BHL variability was negligible.

Conclusion: BHL variability has minimal impact on plan quality; however, there can be loss of coverage when BHLs are systematically shallow within larger gating windows or tolerances. This can be mitigated by restricting lower gating window tolerances or accounting for known uncertainties using a planning structure to account for this motion explicitly.

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