KEY POINTS
- Twenty left-breast cases treated with DIBH and surface guidance were replanned using step-and-shoot tangential IMRT, dynamic-MLC tangential IMRT and two-arc VMAT. All plans prescribed 40.05 Gy in 15 fractions, used the same RayStation platform and were normalized to PTV D50 of 40.05 Gy.
- Dynamic IMRT with 20 control points achieved essentially the same PTV coverage as VMAT: D95 was 38.3 versus 38.4 Gy (P=.218) and V95 96.3% versus 96.6% (P=.571). VMAT remained more conformal, with conformity index 0.83 versus 0.77 (P<.001).
- Dynamic IMRT reduced the low-dose lung bath compared with VMAT. Ipsilateral lung mean dose was 4.8 versus 5.7 Gy, and V5 was 18.6% versus 25.7%; both differences were significant. Mean heart dose was also lower at 1.0 versus 1.2 Gy.
- The contralateral dose difference was substantial relative to the already low absolute exposure. Contralateral lung mean dose was 0.5 versus 0.9 Gy, contralateral breast mean dose 0.7 versus 1.2 Gy, and contralateral breast V3 0.4% versus 1.7% with dynamic IMRT versus VMAT.
- Radiobiological modeling illustrated the trade-off between dose distributions. Predicted ipsilateral-lung pneumonitis probability was actually lower with VMAT (2.2%) than dynamic IMRT (3.7%) because VMAT reduced higher-dose lung exposure, whereas modeled secondary-cancer probability favored dynamic IMRT because of its smaller low-dose bath.
- Beam-on delivery averaged 81.6 seconds with dynamic IMRT versus 141.7 seconds with VMAT, a 42% reduction particularly relevant for repeated breath holds. Step-and-shoot IMRT required 95.4 seconds. Patient-specific QA remained above the institutional 95% acceptance threshold despite greater dynamic-plan modulation.
- The workflow advantage extended beyond speed: tangential IMRT avoided the gantry angles most likely to obstruct the left SGRT camera, whereas VMAT could produce transient 1–2-mm false surface deviations and workflow interruptions. The authors still favor VMAT for complex anatomy when two tangential fields cannot achieve adequate coverage.
CLINICAL TAKEAWAY
For straightforward left-breast DIBH, dynamic tangential IMRT may provide a useful middle ground: target coverage close to VMAT, less low-dose spread and much shorter beam delivery while avoiding SGRT camera-obstruction problems. The trade-off is poorer conformity, and this single-center 20-plan comparison does not establish one technique as universally superior.