2940 - Hybrid IMRT vs. Forward-planned IMRT in Adjuvant Breast Radiotherapy: A Prospective Dosimetric Study

Purpose/Objective(s): Optimal dosimetry in breast cancer (BC) radiotherapy (RT) requires homogeneous planning target volume (PTV) coverage, minimization of hotspots, and reduction of organ-at-risk (OAR) dose. Forward-planned IMRT (F-IMRT) using field-in-field tangents improves homogeneity, whereas inverse IMRT may further enhance uniformity but can increase low-dose OAR exposure. Hybrid IMRT (H-IMRT) combines conventional tangential fields with inverse-planned IMRT to improve dose homogeneity while limiting low-dose OAR exposure. We hypothesized that H-IMRT would reduce high-dose regions within the PTV (V105%) without increasing OAR dose compared with F-IMRT. The primary objective was to assess and compare PTV V105% of the breast and chest wall between F-IMRT and H-IMRT. Secondary objectives were to compare PTV coverage (V90%, V95%, V107%, V110%, D0.01 cc) for the breast, chest wall, and regional nodes; homogeneity index (HI); and OAR dosimetry, including ipsilateral lung, heart (left-sided BC), liver (right-sided BC), and spinal cord.

Materials/Methods: In this prospective single-institution dosimetric study, patients with invasive BC planned for adjuvant hypofractionated RT were included. For each patient, F-IMRT (field-in-field tangents) and H-IMRT plans (80% tangents, 20% two-field inverse IMRT) were generated on the same CT dataset. Targets and OARs were contoured per consensus guidelines. PTV_Eval was created by cropping 5 mm from the skin surface. Normality of paired differences was evaluated using the Shapiro–Wilk test. Dosimetric parameters were compared using paired t-tests or Wilcoxon signed-rank tests as appropriate, with two-sided p<0.05 considered statistically significant; analyses were performed in R.

Results: From 2024–2025, thirty patients were studied; 22 (73%) had right-sided BC, and 50% received breast or chest wall irradiation alone. Mean PTV V105% was significantly lower with H-IMRT (5.85±12.13%) compared with F-IMRT (20.45±16.60%; p<0.001). H-IMRT also reduced V107% and V110% (all p<0.01). The mean D0.1 cc was 42.92±0.83 Gy and 44.26±2.34 Gy, while homogeneity index values were 0.11±0.03 and 0.15±0.05 for H-IMRT and F-IMRT, respectively (both p<0.001). Ipsilateral lung mean V5Gy values were 43.11±14.21% and 42.22±14.17% (p=0.572), and mean V17Gy values were 20.95±7.24% and 20.58±6.83% (p=0.529) for H-IMRT and F-IMRT, respectively. Mean heart dose and spinal cord dose were comparable between techniques. Liver mean dose was slightly lower with H-IMRT (p=0.028).

Conclusion: H-IMRT significantly reduces high-dose regions and improves dose homogeneity without increasing OAR exposure. These findings support H-IMRT as a technically feasible alternative to F-IMRT in adjuvant BC radiotherapy. As this was a planning study, clinical outcome validation is warranted to determine the impact on toxicity and practice.