ESTRO 2024 - Abstract Book

S467

Clinical - Breast

ESTRO 2024

Dosimetric comparison of helical tomotherapy plans in breast cancer patients with silicone implants

Aysenur Elmali 1 , Recep Bozca 2 , Yemliha Dolek 2 , Ozan Cem Guler 2 , Cem Onal 2,1

1 Baskent University, Faculty of Medicine, Radiation Oncology, Ankara, Turkey. 2 Baskent University, Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Radiation Oncology, Adana, Turkey

Purpose/Objective:

Post-mastectomy radiotherapy (RT) in breast cancer patients undergoing immediate breast reconstruction (IBR) with silicone implants poses challenges. The optimal definition of clinical target volumes (CTVs) remains unclear. This study aimed to compare dosimetric outcomes between conventional RT plans with implant sparing RT plans using helical mode (H) and TomoDirect (TD) mode techniques in breast cancer patients with retro-pectoral silicone implants.

Material/Methods:

Forty patients with locally advanced breast cancer, 20 with left-sided tumors and 20 with right-sided tumors; who underwent retro-pectoral IBR and irradiated to the target volumes was 50.4 Gy, administered in 28 fractions of 1.8 Gy were included. Three RT plans were generated for each patient: H (using conventional CTV: including chest wall, skin and implant), TD (using conventional CTVc), and Hs (using implant sparing CTVs). The PTVc is derived from CTVc, whereas PTVs is derived from CTVs, by 5-mm expansion of the CTV in all directions, excluding a 2-mm skin strip. The H, TD, and Hs plans were compared for PTV doses, conformity (CI), homogeneity (HI), organ- at-risk (OAR) doses, and treatment times. HI = [(D2-115 D98)/D50], where D2 and D98 (minimal doses to 2% and 98% of target volumes, respectively) were used as surrogates for maximum and minimum doses.

Results:

The median age of patients was 46-70 years (range: 31 – 54 years). At the time of initial diagnosis, 18 patients had stage II breast 71 cancer and 22 had stage III breast cancer. The median PTVc and PTVs were 915.4 cm³ (453.3 – 1382.0 cm³) and 502.1 cm³ 137 (227.6 – 869.0 cm³), respectively. Median implant volume was 379.7 cm³ (213.8 – 524.0 cm³). All treatment plans successfully met the prescribed volume coverage requirements and adhered to the established dose constraints for organs at risk (OARs). However, the implant-sparing Hs plan exhibited less homogeneity (0.09 ± 0.01) and conformity (0.61 ± 0.10) in delivering doses compared to both H (0.08 ± 0.02; p = 0.007 and 0.78 ± 0.02, p < 0.001) and TD plans (0.07 ± 0.02; p = 0.004 and 0.73 ± 0.08; p < 0.001) (Fig.1). The TD plan demonstrated significant reductions in doses to critical structures such as the lung, heart, contralateral breast, spinal cord, liver, and esophagus when contrasted with the Hs plan. In a direct comparison between the H and Hs plans, the H plan showed a superior ability to limit lung V5 (39.1±3.9 vs. 41.2±3.9 Gy; p<0.001) while having higher V20 (12.3±1.3 vs. 11.5±2.6 Gy; p=0.02) and V30 (7.5±1.6 vs. 4.4±1.7 Gy; p<0.001). Moreover, the H plan outperformed the Hs plan in all heart dosimetric parameters, with the exception of V20. Importantly, the Hs plan significantly reduced mean implant doses (43.4±2.1 Gy) compared to both the H plan (51.4±0.5 Gy; p<0.001) and the TD plan (51.9±0.6 Gy; p<0.001). Implementing an implant-sparing technique to reduce silicone exposure resulted in decreased lung doses. The median treatment duration for the H, TD, and Hs plans, respectively, was 5.4 minutes (range: 4.1 – 8.1 minutes), 6.8 minutes (range: 4.6 – 9.3 minutes), and 5.5 minutes (range: 4.6 – 7.4 minutes). The treatment duration was significantly longer in the TD plan than in the Hs plan (p = 0.001), but no significant difference was observed between the H and Hs plans (p = 0.24).