(P072) Analysis of Composite EQD2 Dose Distribution in Radiotherapy for Cervical Cancer Using Central Shielding Technique

Tomoaki Tamaki, MD, PhD, Shin-ei Noda, MD, PhD, Tatsuya Ohno, MD, PhD, Shingo Kato, MD, PhD, Takashi Nakano, MD, PhD; Saitama Medical University International Medical Center; Gunma University Graduate School of Medicine

PURPOSE: To analyze the three-dimensional equivalent dose in 2-Gy fractions (EQD2) dose distribution of external beam radiotherapy (EBRT) plus intracavitary brachytherapy (ICBT) for cervical cancer using the central shielding technique.

MATERIALS AND METHODS: In a phantom study, a whole-pelvis irradiation (WP) plan was created using the four-field box technique and pelvis irradiation with the central shielding technique (CS) using AP-PA fields with a central block having a 3-cm or 4-cm width. Three patterns of EBRT were created for WP and CS: 30 Gy/15 fractions plus 20 Gy/10 fractions (Plan 30+20); 40 Gy/20 fractions plus 10 Gy/5 fractions (Plan 40+10); and 45 Gy/25 fractions and 0 Gy (Plan 45+0). For ICBT, two plans were created using Point-A prescription: 24 Gy/4 fractions (BTPlan 24/4) for Plan 30+20 and Plan 40+10, and 28 Gy/4 fractions (BTPlan 28/4) for Plan 45+0. The physical doses were converted to EQD2, and the composite EQD2 dose distributions were analyzed.

RESULTS: In Plan 30+20 plus BTPlan 24/4 and Plan 40+10 plus BTPlan 24/4, the area covered with 60 Gy (EQD2) in the lateral direction was larger than that in Plan 45+0 plus BTPlan 28/4. There were no “cold” spots within the lateral axis, which indicates that even with the use of CS, the treatment could provide adequate dose coverage for the central tumor and the parametrial tissue. The Point-A doses of Plan 30+20 plus BTPlan 24/4, Plan 40+10 plus BTPlan 24/4, and Plan 45+0 plus BTPlan 28/4 were 78.0 Gy (CS: 3 cm) or 71.8 Gy (CS: 4 cm), 80.1 Gy (CS: 3 cm) or 77.0 Gy (CS: 4 cm), and 84.1 Gy, respectively. These values were higher than the total Point-A doses reported in previous clinical studies, which omitted the doses provided by CS. On the other hand, the coverage in the anterior-posterior direction was smaller in Plan 30+20 plus BTPlan 24/4 and Plan 40+10 plus BTPlan 24/4 as a result of CS, indicating that the high dose to the bladder and the rectum can be avoided, while the adequate tumor coverage in this direction needs to be assessed carefully.

CONCLUSIONS: Three-dimensional composite dose distribution analysis plays a significant role in the correct understanding of the dose distribution of the combination of EBRT and ICBT for cervical cancer. The use of CS in radiotherapy for cervical cancer provided tumor doses higher than those referred by the Point-A dose in previous reports, with no irregularly “cold” regions around the central target.

Proceedings of the 97th Annual Meeting of the American Radium Society- americanradiumsociety.org