Room: Exhibit Hall | Forum 7
Purpose: To evaluate the impact of ring size on the equivalent uniform dose (EUD) and therapeutic ratio (TR) of high-dose-rate ring and tandem brachytherapy (HDR-RTBT) for primary cervical cancers.
Methods: Three sizes of rings (2.6, 3.0 and 3.4 cm) were used in this study. All sets have the same tandem length (6-cm) and angle (45-degrees). The applicator sets were scanned by CT with a water tank. The plans were made using a clinical protocol. Similar dwelling points patterns and weighting factors were assigned during planning. The plans were normalized at point A (2cm/2cm). After a â€œpearâ€? shape plan is made, the volumes covered by 100% (ISOROI100) and 90% (ISOROI90) of prescription dose in each plan were generated. The ring and tandem volumes were contoured and subtracted from ISOROI100 and ISOROI90 for getting the real tissue volumes covered by those isodose lines. The dose volume histograms of those volumes were exported to a home-made software for further analysis. The modified linear quadratic model (MLQ) was used to calculate the EUDs and TRs.
Results: For the prescription dose of 7.5 Gy at the point A, the average EUD is 9.31 Gy (ranging from 9.28 to 9.33 Gy) for 100% prescription dose isodose line covered volume, and is 8.52 Gy (from 8.51 to 8.54 Gy) for 90% isodose line. The average TR is 2.81 (from 2.78 to 2.85) and 2.60 (from 2.57 to 2.62) for the same isodose lines.
Conclusion: The EUD and TR are not dependent on the ring size. The EUDs are 24% and 14% greater than the prescription dose respectively for 100% and 90% isodose covered volumes, which means the HDR-RTBT is more effective than seen from its prescription dose. The TRs are close to 3, which means the HDR-RTBT could maintain a low level of normal tissue complication.