ESTRO 37 Abstract book

S1036

ESTRO 37

Purpose or Objective Stereotactic radiotherapy, delivering ablative high dose to the target in a single fraction for the maximum local tumor control, requires the rapid dose fall-off outside the target volume to prevent extensive normal tissue damage. Unfortunately, there is no tool to comprehensively evaluate dose gradient near the target volume. We aimed to propose the dose-gradient curve (DGC) as a new tool to evaluate the quality of a treatment plan with respect to the dose fall-off characteristics. Material and Methods The average distance between two isodose surfaces was represented by the dose gradient index (DGI) approximated by the simple equation using the volume and surface area of isodose level. Surface area was calculated by mesh generation and surface triangulation. The DGI of each dose interval was plotted as a function of dose, denoted as the differential DGC (dDGC). The cumulative DGC (cDGC) was defined as a plot of the cumulative DGI generated by summing the DGI from the prescription dose. The performances of the DGCs were evaluated with virtual structures. Results As shown in Figure below, the DGC was generated using an SRS plan. Over the range of dose distribution, the dDGC was plotted as a U-shaped curve in a millimeter scale. The cDGC demonstrated a rotated sigmoid shape, originated from the prescription isodose level. The significant changes in the DGC were observed reflecting the differences in planning situations and various prescription doses.

EP-1908 Assessment of dosimetric impact of respiratory motion during breast IMRT using 4D calculation Y.E. Choi 1 , K. Sung 1 1 Gil Medical Hospital, Radiation Oncology, Incheon, Korea Republic of Purpose or Objective In this study, we aimed to generate 4D dose distributions by incorporating each static dose distribution of three consecutive computed tomography (CT) scans using deformable image registration. The ultimate goal was to evaluate the dosimetric impact of respiratory motion on breast intensity modulated radiation therapy (IMRT) with simultaneous integrated boost (SIB). Material and Methods CT images of ten consecutive patients treated according to our institutional DIBH protocol were retrospectively re- planned using IMRT and 3-dimensinal conformal radiation therapy (3D-CRT). Using the RPM systems for detecting the breathe amplitude, multiple CT scans were performed in distinct respiratory phases: end-expiration (EE), end-inspiration (EI), and deep-inspiration (DI). EE- CT scan was used to develop treatment plan as a reference and then this static dose distribution was projected to EI and DI-CT scans, separately. The prescription was to deliver 50 Gy and 57.5 Gy in 25 fractions to PTV and CTV. The dose distribution projected to EI-CT was deformably registered to EE-CT using Mirada RTx. The ratio of EE to EI used to generate 4D dose distributions was 1:1, 2:1, 3:1, 4:1 and 5:1. The dosimetric parameters of three static dose distributions for each plan (IMRT and 3D-CRT), and 4D dose distributions with different ratio of EE to EI (EE:EI) were compared: conformity number (CN) and homogeneity index (HI) of target volumes, and the dose-volume parameters of the heart, each lung, and contralateral breast.

Conclusion The DGC is a rational method for visualizing the dose gradient as the average distance between two isodose surfaces; the shorter the distance, the steeper the dose gradient. By combining the DGC with the dose volume histogram (DVH) in a single plot with the same dose scale, the DGC can be utilized to evaluate not only dose gradient but also target coverage in routine clinical practice.