Abstract Book

S1034

ESTRO 37

EP-1907 Dose gradient curve: a new tool for evaluating dose gradient K. Sung 1 , Y.E. Choi 1 1 Gachon University Gil Medical Center, Radiation Oncology, Namdong-gu- Incheon, Korea Republic of 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.

titanium clips distributed in a 3D helical pattern inside a bioabsorbable coil and we assessed the potential impact of this new marker on radiation treatment planning in terms of target delineation and resulting dosimetry. Material and Methods All patients had partial mastectomy for T1 N0 M0 stage breast cancer (ductal carcinoma in-situ or invasive ductal carcinoma). One patient cohort was implanted with the BZ implant at the time of partial mastectomy (n=23, BZ), while the other had partial mastectomy alone (n=23, non- BZ). Two clinical staff members independently contoured the volumes for the surgical bed using either the BZ or standard seroma based methods (non-BZ). Radiation treatment plans were then created for each contour set for each patient using the RayStation treatment planning system (TPS) according to the Radiation Treatment Oncology Group (RTOG) protocol 0413. Partial-arc VMAT beams were used for planning and each plan was peer reviewed by an independent planner to ensure its clinical validity. Contours and radiation dose levels were then analyzed using various distance- and volume-based metrics. Results A difference was noted in the ability to effectively contour patients in the BZ vs. non-BZ group, resulting in a significant improvement in agreement between users with conformity indices of 0.83 ± 0.01 for BZ and 0.55 ± 0.21 for non-BZ patients. This resulted in a significantly smaller planning treatment volumes (PTV) of 65.5 ± 4.6cc and 142.8 ± 51.2cc for BZ and non-BZ respectively that reduced the volume of normal breast tissue exposed to radiation (typically from 30% to 15% at the 50% isodose level). The dice similarity coefficient of the 50%, 95% and 100% isodose levels for the BZ and non-BZ groups shown in Figure 1, demonstrating the dosimetric improvement due to reduced inter-user varaibility.

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.

Conclusion We analyzed CT based radiotherapy plans based upon targeting with the BZ surgical marker versus seroma based methods. This study revealed that BZ provides a method to define the location of the surgical tumor bed more consistently between users, and is associated with smaller tumor bed volumes and therefore smaller planned treatment volumes. These findings suggest that the BZ will assist in sparing normal surrounding tissues from radiation exposure that would facilitate a reduction in treatment margins. Additional studies are necessary to confirm these findings and gain a greater understanding of this new implantable marker.