ESTRO 37 Abstract book

S1035

ESTRO 37

mm and 10 mm fixed cones were used to optimize and delivery the plan. Dose was calculated using both ray- tracing and MC dose calculation algorithms. The same plan was copied and applied to the CT image with metallic artifacts. Target dose calculated from different images and algorithms were compared with the actual dose measured using an ionization chamber. 2D dose distributions were measured using radiochromic film and compared with planned dose using gamma analysis with 3%/2mm tolerance. Results Serious metallic artifacts were observed in the CT images with metallic implants presented. The CT numbers in the target volume varying from -1000HU to 867HU. These variations in CT number corresponded to the variation in relative electron density from 0 to 1.47, and in mass density from 0.2 g/cc to 1.53g/cc, respectively, whereas the relative electron density was 1.11 and mass density was 1.09g/cc as measured in the artifact-free images. The gamma passing rates of 2D dose distributions comparison and differences between measured and calculated target doses based on different CT images and dose calculation algorithms were listed in Table I.

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 metallic implants affect the accuracy of dose calculation in Cyberknife stereotactic radiotherapy. Density overriding could improve the accuracy of dose calculation. However, care should be taken when MC dose calculation algorithm was applied. Because Multiplan version 5.3 uses only 3 materials for voxel model in MC electron transport calculation and limited the highest mass density to be 2.69g/cc, whereas the true mass density was 7.8g/cc for stainless steel in this case. The attenuation of dense metallic implants could be under-estimated in MC algorithm and the dose to the target may be over-estimated. This may explain the relatively larger target dose deviation using MC calculation in this study. EP-1906 Assessing the targeting and dosimetric impact of a novel bioabsorbable marker for external beam APBI R. Sims 1 , C. Benjamin 1 , J. Gardner 1 , T. Milne 1 1 Auckland Radiation Oncology, Radiation Therapy, Auckland, New Zealand Purpose or Objective Studies have demonstrated that both hypo-fractionated radiation therapy and accelerated partial breast irradiation (APBI) is safe and effective for patients with early stage breast cancer. APBI has many advantages (e.g. shorter treatment times, lower radiation exposure), however current targeting methods present many challenges. The BioZorb (BZ) marker was developed to indicate the region of surgical tumor removal and visually communicate this to the radiation oncology team for use during planning and treatment. The marker has six titanium clips distributed in a 3D helical pattern inside a bioabsorbable coil and we assessed the potential impact

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. 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