ESTRO 36 Abstract Book

S999 ESTRO 36 _______________________________________________________________________________________________

Purpose of this study was to compare the dose distribution to the organs at risk for different longitudinal margins using a DVH- and NTC-based approach. Material and Methods 10 patients with SCC of the middle or the lower third, who underwent CRT at our institution were retrospective selected. Three planning target volumes (PTV) were created for every patient, with an axial margin of 1.5 cm to the gross target volume (GTV) (primary tumor and PET- positive lymph nodes), analogous to the protocol of the CROSS-trial. The longitudinal margins were 4 cm, 3 cm and 2 cm, respectively. Contouring and treatment planning was performed with the Eclipse 13 planning system (Varian Medical Systems, Palo Alto, CA, USA). For every PTV, volumetric modulated arc therapy (VMAT) plans were optimized. Dose calculation was performed using the AAA algorithm (version 10.0.28) and heterogeneity correction. All plans were normalized to a median prescribed PTV dose of 41.4 Gy with a daily dose of 1.8 Gy. Dose to the lungs, heart and liver were evaluated and compared. Differences of dose parameters were tested for significance with t- test for paired samples. Results Median tumor length was 6 cm with a range of 3 to 10 cm and 8 of the 10 patients (80%) had lymph node metastasis. When using a longitudinal margin of 3 cm instead of 4 cm, all dose parameters (Dmin, Dmax, Dmean, Dmedian and V5-V35), except Dmax could be significantly reduced for the lungs. Regarding the heart, a significant reduction was seen for Dmean and V5, whereas no significant difference was seen for Dmin, Dmax, Dmedian and V10-V35. When comparing a longitudinal margin of 4 cm to a longitudinal margin of 2 cm, not only Dmin, Dmax, Dmean, Dmedian and V5-V35 for the lungs, but also Dmax, Dmin and V5-V35 for the heart were significantly reduced. Nevertheless, no difference was seen for the median heart dose. In addition, the risk of pneumonitis was significantly reduced by a margin reduction of 3 cm and 2 cm. Conclusion The reduction of the longitudinal margin from 4 cm to 3 cm can significantly reduce the dose to lungs, while the reduction to 2 cm can also reduce doses to the heart. Despite clinical benefit and oncologic outcome remain unclear, reduction of the longitudinal margins might provide the opportunity to reduce side effects of CRT for SCC in upcoming studies. EP-1824 Elective breast RT including level I & II lymph nodes: A planning study with the humeral head as PRV J. Van der Leer 1 , K. Surmann 1 , M. Van der Sangen 1 , M. Van Lieshout 1 , C.W. Hurkmans 1 1 Catharina Ziekenhuis, Radiotherapy, Eindhoven, The Netherlands Purpose or Objective The aim of this planning study was to determine a new technique for elective breast radiotherapy and level I and II lymph nodes following the new ESTRO delineation consensus guidelines. According to these guidelines the humeral head should be spared by introducing a planning risk volume (PRV) of the humeral head and connective tissue 10 mm around it. Material and Methods We included ten left sided breast cancer patients in our planning study in Pinnacle 3 v9.8. Each patient was planned with 16 x 2.66 Gy on the breast PTV (PTVp) and the elective level I and II lymph nodes (PTVn). We compared three treatment planning techniques: high tangential field (HTF), 6-field IMRT and VMAT. The HTF technique consisted of two open beams with extra segments and the cranial and posterior border was extended to include PTVn. Some of the leaves were closed

to spare the humeral head + 10 mm around it (hh+10). For the IMRT technique we added four additional fields to the high tangential fields (gantry angle of 330, 30, 80 and 170 degrees) to ensure coverage of the cranial part of the breast and lymph nodes. The caudal border of these additional fields was set 1 cm below the attachment of the clavicle at the sternum. The third technique was a dual arc VMAT from 305 to 180 degrees. The plans were made by inverse planning, achieving a PTVp coverage of V95% ≥ 97% and a PTVn V90% ≥ 95%. Additionally, the dose to the lungs, heart and right breast (OARs) has been minimized. hh+10 was included with an objective of V40Gy < 1 cm 3 for all three techniques. Results HTF resulted in an average PTVp V95% of 97.2% and an average PTVn V90% of 90.4% (see Table 1 and Figure 1). With the additional fields of the IMRT technique the coverage of PTVn increased significantly to on average 98% (p=0.01) while PTVp did not vary significantly (p=0.92). The dose to the OAR was comparable between the HTF and IMRT techniques. When using VMAT the coverage of the PTVn was on average 99.5% (p<0.01 compared to the HTF and p=0.19 compared to IMRT). The dose to the OARs however increased as well. The mean dose to the contralateral breast increased significantly from 0.6 Gy with HTF and IMRT to 2.3 Gy with VMAT (p<0.01 for both).