ESTRO 36 Abstract Book

S450 ESTRO 36 _______________________________________________________________________________________________

PO-0835 PTV margin for pelvic lymph nodes in IGRT guided prostate radiotherapy H.R. Jensen 1 , C.R. Hansen 1,2 , S.N. Agergaard 1 , E.L. Lorenzen 1,2 , L. Johnsen 1 , S. Hansen 2,3 , L. Dysager 3 , C. Brink 1,2 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 University of Southern Denmark, Faculty of Health Sciences, Odense, Denmark 3 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective In recent years irradiation of the pelvic lymph nodes for high risk prostate cancer has received strong interest, as a potential way to increase locale control probability. However the prostate and the pelvic lymph nodes move independently of each other. The purpose of this study is to calculate the additional PTV margin needed for covering the pelvic lymph node region, when performing a registration and setup on the prostate with implanted gold fiducials. Material and Methods All 40 prostate patients treated at the same accelerator in 2015 were included in the study. The majority of the patients had stage T3 disease. All patients had three gold fiducials implanted into the prostate 2-3 weeks before CT simulation, which were used in the daily online IGRT. A total of 1284 cone beam CT scans were analyzed. An automatic gold seed algorithm (used as a surrogate for the prostate) and bone algorithm covering the upper pelvic and lower spine area (used as a surrogate for the lymph nodes) were performed. The deviation between the two registrations was calculated and the population based random and systematic setup error was calculated. To estimate the PTV margin needed the Van Herk margin formula was used M = 2.5 * Σ systematic + 0.7 * σ random Results The setup margin needed for the lymph node region of this patient cohort is 2.1, 6.9 and 6.6 mm for the LR, CC, AP directions, respectively (see table). This margin does not incl. any other uncertainties. The minimum deviation between prostate seed and pelvic bone match is shown as a cumulative histogram in the figure for the individual directions. More than 15.4% of the fractions have a deviation of more than 5 mm, and 5% of the fractions have a larger deviation than 7mm. The largest deviations are seen in the CC and AP direction, and a small deviation in the LR direction. The systematic and random errors are shown in the table.

Conclusion Even though the analyzed IGRT protocol focuses entirely on the gold seed of the prostate the needed margins for the lymph nodes are only slightly larger than 5 mm which in many centers are used as a standard PTV margin. Thus, the additional margin needed to include the lymph nodes is actually somewhat modest. However, the optimal balance between dose coverage of tumor and lymph nodes both in regard to local control and toxicity is still unclear, and needs further investigation. PO-0836 Impact of Deep Inspiration Breath Hold on Left Anterior Coronary dose in Left Breast irradiation. F. Azoury 1 , S. Achkar 1 , N. Farah 1 , D. Nasr 1 , C. El Khoury 1 , N. Khater 1 , J. Barouky 1 , R. Sayah 1 , E. Nasr 1 1 Hotel Dieu de France Hospital - Saint Joseph University, Radiation Oncology, Beirut, Lebanon Purpose or Objective Irradiation of Left breast cancer exposes women to higher doses to the heart and LAD coronary. Blocking the heart in the tangent fields will inevitably cause under dosage in proximity to the tumor bed. Here we evaluate the effect of deep inspiration breath hold (DIBH) on the coverage of the whole breast and the reduction of heart and LAD doses. Material and Methods We performed a dosimetric study on 25 patients treated with DIBH for left breast cancer utilizing RPM (Varian Medical Systems). Treatment plans were generated in Free Breathing (FB) and DIBH. Optimization was done with 3D Field-in-Field technique utilizing two-tangent setup. Care was taken to cover the whole breast volume. Prescription dose was 50Gy in 25 fractions. Planning objectives were: near minimum dose (D98) > 90% (45Gy), near maximum dose (D2) <105% and a median dose of 50Gy. Doses to the heart, LAD and left lung were compiled, left breast coverage was evaluated, and statistical analysis was performed using Student T-test with a 95% Interval of confidence.