Abstract Book

S1235

ESTRO 37

demonstrated differences in all parametres. Only the LT Lung Dmax, RT Lung V20Gy, LT Lung V20Gy, Bilateral Lungs V20Gy, LT Lung V15Gy, Bilateral lungs V15Gy were statistically significant.As the V 20Gy parameter is a universal standard parameter used in determining acceptable lung dose, and with our study confirming a statistical difference in V 20Gy for the RT, LT and Bilateral lungs, this study suggests that the utility of non-rigid registration in clinical scenarios of re-irradiation to the ipsilateral lung where available is necessary. EP-2360 Optimal IGRT strategy for OAR sparing in radiotherapy of the prostate including pelvic lymph nodes A. Van Nunen 1 , D. Schuring 1 , T. Budiharto 1 1 Catharina Ziekenhuis, Radiotherapie, Eindhoven, The Netherlands Purpose or Objective Radiotherapy treatment for lymph node positive prostate cancer includes the pelvic lymph nodes as well as an integrated boost to the prostate. These target areas can move with respect to each other due to changes in rectal filling. Different correction strategies can be chosen to ensure either correct positioning of the prostate or the pelvic nodes, resulting in different PTV margins for both target volumes. Purpose of this study was to determine which correction strategy (and resulting margins) results in the lowest OAR doses in the treatment plans. Material and Methods For 30 patients a planning study was performed in which four different treatment plans were created in the RayStation TPS (Raysearch Laboratories). In these plans, different PTV margins were used reflecting different correction strategies (Table 1): (A) online correction on bony anatomy; (B) offline correction on bony anatomy; (C) online correction on the prostate markers; (D) Using our current standard 1 cm margins around both prostate and pelvic nodes CTV. The CTV-to-PTV margins needed were determined in a previous study, and account for setup errors and baseline shifts between prostate and lymph nodes. Depending on the correction strategy, this results in either larger margins to the prostate or pelvic nodes to account for the baseline shifts. A single-arc VMAT plan was created for all strategies using a fixed planning strategy, and care was taken that the PTV coverage was identical for all plans. For all plans, dose to the PTVs and OARs was evaluated, and a pair-wise statistical analysis was performed on this data to see if a significant difference in OAR dose could be observed.

bowel bag (V 45Gy ) is slightly higher when using online correction on the prostate compared to the other strategies, but well within clinically acceptable limits. ) and femoral heads (D 0.5cc

Conclusion Although the total PTV volume is higher when applying an online correction strategy on the prostate due to the larger margins around the pelvic nodes, this strategy leads to the most optimal sparing of the relevant OARs (rectum, anal canal and bladder), at the cost of a slightly higher dose to the femoral heads and small bowel bag. This correction strategy (and the resulting CTV-PTV margins) is now used clinically in our hospital. EP-2361 What is the gain of breath hold for re- irradiation of recurrent left-sided breast cancer with VMAT? K.F. Crama 1 , J. Visser 1 , N. Bijker 1 , M.W. Kolff 1 , A. Bel 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The use of a breath hold technique for left sided breast cancer is nowadays the standard to reduce high dose regions in the heart, especially when tangential fields are used. For most patients the heart moves away from the field borders when the patient is in breath hold, resulting in less overlap with the tangential fields than during free breathing. Re-irradiation of a recurrence of left sided breast cancer with in most cases a larger target volume, extending more to the medial side and posterior axillary line, often results in a higher heart dose compared with the primary treatment. At our clinic we use for treatment of these recurrences a Volume Metric Arc Therapy (VMAT) technique in free breathing, to get a more conformal plan. It is a completely different technique compared to tangential fields, because a VMAT technique gives a large region a low dose. The question is raised whether breath hold could also reduce the dose to the organs at risk further in

Results For all patients and correction strategies, a clinically acceptable plan was produced. No statistically significant differences were observed in the dose to the PTVs, ensuring that any OAR sparing is not caused by differences in PTV coverage. Figure 1 shows the dose parameters to the rectum, anal canal and bladder for the different strategies. Although online correction on the prostate leads to a higher total PTV volume than both correction strategies on the pelvic nodes, this strategy leads to a significantly lower dose to these OARs. This is due to a reduction of the high-dose PTV volume with respect to the other strategies. The dose to the small