ESTRO 37 Abstract book

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

greater MU to deliver the same prescription as the present study did not seek to address this. EP-2359 Comparing lung dose parametres between rigid and non-rigid registration in advance stage lung cancer X. SUSAINATHAN SELVANATHAN 1 , J.Y.S. CHER 1 , T.Y. CHAN 1 1 National Cancer Institute- Singapore, RADIATION ONCOLOGY, SINGAPORE, Singapore Purpose or Objective In re-irradiation, the cumulative lung dose from both the previous and current radiation plans are of clinical significance. Before the advent of non-rigid registration (also known as deformable registration), radiation plans were fused using rigid registration. This may affect the accuracy of lung dose calculations. Differences in lung doses are expected when using non-rigid registration. Hence, the objective of this study is to evaluate whether there is a difference in lung doses with rigid and non- rigid registration using 5 parametres:D minimum (D min ),D maximum (D max ),D mean ,Volume receiving 20Gy(V 20Gy ) and Volume receiving 15Gy(V 15Gy ). Material and Methods 12 consecutive advance stage lung cancer patients receiving re-irradiation were selected. Eclipse™ Treatment Planning System was utilized to perform rigid registration. Velocity™ was utilized to perform non-rigid registration. In both registration techniques, lungs dose parametres values were extracted from the dose volume histogram (DVH). The paired-sample t test was performed. Results Results are significant if p<0.05.The results are right(RT) lung D min ( Rigid vs Non-Rigid: 71.2cGy vs 69.4cGy, p=0.30), RT lung D max ( Rigid vs Non-Rigid: 5668.6cGy vs 5738.6cGy, p=0.06), RT lung D mean ( Rigid vs Non-Rigid: 1811.9cGy vs 1803.1cGy, p=0.80), RT lung V 20Gy ( Rigid vs Non-Rigid: 40.2% vs 39.0%, p=0.04), RT lung V 15Gy ( Rigid vs Non-Rigid: 46.8% vs 45.7%, p=0.08), left(LT) lung D min ( Rigid vs Non-Rigid: 36.8cGy vs 38.4cGy, p=0.27), LT lung D max ( Rigid vs Non-Rigid: 4986.2cGy vs 4890.9cGy, p=0.03), LT lung D mean ( Rigid vs Non-Rigid: 863.9cGy vs 1005.8cGy, p=0.42), LT lung V 20Gy ( Rigid vs Non-Rigid: 19.1% vs 18.0%, p=0.01), LT lung V 15Gy ( Rigid vs Non- Rigid: 24.1% vs 22.8%, p=0.01), Bilateral lungs D min ( Rigid vs Non-Rigid: 29.5cGy vs 34.8cGy, p=0.11), Bilateral lungs D max ( Rigid vs Non-Rigid: 5818.5cGy vs 5841.6cGy, p=0.04), Bilateral lungs D mean ( Rigid vs Non-Rigid: 1192.3cGy vs 1290.0cGy, p=0.03), Bilateral lungs V 20Gy ( Rigid vs Non-Rigid: 25.3% vs 24.4%, p=0.03), Bilateral lungs V 15Gy (Rigid vs Non-Rigid: 31.0% vs 30.0%, p=0.03). Conclusion To date, studies have demonstrated that non-rigid registration allows greater accuracy in estimating cumulative lung doses as changes in lung volumes, body shape and treatment positions of the patient are better accounted with non-rigid registration. This study 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.