Abstract book - ESTRO meets Asia

S118 ESTRO meets Asia 2018

Purpose or Objective The applicability of the linear quadratic (LQ) model to local control (LC) modeling after hypofractionated radiotherapy to treat lung cancer is highly debated. To date, the differences in predicted outcomes between the LQ model and other radiobiological models, which are characterized by additional dose modification beyond a certain transition dose (d T ), have not been well established. This study aims to compare the outcomes predicted by the LQ model with those predicted by two other radiobiological models in stereotactic body radiotherapy (SBRT) for non-small cell lung cancer (NSCLC). Material and Methods Computer tomography (CT) simulation data sets for 20 patients diagnosed with stage Ⅰ primary NSCLC were included in this study. SBRT treatment plans were designed using a single 30 Gy fraction. Three radiobiological models, including LQ, the universal survival curve (USC) and the modified linear quadratic and linear (mLQL) model were employed to predict tumor control probability (TCP) data. First, the d T values for the USC and mLQL models were determined. Then, the biologically effective dose (BED) and the predicted TCP values from the LQ model were compared with those calculated from the USC and mLQL models. Results The d T values from the USC model were 29.6 Gy, 33.8 Gy and 44.5 Gy, whereas the values were 90.2 Gy, 84.0 Gy and 57.3 Gy for the mLQL model for 1-year, 2-year and 3- year TCP prediction. The much higher d T values obtained from the mLQL model revealed the same dose-response relationship as the LQ model in the low- and high-dose ranges. Additionally, we also found that TCP prediction from the LQ and USC models differed by less than 3%, although the BED values for the two models were significantly different. Conclusion Radiobiological analysis reveals small differences between the models and suggested that the LQ model is applicable for modeling LC using SBRT to treat lung cancer, even when an extremely high fractional dose is used. PO-287 The effect of isocenter dose on tumor local control and toxicities in SBRT for lung cancer B.T. Huang 1 , W.Z. Zhang 1 , P.X. Lin 2 , J.Y. Lu 1 1 Cancer Hospital of Shantou University Medical College, Department of Radiation Oncology, Shantou, China 2 The Second Affiliated Hospital of Shantou University Me dical College, Department of Nosocomial Infection Management, Shantou, China Purpose or Objective The isocenter dose (ID) in the tumor was reported to be highly associated with local control (LC) in stereotactic body radiotherapy (SBRT) for non-small cell lung cancer (NSCLC). However, whether higher ID will impact on the normal tissues remains unclear. The aim of the study is to investigate the effect of ID on LC and related toxicities using radiobiological analysis. Material and Methods Computed tomography (CT) simulation data for 20 patients previously diagnosed with primary stage Ⅰ NSCLC were included in the study. Treatment plans based on four fraction schemes (FS) of 1×30 Gy, 3×15 Gy, 4×12 Gy and 3×18 Gy were designed, respectively. Different ID was generated in each FS to investigate their influence on LC and related toxicities. 3-year LC, radiation-induced pneumonitis (RIP), chest wall pain (CWP) and radiation- induced rib fracture (RIRF) were predicted with radiobiological models.

dose of 43 Gy (range, 42-44) while the novel technique delivered a mean maximum dose of 44 Gy (range, 43-45). Conclusion The novel technique permits the delivery of a substantially higher dose to the posterior cervical lymph nodes in comparison to convential treatment, without compromising dose to the spinal cord. This simple technique could be used to treat patients with squamous cell carcinoma of the head and neck in centres without facilities to deliver IMRT. PO-285 Investigation of set up error in CBCT for whole breast radiation with breast board immobilization S. Yu 1 , S. Wang 2 , Y. Tang 2 , Y. Song 2 , S. Chen 2 , M. Li 2 , J. Jin 2 , Y. Liu 2 , H. Fang 2 , B. Chen 2 , S. Qi 2 , N. Li 2 , Y. Tang 2 , N. Lu 2 , Y. Li 2 1 Beijing Chaoyang Hospital, Department of Oncology, Beijing, China Purpose or Objective To investigate the set-up errors of the whole breast intensity-modulated radiotherapy (IMRT) after breast conserving surgery, and to evaluate the margins from clinical target volume (CTV) to planning target volume (PTV). Material and Methods Thirty patients with left-sided or right-sided breast cancer after breast-conserving surgery in our hospital were enrolled. All of them received hypofractionated whole breast irradiation with breast board immobilization. The kilovoltage cone-beam computed tomography (CBCT) was used to compare the deviations of planning CT and treatment unit and the margin of target volume was calculated. An independent two-sample t-test was used to analyze the differences of set-up errors. Results A total of 151 sets of CBCT images were taken in the whole cohort, and average (5.0±1.3) per patient. The set-up deviations in the x-axis (left-right direction), y-axis (cranial-caudal direction) and z-axis (anterior-posterior direction) were 2.2±1.7 mm, 3.1±2.5 mm, 3.3±2.3 mm, respectively. Margins from CTV to PTV were 6.39 mm, 10.0 mm and 8.57 mm, respectively. The set-up deviations in anterior-posterior direction in the first week was statistically larger than those of the following weeks (3.7±2.5mm and 2.6±1.6mm, p=0.002), and the set-up deviation of the overweight or obesity was significantly higher than patients whose BMI in normal range in z-axis direction (3.9±2.6mm and 2.9±2.0mm, p=0.033). Conclusion The margins ， from CTV to PTV ， between 6 and 10mm were recommended for patients who received hypofractionated whole breast radiation immobilazed with breast board. More frequent imaging verification should be applied in the first week of treatment. PO-286 Comparison of three radiobiological models in stereotactic body radiotherapy for lung cancer B.T. Huang 1 , Z. Lin 1 , W.Z. Zhang 1 , P.X. Lin 2 , J.Y. Lu 1 1 Cancer Hospital of Shantou University Medical College, Department of Radiation Oncology, Shantou, China 2 The Second Affiliated Hospital of Shantou University Me dical College, Department of Nosocomial Infection Management, Shantou, China 2 Chinese Academy of Medical Sciences, Department of Radiation, Beijing, China Physics: Dosimetry, audits and risk assessment Physics: IGRT, patient positioning and mobilisation