ESTRO 37 Abstract book

S1114

ESTRO 37

2 Hospital Universitario HM Sanchinarro, Oncología Radioterápica, Madrid, Spain

approach can be applied to stereotactic radiosurgery and other high precision radiotherapy techniques. Material and Methods The system is comprised of an array of conductive sensors arranged around the patient’s cranium during the SRS treatment. Finite Element Analysis (FEA) was employed to determine a viable sensor array arrangement for 3D detection. Capacitance of the sensor array is monitored continuously and calibrated to patient position. The system is unique in that it is not sensitive to the position of the thermoplastic mask but registers the motion of the cranium. Using an experimental prototype, the first phase of development was focused on detecting translational motion of the cranium. Prototype testing was conducted with the help of a volunteer in a thermoplastic mask. Experiments were performed using a hexapod stage capable of performing 3D motion to simulate patient motion by moving the sensor array relative to the cranium. Four copper sensors (4 by 6 inch) were used in the arrangement shown in figure 1 around the volunteer’s cranium. Motion was introduced in millimeter steps and data were gathered for 3 seconds at each position for all sensors. The average sensor response and the standard deviation were calculated and plotted. Results The results show a sensitivity of over 4 counts/mm for the first 4 mm of motion in the superior direction and a differential signal change of over 10 counts/mm for lateral direction derived from the two lateral sensors. The results of lateral and superior-inferior motion are shown in figures 1 and 2, respectively, demonstrating detection of both superior-inferior and lateral motion with sub millimeter accuracy. Notably, motion in a given direction only affects the signal along the relevant axis, without significant cross-talk. Conclusion The technology described here shows promise in providing real-time, continuous, and sub-millimeter 3D monitoring of the cranium without the need for ionizing radiation or the use of skin as a surrogate.

Purpose or Objective To investigate the frequency and amplitude of baseline or drift of pancreas tumors in stereotactic body radiation therapy (SBRT), using a gating respiratory system radiation therapy with intra-fractional IGRT. Material and Methods 76 fractions of 15 patients with pancreas tumors were treated with SBRT using ExacTrac Adaptive Gating® in a Novalis® linac. This system uses external markers to control the respiratory cycle and internal fiducial markers to measure the movement of the tumor and set up the patient. The tumor position is measured intermittently during the treatment via stereoscopic x-ray images to compensate the baseline drift. Therefore, the accumulative changes in the couch position correspond to the baseline drift in the tumor motion. Results The average change in position of the treatment couch during the treatment time was -0.2 ± 1.4 mm (mean ± standard deviation), 0.1 ± 1.6 mm, and 0.7 ± 3.7 mm in the left-right (LR), antero-posterior (AP) and cranio- caudal (CC), directions respectively. Overall the baseline shift/drift occurs toward the cranial directions. The incidence of a baseline drift exceeding 3 mm was 13%, for the CC direction, within 15 minutes of the start of treatment, and 38% within 30 minutes. On the other hand, the incidence of a baseline drift exceeding 5 mm was 6% for the CC direction, within 20 minutes of the start of treatment, and 21% within 30 minutes. The intra-fractional uncertainties due to baseline drift of pancreas tumors are: Direction M (mm) Σ (mm) σ (mm) LR -0.1 0.3 0.7 AP 0.0 0.4 0.8 CC 0.4 1.1 1.6 M: the overall mean or group systematic error, Σ: the standard deviation (SD) of the systematic error, σ: the SD of the random error. In the absence of intra-fractional IGRT, the baseline drift uncertainties imply the use of increased margins of up to 1 mm in the CC direction, in SBRT for pancreas tumors, when the rest of uncertainties are minimized. Conclusion Real-time monitoring and frequent adjustments of the couch position are suggested to be necesary to compensate for possible underdosage in CC direction due to baseline drift in SBRT for pancreas tumors. EP-2036 Dosimetric issues in pulmonary Stereotactic Body Radiotherapy (SBRT) treatments with Tomotherapy R. Rodríguez Romero 1 , P. Sánchez Rubio 1 , A. Montes Uruén 1 , N. Gómez González 1 1 Hospital Universitario Puerta de Hierro - Majadahonda, Radiofísica y Protección Radiológica, Madrid, Spain Purpose or Objective The irradiation of a moving target alters the dose distribution with respect to that calculated by the treatment planning system. These dose perturbations are due to two distinct effects: dose blurring, which enlarges the penumbra in the movement direction; and the interplay of the target motion with the dynamic irradiation, which may cause significant underdosing or overdosing on the target. The amount of dose variation due to the interplay effect can be magnified when regions of great heterogeneity are irradiated, and treatments are delivered in a few sessions. Thus,

EP-2035 Intrafractional baseline drift in SBRT of pancreas tumors J. García Ruiz-Zorrilla 1 , M.A. De la Casa de Julián 1 , C. Rubio Rodriguez 2 , P. Fernández Letón 1 , J. Martí Asenjo 1 , D. Zucca Aparicio 1 , O. Hernando Requejo 2 , M. López González 2 1 Hospital Universitario HM Sanchinarro, Radiofísica, Madrid, Spain