Abstract Book

S1112

ESTRO 37

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, dynamic irradiation techniques in hypo-fractionated lung treatment are discouraged. This work evaluates the dosimetric variations in hypo- fractionated lung irradiations with helical IMRT ( Tomotherapy ), analyzing the influence of the internal tumor volume (ITV) density and the type of target motion (periodic and non-periodic with drift). Material and Methods Longitudinal respiratory movements, both periodic (sinusoidal and cosine to the fourth power, amplitude: 1.5 cm, period: 6 s) and irregular with baseline drift, were simulated using the QUASAR ( Modus ) phantom. A mobile cedar insert containing two 3 cm diameter acrylic

semi-spheres was used, thus allowing EBT3 radiochromic films to be inserted in.

Dose distributions were calculated with Tomotherapy planning system to deliver 20 Gy to the whole ITV. The CT imaging studies used correspond to the most representative position of each respiratory pattern (mid or exhale) with the original density of the image acquisition and also forcing the ITV density to that of the sphere. Planning parameters compatible with the technical constraints of the unit (rotation period lower than 60 s) were chosen to allow high dose per fraction: 0.100 pitch, modulation factor lower than 2, and field width of 2.5 cm. For each batch of measurements, a no-motion irradiation with rest phantom was performed to consider for output dose variations. The treatment plans calculated for heterogeneous and homogeneous ITV density were delivered for the respiratory patterns proposed. The films were scanned with an Epson 10000XL and using the green channel to obtain the dose with the ImageJ software. Longitudinal profiles were analyzed. Results The films irradiated on the moving phantom showed the expected penumbra widening, but dose variations at the sphere region were less than 3% for periodic patterns, whereas for the irregular pattern were up to 13%. The dose differences observed between homogeneous and heterogenous ITV density were higher for the irregular pattern (4% vs 3%) with a dose increase for heterogeneous case. Conclusion For periodic movements, Tomotherapy hypo-fractionated lung treatments do not yield dose perturbations due to the interplay effect. For irregular movements, the dose increases are restricted to the target area. The influence of the ITV density on dose distribution calculation is negligible, although the magnitude of the interplay dose perturbations is reduced by homogenizing the density. EP-2037 First clinical use of a new surface tracking/biofeedback system: DIBH reproducibility and stability E. Steiner 1 , D.D. Campos 2 , P. Keall 1 , K. Makhija 1 , B. Stanley 2 , T. Yamamoto 2 , M. Daly 2 , Y. Rong 2 1 The University of Sydney, Clinical Medical School - Central- ImageX Institute, The University of Sydney, Australia 2 University of California at Davis Comprehensive Cancer Center, Department of Radiation Oncology, Sacramento, USA Purpose or Objective Deep inspiration breath hold (DIBH) halves the cardiac dose for left-sided breast radiotherapy [Nissen2012]. Visual guidance improves the reproducibility and stability of DIBH [Cervino2009]. This work evaluates the reproducibility and stability of DIBH using the novel