Abstract Book

S1059

ESTRO 37

Figure 1: Sagittal view of the isocenter’s plane and equations. To treat dorsal, lumbar and the remaining cervical regions using the first posterior field, the couch position must be set to 270º. The collimator was set to 90º to define the field union with the jaw instead of using the MLC (higher positioning uncertainty). If a cranial field aperture of L 2 1) and a caudal jaw aperture of L 1 1) are set, it shall be necessary to move the couch longitudinally Δy 1 (Eq.1). The new isocenter would have the same lateral and vertical coordinates (x 0 ,z 0 ) as the previous one. To match the cranial field divergence the gantry angle should be set to α 1 (Eq.2). Following the same procedure the rest of the spinal beams were set; its number would be determined by L 1 i) and L 2 i) . Gantry angles α i+1 and couch displacements Δy i+1 are defined in eq.3 and eq.4. Results Plans generated using two different TPSs showed that beam divergences match exactly (figure 2). Small discrepancies might be encountered between TPSs used because gantry angles and jaw positions are rounded to one decimal. The number of fields used would depend on the jaw apertures selected. A convenient choice would result in small gantry angles, improving the control of beam exits in the sacral spine, preventing a possible gonad irradiation in the case of patients with large spine. An executable code was programmed in Visual Basic 6.0 to accelerate the calculation of α i+1 and Δy i+1 values after providing θ 0, L 1 i) and L 2 i) .

MV. The treatment technique used is based upon four tangential fields, utilizing 6 MV for both internal fields and one external field and 15 MV for another external field. Once the 3DCRT plan was created, the same plan was reproduced employing an estimation of RM given by eq.1., where A i is the amplitude in the i direction; T, T inh and T exh stand for the complete respiratory cycle period, inhalation and exhalation phases periods, respectively.The original plan was divided into 10 subplans. In these plans the isocenter was shifted by a quantity of 1/5·A i , thus simulating different points of the respiratory cycle. MUs are proportional to the time in which the isocenter moves from one position to another.Finally, the plan which took into account the RM was the sum of the 10 isocenter-shifted plans. Values used for breast displacement due to RM were calculated as the mean values of several amplitudes found in the literature, given by (A LAT , A AP , A SI ) = (1, 2.3, 1.6) mm. The complete cycle period T is set to 3.5 s, T exh is calculated as 0.635 · T and T inh =1-T exh . The same procedure was performed multiplying by 5 the amplitudes to quantify the results on a quite unlikely scenario. A dosimetric analysis of the generated treatment plans was performed. For breast PTV homogeneity Index (HI), defined as (D 2% -D 98% )/D avg and Paddick Conformity Factor (FC) were evaluated. For contralateral (left) lung, heart and left breast D 2% was evaluated. For ipsilateral (right) lung, V 20Gy , V 5Gy and D avg were considered. Results Figure 1 summarizes the results for breast PTV. No differences were found in HI or CF considering RM. Moreover, no differences were found when multiplying by five the breathing amplitude.

Figure 1: Results obtained for breast PTV regarding homogeneity index (HI) and Paddick conformity factor (CF). Respiratory amplitudes equations.

Fig. 2. Reconstruction of the treatment geometry in Eclipse. Conclusion A craniospinal treatment methodology is described without the existence of adjacent field overlaps, being quite easy to implement. This technique simplifies craniospinal treatment in the unit as only longitudinal couch displacements are required to match the spinal fields. Furthermore, the radiation therapist could use skin marks as a redundant check of field limits and global positioning. EP-1949 Estimation of the influence of respiratory motion on 3DCRT breast treatments. M. Leonor 1 , A. Prado 1 , R. Díaz 2 1 Hospital Universitario 12 de Octubre, Radiofísica y Protección Radiológica, Madrid, Spain 2 Hospital Universitario 12 de Octubre, Servicio de Oncología Radioterápica. Sección de Radiofísica., Madrid, Spain Purpose or Objective To estimate respiratory motion (RM) effect on dose distributions for 3DCRT treatments in breast tumors evaluating several dosimetric indexes. Material and Methods Dose calculations were performed using Eclipse v.11 (Varian Medical Systems. Palo Alto, CA) with the AAA algorithm (Analytical Anisotropic Algorithm). All plans were generated on a Varian Clinac iX linac with 6 and 15

Table 1: OAR results for 3DCRT, 3DCRT considering

RM and 5 times the RM amplitudes.

Table 1 results state that discrepancies between 3DCRT and RM plans were negligible, showing a subtle increase when comparing 3DCRT with a larger amplitude displacement (5A i ). Moreover, for amplitudes wider than 5A i discrepancies greater than 10% in OAR indexes could be found, although these values are too low compared with the organ limits. Conclusion Indexes evaluated do not differ significantly from the initial plan, where RM was not taken into account. These results bring to light the fact that tangential field breast treatments are quite robust with respect to RM. As a conclusion, RM has a negligible influence on 3DCRT breast treatments.