ESTRO 37 Abstract book

S1060

ESTRO 37

Conclusion A dosimetric comparison based on well-known indexes was performed for ten breast patients with concomitant boost. For breast PTV a better CF was achieved with VMAT or 7F IMRT whether 3DCRT provided the lowest CF. The same thing was observed with respect to the boost PTV. HI was also better for 7F IMRT. For OARs sparing it was shown that 3DCRT provided the lowest values for contralateral breast and both lungs. VMAT spared the heart significantly better than other techniques. EP-1948 Craniospinal irradiation without geometrical beam overlap: An analytical solution. A. Ferrando 1 , A. Prado 2 , R. Díaz 1 1 Hospital Universitario 12 de Octubre, Servicio de Oncología Radioterápica. Sección de Radiofísica., Madrid, Spain 2 Hospital Universitario 12 de Octubre, Radiofísica y Protección Radiológica, Madrid, Spain Purpose or Objective In craniospinal irradiation it is mandatory to verify adjacent overlapping beams if over or underdosed regions are to be avoided. In this work a method for craniospinal treatment with an exact geometrical match of beam divergences is presented. Material and Methods The patient is set over the treatment couch in a cranio- caudal decubitus prono position and hyperextended neck. So as to immobilize the patients a thermoplastic mask is utilized. To design the geometry two treatment planning systems (TPS) were used: Varian Eclipse v11.0 and Elekta Oncentra ® v. 4.1. The cranial region was treated with two lateral 6 MV beams with gantry angles of 90º and 270º. The isocenter was placed in the center of the cranial region. The collimator was set to an angle θ 0 , the inferior jaw aperture was set to L 0 to avoid shoulder irradiation and the couch angle was set to RISO= arctg[L 0 /(cosθ 0 ·SAD)] (gantry=270º). The opposed field (90º) is adjusted using mirror angles to match beam divergences (figure 1).

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) .

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 MV. The treatment technique used is based upon four

Figure 1: Sagittal view of the isocenter’s plane and equations.