ESTRO 36 Abstract Book

S851 ESTRO 36 _______________________________________________________________________________________________

conformity, homogeneity and gradient score indexes were calculated and compared with published reports of HT radiosurgery. Furthermore, the V 12Gy of the healthy brain was also evaluated. A mask-based fixation system has been chosen, because of its high intracranial repositioning accuracy (always <1mm, further details have been discussed in another our work). Results The mean D 98% , D mean and D 2% of the PTV were (90.0±3.5)%, (99.4±1.8)%, and (106.9±3.1)% respectively. The minimal dose to the target was slightly lower than the objective, however it was sacrificed in order to have low doses to the healthy brain. The mean values of conformity, homogeneity and gradient score indexes were 1.4±0.2, 1.08±0.03 and 57±10 respectively and all indexes were comparable with published results (Table 1). The V 12Gy of the healthy brain was strictly respected except in two cases, where multiple lesions were treated and the prescription was 20Gy and 21Gy. The median V 12Gy was 8.8cc. No acute toxicity of any kind was recorded. The mean treatment time was 15±4minutes with a maximum of 46minutes for a patient treated with two consecutive plans irradiating 4 lesions. Conclusion HT radiosurgery for single and multiple brain metastases appears feasible with satisfying dosimetric results. Moreover, it appears to have encouraging clinical outcomes and the use of non-invasive set-up improves the treatment cosmetic and patient comfort. EP-1579 Practical dosimetrical issues in Intraoperative electron radiation therapy S. Wadi-Ramahi 1 , F. Alzorkany 1 , B. Moftah 1 , A. Alsuhaibani 2 1 King Faisal Specialist Hospital and Research Center, Biomedical Physics, Riyadh, Saudi Arabia 2 King Faisal Specialist Hospital and Research Center, Radiation Oncology, Riyadh, Saudi Arabia Purpose or Objective Our intraoperative radiation therapy (IORT) service is an active one treating around 50 patients/year. Sarcomas and gastric tumors are the most we treat with IORT. Certain cases pose clinical challenges, such as the use of cone blocking, abutting treatment fields, retreats, and bone partially covering the treatment area. The purpose of this work is to discuss some of these situations and our proposed solutions used clinically at the time. Material and Methods We have the Mobetron® linac (IntraOp, Ca) which can deliver 6, 9 and 12MeV high dose rate electron beams, and an array of cones ranging in diameter from 3 to 10 cm for zero, 15 and 30 deg bevel angles. Only three clinically- faced dosimetrical issues will be discussed here: a)Use of lead shielding to conform to an irregularly shaped tumor, b)The presence of bone in the treatment field as in intraarticular sarcoma and pelvic tumors, c)Abutting two electron cones to treat a long tumor bed. Results a) Following a case requiring ~30% of the area to be blocked, we measured the output for 30% blocked cones using a 0.125cc chamber in water. Each output was normalized to that of the 10cm open cone to calculate

OF blocked

. For cones with diameter > 6 cm the OF blocked was

within -5% of the OF open had a decreased OF blocked

, for those with diameter <6 cm reaching almost to 80% of the

OF open . This behavior was noted for all energies, Fig.1. The for treatment is a physics decision, taking into account the amount of blocked area of the cone, cone size and beam energy. Noting that the OF blocked were measured for 30% blocked area. b) For a tumor that is partially shielded by bone and treated with a single en face electron beam requires balancing penetration through the bone with exposing normal tissue. The thickness of the bone in a plane parallel to the direction of the beam is measured and then converted to effective depth by using the coefficient of equivalent thickness (CET) using the electron density of bone, 1.65electron/mL. This distorts the shape of the tumor by introducing deep arms, Fig2a. One such case was a sarcoma extending to the intraarticular space and the humeral head covering part of the field. The energy in this case is chosen such that 90% isodose is at the deepest part. Shielding of the normal tissue underneath the shallower parts of tumor is discussed and applied according to practicality of site. C) For long tumors, abutting circular cones at the surface will leave lateral parts under-dosed, while overlapping the cones will introduce high dose, Fig2b. The clinical decision for these cases is to abut at the surface to avoid hot spots in the overlapped area. Overlap of low doses from each cone at depth is deemed acceptable. use of the OF blocked