Abstract Book

S1002

ESTRO 37

shielding (1 mm lead equivalent) was applied to each phantom using lead rubber sheets from decommissioned lead aprons (Mavig Multiplex, Mavig GmbH). Furthermore, a 1 mm rectangular lead plate was placed between the treatment couch and the phantom. Coplanar double-arc VMAT plans (6 MV x-rays) were made in Varian Eclipse 13.0, with the target situated in the superior fossa. For each phantom, plans were made for two different spherical sizes of PTV (2 cm and 5 cm diameter) to investigate the dependence of PTV size on out-of-field dose. Each treatment plan should deliver a dose of 54 Gy to the PTV in 30 fractions, using a Varian Truebeam linear accelerator. The clinical equivalence of the plans for small and large PTV was validated by comparing the plan conformity index. For each phantom, out-of-field dose measurements were performed for small and large PTV, with and without lead shielding of the phantom’s torso (see Figure 1). TLDs were calibrated using a Sr-90 source prior to the measurements. Six TLDs were inserted at each organ location and subject to three fractions of 1,8 Gy delivered to the PTV. The doses to the TLDs were subsequently read using a Mirion RE-2000A TLD reader.

lead shielding in paediatric radiotherapy of brain tumours. A 1 mm thick lead blanket can be considered light enough to be endured by a paediatric patient, and can be easily obtained from commercially available materials. The resulting dose reductions will reduce the probability for secondary cancer incidence in radiosensitive organs. EP-1857 Feasibility of Gafchromic EBT3 film for measuring out-of-field dose in radiotherapy T. Viren 1 , A. Saikkonen 2 , J. Niemelä 2 , J. Keyriläinen 2 , S. Salomaa 3 , J. Seppälä 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 Turku University Hospital, Department of Oncology and Radiotherapy, Turku, Finland 3 University of Eastern Finland, Department of Environmental and Biological Science, Kuopio, Finland Purpose or Objective The aim of the present study was to evaluate the feasibility of Gafchromic EBT3 film for the measurement of out-of-field (OF) doses. Material and Methods Gafchromic EBT3 films (ISP Inc., USA) were calibrated using method described by Peet et al. 1 . Subsequently, stack of two films (size: 5x15cm) were attached in to a custom made holder within a water phantom (Blue phantom 2 , IBA Dosimetry GmbH, Germany). The short side of the films were positioned at the central axis (CA) of beam (SSD=785mm and IRIS collimator with diameter (Ø) of 50mm and 15mm) from CyberKnife (Accuray Inc., USA) treatment machine at dose maximum depth (d max =15mm) and 20000MUs (roughly corresponds to the number of MUs delivered in prostate treatment) was delivered. Next, film holder was moved 75 mm away from CA at y-direction and measurement was repeated with new films. The measurements were repeated once and the final beam profile was determined as a mean of four films. RPL measurements were conducted simultaneously with first film measurements. RPL dosimeters (Dose Ace GD-302M, AGC Techno Glass Co., Japan) were attached to film holder at distances of 50, 100, and 200 mm; and 100 and 200mm from CA for 50mm and 15mm collimators, respectively (Fig 1). Subsequently, RPL dosimeters were read using calibrated read-out device. Finally, beam profiles corresponding to the film measurements (x and y directions at CA, length of scan 460mm) and absolute dose at d max were measured in water phantom using Farmer type ICs (profiles: TW30013, PTW-Freiburg GmbH, Germany; absolute dose: NE 2571, NE Technology Ltd, UK) and electrometers (profile: CCU, IBA Dosimetry GmbH, Germany; absolute dose: NE 2503/3 NE Technology Ltd., UK). For comparison, dose distributions corresponding the measurements were calculated in water phantom created in MultiPlan TPS (Accuray Inc., USA). Monte Carlo (MC) algorithm was used. Finally, the film measurements were compared to IC and RPL measurements and to dose calculation. Results The film measurements agreed to doses measured with IC in the OF area of a single beam in water (Fig. 1, Table 1). The doses with RPL measurements were in line with doses measured with IC and film in the OF area, however, higher difference was detected near the field edge. The MC calculation underestimated the dose in comparison to measurements.

Results Measured organ doses are presented in Table 1 for both phantoms and both PTV sizes, reported for the total treatment dose (54 Gy to the PTV). As expected, measured doses were higher for the large PTV compared to the small PTV for both phantoms. Shielding of the phantoms yielded a dose reduction of 8-18 % for thyroid, 15-18 % for mamma and 16-23 % for testes. The dose reduction in the three investigated organs was statistically significant (Student’s t-test, significance level p<0,05) in all cases except for the thyroid for the large PTV in the 5-year old phantom. No systematic differences in dose reduction were seen with PTV size or child age.

Conclusion Application of a 1 mm lead equivalent shielding of the phantoms resulted in a systematic decrease in the measured dose to the thyroid, mamma and testes, suggesting a clinically relevant application of out-of-field