ESTRO 37 Abstract book

S1002

ESTRO 37

Results Multiplan MC algorithm underestimated the out-of-field mean dose in all of the three measurement planes. Up to 36.4% difference was detected between measured and calculated mean dose and the difference increased as a distance from high dose region (Tab. 1). After applying the correction factor for the calculated out-of-field region the difference between calculated and measured mean doses decreased significantly (Tab. 1).

These values are within ±34% of those reported for CT scans 0.027, 0.024, and 0.020 (mSv/mGy.cm) for the body protocols, respectively, whereas CC E values of the head scan were about double of that for CT head scans (0.0020 (mSv/mGy.cm)).

Conclusion The results demonstrate that it is recommend taking into account beam width of CBCT scans when using conversion coefficients to estimate E values. The use of the coefficients reported for CT scans leads to relatively small variations for the body scans, but large differences for the head scan. EP-1855 Accuracy of MultiPlan Monte Carlo dose calculation algorithm to determine whole body mean dose T. Viren 1 , S. Salomaa 2 , J. Seppälä 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 University of Eastern Finland, Department of Environmental and Biological Science, Kuopio, Finland Purpose or Objective The aim of the present study was to determine accuracy of MultiPlan (Accuray, USA) Monte Carlo (MC) based dose calculation algorithm for evaluation of whole body mean dose in stereotactic body radiotherapy (SBRT) treatment of prostate cancer. Material and Methods Gafchromic EBT3 films were calibrated using method introduced by Peet et al. 1 (Fig 1). Unirradiated films (8x10 -inch sheet) were first scanned in RGB format (Epson Perfection V700, Epson, Japan). Next, stacks of two films (8x10 inch) were placed in three separate planes (0, 7.5cm and 15cm from target plane) inside anthropomorphic phantom (Fig 1). Planning CT image was acquired and a typical SBRT treatment plan of prostate cancer (5x7.25Gy) was calculated in the phantom with MC algorithm (MultiPlan v5.2, Accuray, USA). Next, the plan was delivered into phantom with CyberKnife treatment machine (Accuray, USA). All five fractions were delivered in one session after which the measurement was repeated for fresh set of films. The irradiated films were scanned 24 hours after the irradiation and the film optical density was converted to absorbed dose using calibration curve. Final dose distribution in each measurement plane was calculated as a mean of four films. Measured mean dose in each plane was subsequently, determined and compared with dose calculation. Only dose levels <2Gy were included in the analysis. Previously, based on extensive measurements of CyberKnife system performed in water phantom, it was observed that the studied MC algorithm underestimates phantom mean doses on average by 34.2%. To investigate the possible improvement of the whole body mean dose calculation accuracy the out-of-field region dose distributions (<0.5Gy) were up scaled by a factor of 0.342 and the doses were evaluated.

Figure 1. A) Film calibration curve. B) EBT3 films placed in 3 planes inside the phantom. C) Measured and D) calculated dose distribution in plane 7.5cm from center of target. E) Calculated dose distribution with dose correction applied to low dose region. Table 1. Difference (%) between measured and calculated mean dose in amorphous phantom with and without correction of out-of-field doses.

Conclusion MultiPlan MC algorithm underestimates whole body mean dose. For this particular case one constant correction factor applied for out-of-field regions resulted in satisfying dose calculation accuracy in the determination of mean dose to amorphous phantom. By taking into account the beam size and the corresponding MUs could yield more robust correction for mean dose. References 1. Peet SC et al. Medical Physics 54, 2016 EP-1856 Reducing out-of-field organ doses during paediatric brain cancer radiotherapy using lead shielding A.R. Beierholm 1 , D.E. Nygaard 2 , E. Lagoni Juhl 3 , R. Hansen 4 , J. Hansen 4 1 Danish Health Authority, Radiation Protection, Herlev, Denmark 2 Metropolitan University College, Bachelor’s Degree Programme in Radiography, Copenhagen, Denmark 3 Copenhagen University Hospital, Department of Oncology, Copenhagen, Denmark 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Radiotherapy exposes organs located out-of-field to dose from leakage, collimator scatter and patient scatter. This study examines how the out-of-field dose to selected radiosensitive organs is affected by applying a 1 mm lead shielding during delivery of volumetric-modulated arc therapy (VMAT) for paediatric brain cancer. Material and Methods Out-of-field dose to the thyroid, mamma and testes was measured using thermoluminescence dosimeters (TLD- 700) inserted in two CIRS ATOM anthropomorphic phantoms resembling a 1-year and 5–year old child. Lead shielding (1 mm lead equivalent) was applied to each