P140] Flattening filter-free radiotherapy for paediatric brain cancer: Impact on the dose to selected out-of-field organs

Abstract

Purpose Removal of the flattening filter has been shown to alter the out-of-field dose during radiotherapy. In this study, the dose to selected out-of-field organs is compared for conventional flattened and flattening filter free (FFF) volumetric-modulated arc therapy (VMAT) of paediatric brain cancer. Methods Out-of-field doses to the thyroid, breast and testes were measured using thermoluminescence dosimeters inserted in two anthropomorphic phantoms resembling a 1-year and 5-year old child. Coplanar VMAT plans were prepared for 6 MV and 6 MV FFF photon beams, for two different PTV sizes (2 cm and 5 cm diameter), simulating post-operative radiotherapy of an ependymoma in the posterior fossa. The phantoms were subject to three VMAT fractions of 1,8 Gy each, delivered using a Varian TrueBeam linear accelerator. Some additional irradiations were performed for the 1-year phantom with the head of the phantom removed, to investigate the contribution from internal scatter to the total out-of-field dose. Results For the testes, measured doses showed a statistically significant decrease (Student’s t-test, significance level p 0,05) of 21–42% for FFF compared with conventional plans, for both phantoms as well as both PTV sizes. Doses to the breast showed a significant decrease of approximately 12% for the 5-year phantom for the small PTV, but other breast doses did not show a significant difference with and without the flattening filter. Doses to the thyroid showed a significant increase of approximately 18% for the 1-year phantom for the small PTV, with other dose differences being insignificant. However, doses to all three organs were significantly reduced (19–37%) when removing the head of the 1-year phantom, minimizing the contribution from internal scatter. Conclusions The results show that FFF VMAT of paediatric brain cancer leads to reduced doses to organs far from the field compared to VMAT with conventional flattened beams, while doses to organs close to the field seem to increase. The latter finding can be explained by an increase in internal scatter, which is possibly caused by the softer energy spectrum of the 6 MV FFF beam for the investigated linear accelerator model compared to conventional 6 MV.