Out-of-field dose measurement in photon and proton craniospinal irradiation of paediatric patients – EURADOS WG9 phantom study

Background and Objectives Out-of-field doses, caused by stray radiation, may increase secondary cancer risk for radiotherapy patients. Children are of particular concern due to high radiosensitivity and long-life expectancy. Therefore out-of-field dose measurement of different radiotherapy techniques is extremely important for risk assessment and radiation protection. Craniospinal (CS) irradiation increased tremendously survival rate for the patients with medulloblastoma which is the most common malignant brain tumour in children. This study, carried out within EURADOS Working Group 9 (Radiation Dosimetry in Radiotherapy), evaluates and compares out-of-field doses for paediatric medulloblastoma treatment using photon and proton CS irradiation. Materials and Methods Photon and proton irradiations were performed on Siemens Artiste linac at UCHCSM UHT (Zagreb) using 3D conformal radiotherapy (3D-CRT) and at CCB (IFJ, Krakow) using pencil beam scanning (PBS) technique (IBA Proteus 235 gantry) respectively. CS treatments of the same target volume (whole brain and spinal cord irradiation + 0.5 cm margin) in CIRS anthropomorphic phantoms representing 5-year and 10-year old children were performed. For 3D-CRT, 6 MV photon beams were used for brain and 6 MV + 18 MV for spinal cord irradiation. The stray doses were measured with three types of thermoluminescent detectors (MTS-7: 7LiF (Mg, Ti) and MTS-6: 6LiF (Mg, Ti) for 10-year ; MCP-n: LiF(Mg, Cu, P) for 5-year) and radiophotoluminescent (RPL) detector type GD-352M. The neutron contribution during PBS was determined with poly-allyl-diglycol-carbonate (PADC) nuclear track detector in 10-year phantom. For 3D-CRT dosimeters were placed in all out-of-field organs, while for PBS dosimeters were put in the selected radiosensitive organs close to spinal cord. Results and Discussion For out-of-field doses during 3D-CRT CS radiotherapy excellent agreement between RPL and MCP-n detectors was confirmed: the RPL/MCP ratio for the same point in 5-year phantom was on average 1.02  0.15. Due to the use of 18 MV photon beams, higher doses were measured with MTS-6 than with MTS-7: the highest gamma-equivalent neutron dose per target dose in 10-year phantom were 293 and 181 mGy/Gy for esophagus and TL/spine respectively. Mean organ doses for both phantoms were comparable with exception of pancreas, stomach and four organs (uterus, adrenals, mandible, testes) with a factor 3.5, 2 and 1.5 higher values for 5-year phantom. This finding is expected due to the closer proximity of organs to the central spine axis in the 5-year phantom. Comparison of stray radiation doses for proton and photon radiotherapy for 10-year phantom is shown in Figure 1. For points in the selected organs shown in Figure 1, gamma doses (RPL) are 2-3 orders of magnitude lower for PBS in comparison to 3D-CRT (MTS-7). For PBS, measured neutron dose equivalents (PADC) are a factor of 5 higher than gamma doses (RPL). In other points (closer to the central spinal axis) of the selected organs (including lungs, thyroid, esophagus), PADC were over-irradiated while measured gamma doses showed a factor 2 to 2000 lower values in comparison to 3D-CRT. Conclusion For measured points CS treatment with protons showed lower out-of-field doses for paediatric patients in comparison to photons.