The scatter photons and photoneutrons from high energy photon beams (more than 10 MV) will increase the undesired dose to the patient and the staff working in linear accelerator room. This undesired dose which is found at out-of-field area can increase the probability of secondary malignancy. The purpose of this study is to determine the equivalent dose of scatter photons and neutrons generated by 3 different treatment techniques: 3D-conformal, intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). The measurement was performed using two types of the optically stimulation luminescence detectors (OSL and OSLN) in the Alderson Rando phantom that was irradiated by 3 different treatment techniques following the actual prostate cancer treatment plans. The scatter photon and neutron equivalent dose were compared among the 3 treatments techniques at the surface in the out-of-field area and the critical organs. Maximum equivalent dose of scatter photons and neutrons was found when using the IMRT technique. The scatter neutrons showed average equivalent doses of 0.26, 0.63 and 0.31 mSv․Gy -1 at abdominal surface region which was 20 cm from isocenter for 3D, IMRT and VMAT, respectively. The scattered photons equivalent doses were 6.94, 10.17 and 6.56 mSv․Gy -1 for 3D, IMRT and VMAT, respectively. For the 5 organ dose measurements, the scattered neutron and photon equivalent doses in out of field from the IMRT plan were highest. The result revealed that the scatter equivalent doses for neutron and photon were higher for IMRT. So the suitable treatment techniques should be selected to benefit the patient and the treatment room staff.

Background: The Monte Carlo (MC) simulation is an effective tool for determining the absorbed dose in small field sizes. To calculate accurate results, the MC simulation requires precise geometric and material descriptions of the linear accelerator head. Due to proprietary information issues, the description of the Varian TrueBeam™ linear accelerator (Varian Medical Systems, Palo Alto, CA) head geometry and material information are not available. Instead, the manufacturer provided a phase-space file just above the jaw for each photon energy level. Although several studies have validated the accuracy of this phase-space file, to the best of our knowledge, there are no reported data for a small field size (<2x2 cm2) of 6 MV photon beams. Objectives: The purpose of this study was to evaluate the Varian TrueBeam™ phase-space file of the 6 MV photon beam provided by the manufacturer for the Monte Carlo (MC) simulation in small field dosimetry. Materials and methods: The TrueBeam™ linear accelerator was simulated using an EGSnrc MC code with a Varian phase-space file as the input. The simulation was compared with the measurement using percent depth dose (PDD) and beam profile, and the field output factor (FOF) for the 0.6x0.6, 1x1, 2x2, 3x3, 4x4, 6x6, and 10x10 cm2 field sizes. Results: The agreement between the measurements and simulated PDD data was under 2.2% beyond the buildup region. The distance to agreement (DTA) in the buildup region was within 1.0 mm. The simulation data presented identical profiles with the measurement within 1.0% of the dose difference or 1.2 mm of the DTA. The mean dose difference in the radiation field was ≤1.5% for the ≥1x1 cm2 field size. The largest deviation was observed in the 0.6x0.6 cm2 inline beam profile. The deviation of the penumbra and full width at half maximum (FWHM) between simulation and measurement was <2 mm. The agreement of the simulated and measured FOF was within 1.0%, except for the 0.6x0.6 cm2 field size. Conclusion: Overall, the MC simulation demonstrates data that is consistent with the measurement for the ≥1x1 cm2 field sizes. These data assure that the 6 MV Varian phase-space file can be used as a radiation source for accurate MC dose calculation in a small field. However, a large discrepancy in beam profiles was observed at the 0.6x0.6 cm2 field size due to the different primary source sizes among TruebeamTM machines.

Background: Radiotherapy in cholangiocrcinoma has to overcome organ tolerance of the upper abdomen. Hi-technology radiotherapy may improve conformity and reduce dose to those organ. Objective: Quantitatively compare the dosimetry of conformal dynamic arc radiotherapy (CD-arcRT) and intensity modulated radiotherapy (IMRT) in unresectable cholangiocarcinoma. Material and methods: Eleven cases of unresectable cholangiocarcinoma were re-planned with IMRT and CDarcRT at King Chulalongkhorn Memorial Hospital between 20 September 2004 and 31 December 2005. Both the planning techniques were evaluated using the dose volume histogram of the planning target volume and organ at risk. The conformation number and dose to critical normal structures were used to determine the techniques. Results: IMRT technique was significantly conformed to the planning target volume than CD-arcRT in term of conformation number. For critical structure, IMRT significantly reduced the radiation dose to liver in terms of mean liver dose, V30Gy and V20Gy of the right kidney. Conclusion: The advantage of IMRT was more conformity and reduced dose to critical structure compared with CD-arcRT, but there was no difference between these techniques in terms of V20Gy of left kidney and maximum dose to the spinal cord.