Photoneutron dose estimation in GRID therapy using an anthropomorphic phantom: A monte carlo study

Background: In the past, GRID therapy was used as a treatment modality for the treatment of bulky and deeply seated tumors with orthovoltage beams. Now and with the introduction of megavoltage beams to radiotherapy, some of the radiotherapy institutes use GRID therapy with megavoltage photons for the palliative treatment of bulky tumors. Since GRID can be a barrier for weakening the photoneutrons produced in the head of medical linear accelerators (LINAC), as well as a secondary source for producing photoneutrons, therefore, in terms of radiation protection, it is important to evaluate the GRID effect on photoneutron dose to the patients. Methods: In this study, using the Monte Carlo code MCNPX, a full model of a LINAC was simulated and verified. The neutron source strength of the LINAC (Q), the distributions of flux (φ), and ambient dose equivalent (H*[10]) of neutrons were calculated on the treatment table in both cases of with/without the GRID. Finally, absorbed dose and dose equivalent of neutrons in some of the tissues/organs of MIRD phantom were computed with/without the GRID. Results: Our results indicate that the GRID increases the production of the photoneutrons in the LINAC head only by 0.3%. The calculations in the MIRD phantom show that neutron dose in the organs/tissues covered by the GRID is on average by 48% lower than conventional radiotherapy. In addition, in the uncovered organs (by the GRID), this amount is reduced to 25%. Conclusion: Based on the findings of this study, in GRID therapy technique compared to conventional radiotherapy, the neutron dose in the tissues/organs of the body is dramatically reduced. Therefore, there will be no concern about the GRID effect on the increase of unwanted neutron dose, and consequently the risk of secondary cancer

Investigation of the Field Size Effect on Wedge Field Isodose Curves Angle for Two Energies; 6 & 18 MV, produced by VARIAN 2100C Linac

Introduction: Nowadays,  considerable developments  in  the field  of  radiotherapy have  been  achieved.  They  include  the  advances  made  in  the  equipments  and  treatment  planning  techniques  which  require  highly complex calculations. Such achievements have made it possible to treat cancer patients not only  with  higher  radiation  dose  but  also  with  higher  precision  and  consequently  increasing  the  chance  of  curing the cancer. However, the conventional techniques requiring physical wedge are still being used  but with a lesser frequency. One of the wedge parameters needed to be measured is the wedge angle. It is  the angle that the horizontal line creates with the tilted isodose curve at a specific depth and for a certain  field size.   In this study, the variation of wedge angle for different field sizes was evaluated using dosimetric and  mathematical method.  Material and Methods: For the wedge fields with a dimension of 6×6 to 20×20 cm 2 , the wedge angle  for  two  photon  energies  of  6  and  18  MV  was  measured  by  the  dosimetric  method.  For  these  measurements, the conventional wedges having the nominal wedge angle of 15, 30, 45 & 60 were used.  The theoretical method suggested by Saw et al. is also used to indirectly calculate the slope of isodose  curve  by  the  dose  profile  and  percent  depth  dose  data.  The  dose  profile,  percentage  depth  dose  and  isodose curves were drawn for all the field sizes and the tilt of isodose curve at 10 cm depth, according to  international definition, is considered as the wedge angle. The data were obtained using the theoretical  equation of wedge angle and it was compared to the dosimetric data.  Results: The result obtained in this work shows that the wedge angle increases with the field size. For a  6×6 cm 2 field size, the calculated wedge angle has the highest difference in comparison to the nominal  wedge angle. The difference is equal to 14.7 degree for a 45° wedge and a 6 MV photon. The highest  difference for a 45° wedge angle, a field size of 10×10 cm 2 and a 6 MV photon is 9.2 degree. Comparing  the calculated and measured wedge angles shows a maximum difference of 4 degree for 6 and 18 MV  photon beams.  Discussion  and  Conclusion:  The  wedge  angle  varies  with  field  size.  In  order  to  get  a  better  dose  distribution in the conventional radiotherapy, it is necessary to use the appropriate wedge angle which  generates the desired slope for the isodose line and for the specific field size