Introduction:
Three physical conditions caused an external beam designated as a small field. The first is the loss of lateral charged particle equilibrium. The second is the effect of collimators on the primary photon source occlusion. The third condition is the size of the detector which is large compared to the field size. The most obvious perturbation effects in last condition is volume averaging. The aim of this study was to investigate dose distribution for small field sizes using NIPAM polymer gel dosimeter through MRI technique.
Materials and Methods: NIPAM polymer gel production: To fabricate NIPAM gel dosimeter at first, the gelatin (5 wt %) completely dissolved in 90% (80.1 wt %) of the water at 50 °C. Approximately 3 wt. % of Bis was dissolved at 50 °C. The N-isopropylacrylamide (3 wt%) was added after the gelatin–cross linker mixture was cooled to almost 37 °C. THPC (10 mM), as the antioxidant, was mixed with the %10 remnants of water, and was added to the solution. After preparation, the gel tubes were stored at temperature of 4- 7 °C in a refrigerator for 12 h.
Irradiation: Irradiation experiments were performed by a 6 MV X‑rays generated by a 2100C/D Varian linac. The vials filled with calibration NIPAM gel dosimeters were positioned fixedly in floor a 20×20×15 cm3 water phantom. Also, 5 cm Perspex slab was placed under the water phantom to ensure a full photon scattering condition.
For small field investigating a Perspex cubic phantom with dimensions of 7×7×7 cm3 filled with NIPAM polymer gel dosimeter. The phantom was irradiated using three perpendicular field with size of 1×1 cm2.
Response readout: The response of NIPAM polymer gel dosimeter was readout using a 1.5 T MRI scanner (Siemens Avanto, Germany). To scan the polymer gels a 32-echo Carr-Purcel-Meiboom-Gill (CPMG) pulse sequence was applied.
Results: Calibration curve of NIPAM polymer gel dosimeter was obtained. Equation (1) represents the variation of the R2 of the NIPAM polymer gel dosimeter a function of absorbed dose.
R2=0.208×Dose+1.925                                                                   (1)
Radiation dose profile in depth of 35 mm in NIPAM polymer gel dosimeter was compared with that calculated using Eclipse™ treatment planning system.
Conclusion: Results confirmed an accordance between dose profiles measured using polymer gel dosimeter and that calculated by TPS. Results showed a maximum difference of 3.8% between dose profiles calculated using TPS and measured by polymer gel dosimeter.