To evaluate the accuracy of monitor unit calculated by the treatment planning system (TPS) algorithm using an inhouse designed head and neck phantom. The designed in-house phantom was constructed in the shape of a block using a Plexiglas of density 1.16g/cm3 and diameter 0.3mm. The phantom has provision for five hollow inserts for the ionization chamber and tissue equivalent materialsmimicking different biological tissues such as the bone, Brain, AdiposeandTrachea.The percentage compositions by mass of various chemical components were mixed together to mimic each biological tissues.9.5% of carbon; 87.3% of water to mimic the brain with 3.324e/kg electron density, 37.03g of carbon; 4.83g of hydrogen 35.66g of oxygen; 15.3g of carbon for bone with 3.353e/kg electron density, 68.10g of carbon; 19g of oxygen; 11.02g of hydrogen for adipose with 3.100e/kg electron density, 13.9g of cabon; 71.5g of oxygen; 10.4g of hydrogen for trachea with 3.336e/kg electron density. The phantom was filled with water and scanned with a Hi-Speed CT-scannerand the images were transferred to the ELEKTA-Precise treatment planning system.Several simple treatment plans of multiple beams were made with the designed phantom using the Area Integration Algorithm configured to give 1.0 Gy at the iso-centre. Measurements of Monitor units were conducted using 6 MV photon beams from the ELEKTA-Precise clinical linear accelerator with iso-centric set up. A pre-calibrated NE 2570/1 farmer-type ionization chamber along with its electrometer was used to determine the absorbed dose. Necessary corrections for temperature, pressure, polarization, recombination factor were effected on the ionization chamber response. The result obtained show that the deviation between the monitor unit of the newly designed head and neck phantom and the solid water phantom were within the ±4% accepted limit. The result show that the monitor unit calculated by the treatment planning system algorithm using the in-house designed head and neck phantom is accurate and that the phantom can be used successfully for routine verification exercises.
