ANALYZING THE CONSISTENCY OF BEAM QUALITY AND OUTPUT DOSE/MU FOR PHOTON BEAMS OF ENERGY 6 MV AND 15 MV OF A MEDICAL LINAC USED FOR RADIOTHERAPY AT INSTITUTE OF NUCLEAR MEDICAL PHYSICS, AERE, SAVAR.

Radio Therapy, Medical Physics

Medical Linac (linear accelerator) delivers ionizing radiation which is used for cancer management. In radiotherapy, it is mandatory to provide maximum dose to the cancerous tumor and minimum dose to the surrounding normal tissues. So it is required to assess the consistency of beam quality and beam output dose/MU (Monitor Unit). The output dose of Linac (Model: CLINAC-iX) of photon beam of energy 6 MV and 15 MV installed at Institute of Nuclear Medical Physics, AERE Savar has been measured with FC65-P ionization chamber using 1D Water Phantom. In this study, data collection period was from October 2019 to September 2022 in accordance with the TRS-398 dosimetry protocols. Over the period of 36 months we found that TPR20,10 for 6 MV photon beam was 0.67 corresponding and TPR20,10 for 15 MV photon was 0.76. Which showed that the values of beam quality TPR20,10 never changed when the values are rounded to two decimal points. The measured output dose/MU was found in between 0.998 to 1.023 cGy for 6MV photon and 0.998 to 1.032 cGy for 15MV photon. The dose/MU for all radiation beams was maintained within 2% of deviation from the recommended value 1cGy/MU at Dmax position over the period of 36 months. In our study, the observed deviations of output dose/MU were maintained in tolerance interval instructed by the protocol. If the tolerance crossed the machine tuned of the monitor unit for getting 1 cGy/MU at the Dmax position in water. Here, absorbed dose to water based protocol was followed which reduced the uncertainty of the accuracy of the dose determination.

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