Dosimetric Measurement of 60Co Teletherapy Beam by Various Detectors and Comparison of Absorbed Dose to Water with OSL Technique

Radiotherapy plays a leading role in the treatment of cancer patients in all over the world. External beam radiotherapy includes high-energy photon and electron beam linear accelerator, tele-cobalt therapy, cyclotron-based proton beam therapy, reactor-based boron neutron capture therapy, low energy x-ray therapy etc. The outcome of the radiotherapy is highly dependent on how the precisional dose is delivered to the tumor which should not exceed 5% of the prescribed dose including all types of uncertainties involved in the treatment procedure such as dosimetry, treatment planning, and dose stability of the treatment unit. This implies that dose measurement should be accurate which lies within ±3.5% as per ICRU recommendation.

In conventional radiotherapy, radiation fields are characterized between 4 cm ´ 4 cm to 25 cm ´ 25 cm.  A field size below 4 cm ´ 4 cm is considered a small field. The dosimetry of small fields is a challenging issue. The motive of this study is to compare dosimetric measurements- absorbed dose, temperature and pressure correction factor (K_TP) polarity correction factor (k_pol), ion recombination correction factor (k_s), D_max(ref) and PDD among three ionization chambers- Farmer (FC65-G), Semiflex (31010) and further study of radiation dosimetry in tele-cobalt therapy and linear accelerator. For radiation dosimetry, three suitable ionization chamber selection (FC65G, semiflex, pinpoint) and 3D scanning blue Phantom2 were used for the absorbed dose to water measurement along with 2 × 2 cm² to 20 × 20 cm² field sizes and the reference field size 10 × 10 cm². The absorbed dose to water was measured by the IAEA dosimetry protocol TRS-398 and TRS-483 protocol. The percentage depth dose was calculated with each chamber together with Optically Simulated Luminescence (OSL) dosimeter. The dose rate measured and compared between ionization chambers in tele-cobalt therapy and Linear accelerator was performed with a single-element OSL dosimeter.  The comparison of different field sizes with the standard field ensured the accuracy of the dose deposition of radiation fields. The use of different ionization chambers showed how the dose rate varies when used in combination with different field sizes and ionization chambers. The energy correction factor was measured from the dose measurement from the ⁶⁰Co teletherapy system and linear accelerator. The conclusion drawn from this work is that there is no ideal detector with all of the features required for radiation dosimetry. This study recommends combining different types of detectors rather than only one to collect the essential data because each form of detector has its own limitations. The details of this measurement data are analyzed and summarized which will help to define radiation dosimetry for various square field sizes. The experimented energy correction factor will be further helpful for remote measurement.

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