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A STUDY OF THE ABSORBED DOSES IN DIFFERENT PHANTOM MATERIALS AND FABRICATION OF A SUITABLE PHANTOM

This is a study in between Solid water phantom and Paraffin wax Phantom.

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A STUDY OF THE ABSORBED DOSES IN DIFFERENT PHANTOM MATERIALS AND FABRICATION OF A SUITABLE PHANTOM

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  1. M. A. Rahman1, M. M. Rahman2, M. A. Hai2, M. A. Islam1 1Dept.of Physics, Rajshahi University; 2Cancer Centre, KYAMCH, Sirajgang A STUDY OF THE ABSORBED DOSES IN DIFFERENT PHANTOM MATERIALS AND FABRICATION OF A SUITABLE PHANTOM Presented By Md. AshikurRahman Scientific Officer DRiCM, BCSIR Dhaka

  2. What is CANCER ? Cancer is a large group of diseases (over 200) characterized by uncontrolled growth and spread of abnormal cells. • Surgery • Chemotherapy • Radiation therapy • Teletherapy • Brachytherapy TEREATMENT OPTIONS

  3. AIM OF RADIATION DOSIMETRY • The prescribed dose in radiation therapy has to be converted into machine monitor units for patient treatment. • It is necessary to plan how we deliver the prescribed dose to a patient. • The dose distribution inside the patient cannot be measured in the body of the patient himself. • Hence, the patient needs to be replaced by a tissue-equivalent material.

  4. Patientis replaced by tissue-equivalent materialto calculate the dose inside the body Tissue-equivalent material or Phantom

  5. RADIATION DOSIMETRY • Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue • Scientific determination of amount, rate, and distribution of radiation emitted from a source of ionizing radiation • Measuring the radiation-induced changes in a body or organism and • Measuring the levels of radiation directly with instruments.

  6. PHANTOM • A phantom is a substance which is made of tissue equivalent materials. • A phantom represents the radiation properties of the patient and allows the introduction of a radiation detector into this environment, a task that would be difficult in a real patient. • A very important example is the scanning water phantom. • Alternatively, the phantom can be made of slabs of tissue mimicking material or even shaped as a human body (anthropomorphic).

  7. Fig. Image of water phantom Fig. Image of PMMA phantom Fig. Image of solid water phantom

  8. Suitability of different Phantoms Water phantom is widely used for dosimetry purposes and it is recommended as the reference medium for absorbed dose measurements for both photon and electron beam by many international institute. Although water phantoms are standard for absolute dosimetry but those are inconvenient for both linac and cobult-60 machine. Due to the problems of water phantom they are replaced by solid phantoms which have the same properties as like as water with slight variation.

  9. Cost effective Phantom • Solid phantoms are expensive and procured with difficulty from abroad. • It would be advantageous if an inexpensive and locally available tissue equivalent material is available. • This is particularly relevant for developing countries like ours. • Paraffin wax is found to have density and electron density similar to that of water with a slight variation and we decided to fabricate a phantom using paraffin wax.

  10. Fig. Paraffin wax phantom without bubbles Fig. Paraffin wax phantom with bubbles CT scan image of phantom-1 (without bubbles) • CT scan image of phantom-2 • (with bubbles)

  11. FABRICATION OF PARAFFIN WAX PHANTOMS Two paraffin wax phantoms were made one was with bubbles and another without bubbles. In this study our object is to justify the suitability of the paraffin wax phantoms and find out a scaling factor between those readily available solid phantoms with water phantom

  12. Table 1: Properties of used phantom materials

  13. After making proper dimension, two paraffin wax phantoms were drilled with a d.c drill machine and the holes were similar to the chamber dimension The phantoms were then placed on a CT scanner for checking internal integrity

  14. Temperatures inside phantoms were collected by digital thermometer and room pressures were taken with the help of a barometer. Then the phantoms were placed on the couch of the linear accelerator. The phantoms surface center were aligned with the central axis of the beam from the gantry at zero degree angle. The distance between the phantoms surfaces to the source were made 100 cm with the help of optical mark reader.

  15. The monitor unit of the linear accelerator was made 100, SSD = 100 cm, Field size = 10 × 10 cm2 Fig. Optical mark reader indicate 100 cm SSD The readings were taken three times for each of 6 MV and 15 MV photon beam with the help of an electrometer in every phantom. Fig. Linear accelerator

  16. Two types of ionization chamber were used in the experiment one was Farmer and another was Semi-Flex. Fig. Farmer type ionization chamber Fig. Semi-Flex type ionization chamber Absolute dosimetry protocols TG-51 and IAEA TRS 398 was followed.

  17. Correction factor The ratios between the doses in the water phantom and those in other phantoms are scaling factors. Naturally this scaling factor is unity for water. A number close to 1 would be a good ratio. Although slight variation also acceptable. Various correction factor was calculated for the determination of absorbed dose in different phantoms.

  18. Absorbed dose to water at the reference depth, zref, in a water phantom irradiated by a beam of quality Q is DW, Q = MQ ×ND, W × kTP × kS × kpol × kQ, Q 0 MQ → Monitor reading ND,W → Calibration factor in terms of absorbed dose to water kTP → Temperature pressure correction factor kS→ Ion recombination correction factor kpol → Polarity correction factor kQ,Q→ Chamber specification factor 0

  19. Correction for temperature and pressure Polarity correction factor Ion-recombination correction factor Chamber specification factor And,

  20. Table 2: Scaling factor for different phantom materials for 6 MV photon beam

  21. Table 3: Scaling factor for different phantom materials for 15 MVphoton beam

  22. Research findings Deviation from water phantom is minimum for Paraffin wax phantom and maximum for PMMA phantom. Although in the case of solid water phantom this deviation should be least but it varies about 6% , where the deviation of Paraffin wax phantom is around 1% The cost of solid water phantom is at least 3000 dollars, where the cost of Paraffin wax phantom is ONLY 35 DOLLARS

  23. Conclusion • Now it is seen that a Paraffin wax phantom is much more suitable than other solid phantoms due to their availability, price and process of fabrication. • The scaling factor is a multiplication factor it will help us to use any solid phantom and also convert the absorbed dose in solid phantoms equivalent to water phantom.

  24. Thank You all

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