Absolute Dosimetry for the Linear Accelerator

Absolute dosimety

1. ABSOLUTE DOSIMETRY Presented By Md. Ashikur Rahman Scientific Officer INMP, AERE Savar , Dhaka. 12. 03. 2019

2. 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 .

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. Medical dosimetry is the calculation of absorbed dose and optimization of dose delivery in radiation therapy. It is often performed by a professional health physicist with specialized training in that field. Absolute dosimeter produces a signal from which the dose in its sensitive volume can be determined without requiring calibration in a known radiation field.

5. Radiation dose delivered to the target and surrounding tissues is one of the major predictors of radiotherapy treatment outcome . It is generally assumed that the dose must be accurately delivered within +/-5% of the prescribed dose to ensure the treatment aims are met . Minimum dose to tumour = 95% Maximum dose to tumour = 107%

6. ABSOLUTE Dosimetry is a direct measure of ionization or absorbed dose under standard conditions, which are things like calorimetry [measure energy deposited which eventually appears as heat], electrons released (in an ionization chamber where electronic charge is measured), or ion formation where the number of valence changes in a known amount of ions is directly related to the number of electrons (chemical dosimeter). Because of the need for accuracy, absolute dosimetry informs a standard and is usually tied to a single government based agency responsible for 'the standard'. ABSOLUTE DOSIMETRY

7. Absolute dosimetry is a technique that yields information directly on absorbed dose in Gy . This absolute dosimetric measurement is also referred to as calibration. All further measurements are then compared to this known dose under reference conditions. Of tremendous importance: If the absolute dosimetry is incorrect EVERYTHING will be wrong

8. DOSIMETRY CONCEPTS AAPM (American Association of Physicists in Medicine) Task Group-21 (1983) TG-51 (1999) IAEA (International Atomic Energy Agency) Technical Report Series 277 (1997) TRS-398 (2000)

9. Farmer Chamber FC65-P Dose-1 Electrometer

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

11. Table 1: Properties of phantom materials Material name Chemical Composition Mass density (gm/cm 3 ) Number of Electrons/g (×10 23 ) Water H 2 O 1.00 3.34 Solid water Epoxy resin-based mixture 1.00 3.34 PMMA (C 5 O 2 H 8 ) n 1.16 - 1.20 3.24

12. Absorbed dose to water at the reference depth, z ref , in a water phantom irradiated by a beam of quality Q is D W, Q = M Q ×N D, W × k TP × k S × k pol × k Q, Q M Q → Monitor reading N D,W → Calibration factor in terms of absorbed dose to water k TP → Temperature pressure correction factor k S → Ion recombination correction factor k pol → Polarity correction factor k Q,Q → Chamber specification factor

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