In high energy x-ray beam therapy, number of monitor units for treatment of a patient is Calculated using empirical functions of central axis dosimetry parameters, such as percentage depth dose (PDD), tissue maximum ratio (TMR), and quantities that express the field size dependence of scattered radiation - the collimator scatter correction factor (Sc), the phantom scatter correction factor (SP) and the total scatter correction factor (Scp). For rectangular fields, the interchange of upper and lower collimator jaws affects the numerical value of collimator scatter factor which is commonly known as collimator exchange effect (CEE). Collimator exchange effect of Siemens MLC for 6 and 15 MV open photon beams was determined experimentally. The measurements were carried out using a PMMA mini phantom and an ionization chamber at the depth of dose maximum (dmax=1.5 cm), 5 and 10 cm for 6 MV photon beams and at the depth of dose maximum (dmax = 3.0 cm) and 10 cm for 15 MV photon beams, respectively. Results of these measurements indicate that the secondary collimator of Primus linac exhibits the collimator exchange effect. It is also observed that the value of Sc for those rectangular fields, the longer side of which are defined by y-jaws are higher than the value of corresponding rectangular field, the longer side of which are defined by x-jaws. The collimator exchange effect of this collimator system may therefore be attributed to the radiation backscattered to the monitor chamber by upper jaws of the secondary collimator. Further, as the elongation ratio (ratio of length and width of rectangular field) of the rectangular field increases, the percentage difference of Sc values between the two corresponding rectangular fields also increases in the beginning and then tends towards saturation. The maximum percentage difference between the Sc of two corresponding rectangular fields at 1.5,5.0 and 10.0 crn depths for 6 MV photons are 1.66,1.44 and 1.1 9%, respectively. While the maximum percentage difference between the Sc values of corresponding rectangular fields at 3.0 and 10.0 cm depths for 15 MV photons are 1.74 and 1.38%, respectively.

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The radiation output calibration of external beam radiotherapy machines based on the ionization chamber measurement is an important task of the medical physicists. Earlier, an in-air calibration measurement was practiced for 60Co beam and the water calibration methods were limited to linear accelerators only. In order to have a trace ability of dosimetry between calibration laboratories and users, various protocols were introduced over two decades. The absorbed dose to air calibration) factor was explained in TG-21 and TRS-277 protocols which were later replaced by absorbed dose to water calibration (ND,w)in TG-51 and TRS-398 protocols. In this context, we have compared in-air measurements with in-water measurements using above protocols for the 60Co and I37Cs beams, and the results agree within &2%. The photon beam output values measured using TRS-277 and TRS-398 formalisms were within +2% for the 60Cob beam, 6 MV and 18 MV x-ray photon beams. The measurements using a cylindrical chamber (TRS-277) and parallel plate chamber (TRS-381) have shown good agreement for high energy photons and electrons. The TLD audit carried out for all the photon beams and the results are within &5%.

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Ferrous ammonium sulphate benzoic acid xylenol orange dosimeter (FBX) developed to detect and measure radiation doses in the clinical range recommended the use of triple distilled water for its preparation and its shelf life was claimed to be two weeks. To be of use as a clinical dosimeter for routine use, preparation of this dosimeter with distilled water generated in the pharmacy department of our hospital was tried. The rapid auto-oxidation encountered forced us to undertake a systematic experimental study. The modification made in the concentration of sulphuric acid significantly increased its shelf life and hence it is referred to modified FBX (MFBX). Doses measured under various conditions were checked against ionization measurements with secondary standard dosimeter to show that MFBX can be used with confidence as a reliable clinical dosimeter. It is shown that measurement of doses with volumes as small as 0.6 - 0.7 ml is possible. Besides, MFBX can be used for output check as well as for verifying beam parameters of teletherapy units.

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The International Atomic Energy Agency (IAEA) was created in 1957. It had 56 members in that year, including India. IAEA has now a membership of 134 states. Its headquarters is in Vienna. The mandate for IAEA has been twofold: (i) to promote the peaceful applications of atomic energy in various fields; and (ii) to evolve safeguards (through Bilateral and Multilateral Agreements) to ensure that nuclear materials, instruments and technologies are not clandestinely diverted for non-peaceful uses. Special attention was to be paid to the growth of nuclear energy applications in the developing world.

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Blocked cardiac vessels pose a major clinical problem in modern world. Dilating blocked vessels by balloon angioplasty results in restenosis, mainly due to the excessive tissue repair. Use of radiation after angioplasty may help in arresting or preventing restenosis. Novoste BetaCath device uses 90Sr beta source train to deliver radiation dose to the injury site. A series of tests have to be carried out when a new device is received in the department and also just before and immediately after each treatment. The dose delivered at the prescription point depends upon the vessel diameter and the presence or absence of a stent. 57 patients were treated at the Royal Prince Alfred Hospital (RPAH) between July 1999 and November 2002. Of these, 14 (25%) patients had reference vessel diameter (RVD) between 2.7 and 3.3 mm, while 43 (75%) had RVD between 3.3 and 4 mm. The follow-up results for all of these patients have been very good. Only one radiation incident has been experienced at RPAH with no noticeable radiation exposure to the staff involved.

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