Characteristics of the electron beams of energies 6,9,12,16 and 20 MeV produced by Clinac 1800 have been studied. The characteristics include percentage depth dose, isodose distribution, and depth of maximum dose, surface dose, photon contamination, uniformity index and penumbra. Most of the measurements are carried out using semiconductor detectors while small volume ionization chambers and plane-parallel chamber served as standards for comparison. Isodose curves were drawn using the film densitometry method. Range-energy are obtained from the observed depth dose data. These parameters vary from machine to machine and require to be ascertained for individual units. Though the observed parameters differ to a considerable extent from their theoretically predicted values, yet they generally follow the trend experimentally observed by others for similar type of units.

Energy parameters of three linear accelerators manufactured by different companies have been studied. All three machines produce 6 MV X-rays. The energy parameters specified by different manufacturers and different protocols were measured and evaluated. Profile measurements were carried out to find out the quality of the beam in the off-axis. These are affected by the beam production system which are also compared. Relative measurements were carried out using a radiation field analyser and the absolute measurements were carried out using 0.6 cc ion chamber and Baldwin Farmer dosemeter. Generally, the manufacturers of the linear accelerators specify % DD measurements at two different depths one at dmax and the other one at 10 cm depth or the depth at which 50% of the maximum dose occurs. The studies conducted on three different machines indicate that they are within the specifications but less than the quoted nominal energy of 6 MV. Atleast, % DD measurements should be carried out at two different depths one at a smaller depth and the other one at a larger depth. Profile measurements are within the specifications.

The evaluation response of tumours requires quantification of tumour mass in terms of volume or maximum diameter. In normal practice changes in maximum diameter of tumour are assessed visually on follow-up CT films. This may lead to erroneous results. Twenty one patients of testicular cancer in stage I1 A to I1 D with retroperitoneal lymphadenopathy were scanned by CT for 3 to 5 times during the course of treatment. Tumour size was assessed in terms of diameter as per practice followed routinely. Avery important observation was that actual measurement of diameter instead of visual assessment resulted in change of stage of cancer in 42.8% (9 out of 21) cases. Moreover, in 6 out of these 21 cases (28%) there occurred a change in stage from I1 C to II D which assumes significance due to change in treatment protocol from BEP (Bleomycin. Etoposide, Cisplatinum) to BOP-VIP (Bleomycin, Vincristin, Cisplatinum, VP-16, Ifosfamide, Cisplatinum). Tumour volume estimated on the basis of visual method differed considerably from the calculated one. In 16 out of 21 scans, the difference was between 6.4% to 42.9%. The acceptable difference of 5% was seen only in 4 out of 21 cases indicating the importance of volume measurement. The method of volume estimation was validated and found to be within + 5% of actual volume. The correlation between diameter and volume shows that tumours with similar range of diameter have 20% to 100°/o more volume. Many times increase in diameter was found associated with actually reduced tumour volume, difference being as much as 515%. This has not been documented earlier in clinical situations. Thus, the study underscores the role of measuring the diameter of tumour for staging and highlights the need for actual volume estimationm rather than depending on maximum transverse diameter alone in follow-up studies.