Journal of Medical Physics
: 2011  |  Volume : 36  |  Issue : 3  |  Page : 184--185

Review of Ph.D Thesis ''Dosimetric monitoring methods for megavoltage photon and electron beams in clinical radiotherapy''

R Ravichandran 
 Chief Medical Physicist, National Oncology Center, Royal Hospital, Muscat, PB1331, PC 111, Sultanate of Oman

Correspondence Address:
R Ravichandran
Chief Medical Physicist, National Oncology Center, Royal Hospital, Muscat, PB1331, PC 111
Sultanate of Oman

How to cite this article:
Ravichandran R. Review of Ph.D Thesis ''Dosimetric monitoring methods for megavoltage photon and electron beams in clinical radiotherapy''.J Med Phys 2011;36:184-185

How to cite this URL:
Ravichandran R. Review of Ph.D Thesis ''Dosimetric monitoring methods for megavoltage photon and electron beams in clinical radiotherapy''. J Med Phys [serial online] 2011 [cited 2019 Oct 20 ];36:184-185
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Full Text

The thesis by S Sathiyan is written based on the experimental research work carried out at the Kidwai Memorial Institute of Oncology (KMIO) (Regional Cancer Treatment and Research Center) of Government of Karnataka, in the high energy linear accelerators and tele-cobalt machines under guidance of Dr. M. Ravikumar. The author has investigated calibration and quality assurance methods related to clinical beam parameters involved in the treatment of cancer patients with high energy megavoltage photons and electrons. The thesis consists of seven chapters, highlighting the results and discussion in chapters 4, 5, and 6 with summary and conclusion in chapter 7. There are about 250 references documented in support of his methods and discussion of results. The thesis is presented systematically, outlining the physics of photons and electron beams, relevant to the experimental work and summarized important experimental results in two major areas, megavoltage photons and megavoltage electrons.

Photon beam parameters of tele-isotope machines relevant to clinical radiotherapy are collimator scatter, penumbra, and rounded isodose curves. A method to experimentally quantify primary, scatter, and recoil electron components in beam intensity is highlighted, and relevance of this method was highlighted in making clinically useful photon beams with a short distance type head and neck cobalt machine. The author has succeeded in producing a more flattened cobalt beam similar to a 6 MV X-ray beam using an unit density wax compensator mounted at the collimator level. The method of achieving flatness will have applications if similar flatness is required in a wide beam as encountered in total body treatments. With multiple beams, this flattened isodose curves might produce more homogeneous dose in a planned target volume (PTV). In countries with more populations, Cobalt-60 isotope machines still find applications in cancer management, and therefore these findings are of significance.

Exit dose measurements have also been carried out in phantoms for 6 MV, 18 MV, and telecobalt beams. Reductions in relative exit doses in the absence of backscatter materials were quantified using parallel plate chambers. Perturbation effects in high energy photon beams due to the presence of metal interfaces for aluminum, steel, copper, cadmium, and lead metals were studied. Forward and backward dose perturbation effects were separately measured. These measured values have applications to develop algorithms for theoretical estimates of absorbed dose. Physics of enhanced dynamic wedges (EDW) for 6 MV and 18 MV beams were studied for 'Varian' linear accelerator and their efficacy over hard wedges were evaluated.

IAEA protocols TRS 381, and TRS 398 for determination of absorbed dose were implemented and deviations arising due to air-kerma related protocols were determined. TLD verification of delivered doses was carried out in seven teletherapy machines and recommendations of establishing secondary standards at the institutional level were highlighted. Beam modeling in terms of the equivalent square field concept has been investigated to determine and validate correction factors for x and y collimator jaws definitions of field. Methods for treatment verifications of intensity modulated radiotherapy plans were standardized using calibrated 2D-ion chamber array matrix commercially available with suitable software, and established their efficacy over conventional method film dosimetry. Photon fluence patterns at selected planes have been checked against planned dose delivery with treatment planning systems.

High energy electron beams of energies 6 MeV to 22 MeV were studied in terms of absorbed dose delivery, physics of interactions in phantom materials, and forward and backscattering effects. [Ehancement dose factors were established for high atomic number metal inhomogeneities which were quantified]. Dose calculation methods using these factors in proximal tissue were also explained. Plane parallel chambers had been cross calibrated in electron fields and better agreement in absorbed dose was reported in comparison to the conventional method of cobalt-60 calibrations of these chambers.

This thesis has addressed many important aspects of clinical physics and added a lot of experimental data useful for the department and also for future applications in modeling interactions of photons and electrons in tissue equivalent media. The materials and illustrations are of good standard, and overall presentation makes it worth preserving in leading medical physics departments as reference material. The recommendations made by the author for future research workers have good scope and are worth pursuing.