Syed F. Akber

Assessment of tumor control probability for high-dose-rate interstitial brachytherapy implants

Summary Aim The study was designed to propose a novel concept of biologically effective equivalent uniform dose to calculate tumor control probability for HDR implants. Materials and Methods The expression of biologically effective equivalent uniform dose was derived for non-uniform dose distribution in HDR implants using quality indices and voxel-based tumor control probability. Results The results of this study show that high dose regions of the implant have higher tumor control probability. But these regions may also have a large number of normal cells and consequently may lead to severe normal tissue complications. If tumor coverage was not proper then the overall tumor control probability would be low and might result in tumor recurrence. Higher values of external volume index, dose non-uniformity ratio and overdose volume index were related to higher normal tissue complication rates outside and inside the implants. Conclusion The present concept may provide an alternative approach to calculate tumor control probability for HDR implants.

The quality factor of x-rays/gamma rays as one is realistic?

The quality factor for x-rays, gamma rays and electrons assigned as one need to be revised. It is observed that as the energy decreases, mean lethal radiation dose (D o )decreases as well and become more potent. It is therefore proposed that radiation quality in biological systems should be assessed in the mitotic phase of the cell cycles. Furthermore, based on the mean lethal radiation dose within specific energy range, an appropriate quality factor of x-rays, gamma rays and electrons should be assigned.

Correlation between mean lethal dose and organ weight

The assessment of mean lethal dose (D o ) with organ weight in human organs yields a correlation coefficient of 0.94. Results indicate that as the organ weight increases, the D o decreases.

A new approach to cancer treatment

A new approach is applied to correlate different phases of the HeLa cell S-3 with mean lethal ionising radiation dose (Do) along with nuclear magnetic resonance water-proton spin-lattice relaxation time (T1). This information can be used to pin-point the mitotic phase of the cells in vivo. This enables us to apply ionising radiation treatment at that particular time. This will increase the efficacy of radiation treatment in cancer patients.

Tissue weighting factor and its clinical relevance

Since 1977, the International Commission on Radiological Protection (ICRP) published, revised and updated tissue weighting factors (TWFs) in human tissues/organs. 1–3 TWFs are based on estimates of the radio-sensitivity of each organ. However, TWFs change every decade or so as if it is a variable quantity (Table 1). TWFs are used in the calculation of the effective dose that is not a real quantity 4 but a conceived quantity proposed by ICRP. In computing TWFs, ICRP did not take into account the body weight, organ weight and gender difference. The value of TWFs ICRP provided: it is interesting to note that not a single biophysical factor correlate with TWFs. Radiation sensitivity of human organs varies as a function of organ weight. 5 Smaller organs have lower radiosensitivity and in turn higher radiation tolerance dose (TD50). 5 As the organ increases in size and weight by assembling many cells of different functions, TD50 decreases. TWFs calculated in Tables 2 and 3, provide a new perspective. First of all, all the variables such as gender difference, organ weight and body weight are taken into account. As Woodward and White eloquently wrote: ‘The need for reliable composition and density data of human organs is a prerequisite in theoretical dosimetry involving radiation interactions in human tissues. Uncertainties in elemental compositions and mass densities of the body tissues will lead to reduced confidence in the relevance of the calculated and measured doses. Uncertainties in the elemental composition of each organ of ah uman body may affect the dosimtery of low- and high-energy photons. The concentrations of high atomic number elements in a tissue will strongly influence photoelectric absorption, while hydrogen content will affect the Compton scattering’. 7 ICRP did not take these factors into account in computing the TWFs and therefore effective dose (ED) cannot be relied upon. ICRP also assume that there is no difference between kV and MV energies nor there is any difference in electron density or mass density 6 in different organs. The ratio of organ weight to body weight is a close approximation of all the factors given in Table 2. The TWFs in the present case is calculated as

Oxygen in S phase of a cell cycle

It isgenerally agreed that the S phase is radioresistant because of its deficiency of molecularoxygen. This conclusion is based on the oxygenenhancement ratio (OER) and not by measur-ing the actual molecular oxygen content in eachphase of the cell cycle (Table 1).The OER is calculated by radiation dosedelivered under hypoxic and under aeratedcondition to achieve the same biological endpoint. The OER at high doses yield a valuebetween 2?3 and 3?5 for X-rays and g rays.We abstracted the data from the literature. It isinteresting to note in Figure 1, as the cell ages,water proton NMR Spin Lattice relaxation time(T

The clinical efficacy of effective dose

Effective dose (E) is proposed by the International Commission on Radiological Protection (ICRP) in 1975 to replace effective dose equivalent. Effective dose as defined by the ICRP is a dose quantity of health determinant due to scholastic effects from being exposed to low doses of ionising radiation. One of the goals of ICRP in proposing E is to quantitate radiation exposure for establishing and providing ionising radiation dose limits for radiological protection. E reflects the health determinant aspect from an ionising radiation exposure to any section of the human body whether it be external or internal exposure to uniform body ionising radiation exposure for a reference person.

Partial volume characteristics of ionization chambers in kilovoltage x-ray exposure measurements

The partial volume (spatial) response of four ionization chambers (Keithley) in kilovoltage X-ray beams, generated by the Philips Super 80CP X-ray unit, was assessed. The volume of the ionization chambers were of 10 cm 3 , 15 cm 3 , 150 cm 3 , and 600 cm 3 used with Keithley electrometer Model 35040. The beam output was measured using a monitor chamber (Radcal 6.0 cm 3 ) placed close to the collimator. The source to chamber distance was kept constant at 1 m. For the measurement of the response of ionization chambers of 15 cm 3 , 150 cm 3 , and 600 cm 3 , a slit of 2.0 mm width was made in a lead sheet of 3.2 mm thick and size of 30 × 30 cm 2 and was placed on the ionization chamber. The measurements were made for 81 kVp, 400 mA, and 0.25 s and the slit was moved at an increment of 2.0 mm over the entire length of the chamber. For the measurements of the ionization chamber of 10 cm 3 (CT chamber), the beams of 120 kVp, 200 mA and 0.2 s were generated, and a slit of 5 mm width was made in a similar lead sheet that was moved at an increment of 5.0 mm. From the result it appears that the sensitive volumes of the ionization chambers affect the response of the ionization chamber to incident radiation.

A novel approach to assess mean lethal radiation dose with water proton spin lattice relaxation times

The assessment of mean lethal radiation dose (D0) in human organs, using multi-target and linear quadratic models, with water proton nuclear magnetic resonance spin lattice relaxation time yields a correlation coefficient of 0.90 and 0.82, respectively. Results of this study reveal that as the spin lattice relaxation time increases, the D0 decreases.