Colin G. Orton

Depth dose in skin for obliquely incident 60Co radiation

Abstract Clinical observations show that much of the skin-sparing effect of 60Co radiation is lost if the beam is incident obliquely on the skin. This is most evident with tangential irradiation. LiF thermoluminescent powder was used to determine the build-up of dose in the superficial layers of skin of a tissue-equivalent phantom. Dose measurements at several skin depths were made for various values of angle of incidence, field size and SSD. The dose to the basal layer of the epidermis increases with increase in both field size and angle of incidence, but is independent of epidermal thickness. The maximum dose to the dermis occurs for angles of incidence between 60 deg. and 90 deg. and varies little with field size and dermal thickness. These results can be related to observed skin reactions.

What minimum number of fractions is required with high dose-rate remote afterloading?

Dr Orton has presented a useful mathematical analysis which I agree with insofar as it represents an interesting way of finding the theoretical maximum dose/fraction which can be used when switching from a protracted LDR treatment to a fractionated HDR regimen. However, the assertion that his particular numerical example (Example 2 of his letter) represents a “more rigorous application of the LQ model” is misleading, since his predictions are only valid for tissues which can be described in terms of the parameter values he has chosen to use and, on this point, he has misquoted the use of the damage repair constants in my own paper (Dale, 1985).

Definition of T in the NSD equation

We have received several comments concerning the definition of the “overall treatment time” (T days) as used in our recent British Journal of Radiology article “A simplification in the use of the NSD concept in practical radiotherapy” (Orton and Ellis, 1973). It has been correctly observed that the value of T as used in our article is not identical to that defined originally (Winston, Ellis and Hall, 1969). Specifically, the time T for N fractions was previously defined as the interval between the first and Nth fractions, whereas we have defined T for N fractions as the “interval between the first and (N + l)th fractions”. For example, if five successive daily fractions are given, T=five days and not four as previously defined. Similarly, for a patient treated once weekly for ten weeks, T=70 days (and not 63).

Fractionated high dose rate versus low dose rate cervix cancer regimens

I found the article on fractionated high dose rate (HDR) versus low dose rate (LDR) regimens for intracavitary brachytherapy for cervix cancer by Brenner and Hall (1991) to be most interesting, especially their use of 38 different sets of linear-quadratic (L-Q) model parameters in their computations. Their ability to fit the L-Q model to the observed late complications data from the Christie Hospital LDR clinical trial (their Fig. 8) was especially impressive.

The NSD formula and the Oxford pig skin experiments

In a recent paper (Berry et al., 19741) pig skin experiments led to the conclusion that the NSD formula failed to correctly represent iso-effect doses in the range of 6–30 fractions. Unfortunately, the authors did not discuss the statistical significance of their results. In fact, my own analysis of their data shows that precisely the opposite conclusion could be drawn, namely, the NSD formula does represent iso-effect doses calculated from their results within experimental error.

Low-temperature response of LiF-Teflon dosimeters.

Lithium fluoride-Teflon microrods irradiated at 77 K show a 20% decrease in sensitivity compared to samples irradiated at room temperature. Measurements were made at60 Co and 50 kVp energies in both the linear and supralinear response regions. The observed decrease in sensitivity is attributed to a decrease in the number of electrons trapped in levels contributing to the main glow curve peak. A mechanism of competing trapping states present only at low temperatures can explain the general features of the data. Some electrons, normally trapped in the main glow curve energy levels, are instead bound to lower temperature competing traps. These do not contribute to the integrated thermoluminescent intensity because of rapid fading at room temperature.

Victoreen R-meter Linearity and the Stem/End-Cap Effect

Victoreen R-meters exhibit a nonlinear exposure response for high-energy radiations if the stem and end cap are included in the beam. For 12 chambers tested, the average sensitivity range was 7.5 ± 0.9% for 60Co radiation. If the rear of the stem and end cap are excluded from the primary radiation beam, nonlinearity is substantially reduced. Recommendations are made on the use and calibration of these instruments.