Dose-rate effects in biologically targeted radiotherapy

Design and testing of a microcontroller that enables alpha particle irradiators to deliver complex dose rate patterns

There is increasing interest in using alpha particle emitting radionuclides for cancer therapy because of their unique cytotoxic properties which are advantageous for eradicating tumor cells. The high linear energy transfer (LET) of alpha particles produces a correspondingly high density of ionizations along their track. Alpha particle emitting radiopharmaceuticals deposit this energy in tissues over prolonged periods with complex dose rate patterns that depend on the physical half-life of the radionuclide, and the biological uptake and clearance half-times in tumor and normal tissues. We have previously shown that the dose rate increase half-time that arises as a consequence of these biokinetics can have a profound effect on the radiotoxicity of low-LET radiation. The microcontroller hardware and software described here offer a unique way to deliver these complex dose rate patterns with a broad-beam alpha particle irradiator, thereby enabling experiments to study the radiobiology of complex dose rate patterns of alpha particles. Complex dose rate patterns were created by precise manipulation of the timing of opening and closing of the electromechanical shutters of an α-particle irradiator. An Arduino Uno and custom circuitry was implemented to control the shutters. The software that controls the circuits and shutters has a user-friendly Graphic User Interface (GUI). Alpha particle detectors were used to validate the programmed dose rate profiles. Circuit diagrams and downloadable software are provided to facilitate adoption of this technology by other radiobiology laboratories.

Spatial distribution of Auger electrons emitted from internalised radionuclides in cancer cells: the photoresist autoradiography (PAR) method

Microdosimetric evaluation of Auger electron-emitting radionuclides involves a detailed evaluation of energy deposition at a nanometre scale. To perform Monte Carlo modelling of such energy deposition, accurate information regarding the spatial distribution of the radionuclide is required. A recent addition to the methods for determining the spatial distribution of cellular internalised radionuclides is based on detection in a polymer photoresist (e.g. polymethyl methacralate), followed by atomic force microscopy analysis of the resultant 3D pattern. In comparison with present practice, the method offers greater spatial resolution and improved quantification. The volume of the pattern is proportional to the total dose, thereby permitting assessment of variability of accumulated activity, while the variation in depth across the pattern reflects the lateral spatial distribution in the local fluence per unit area. An added advantage is the similarity in response to ionising radiation of an organic polymer compared to that of biological material. A pattern in the resist from radiation emitted by a radionuclide treated cell gives additional spatial information about the energy deposited in the resist.

Survival of tumor and normal cells upon targeting with electron-emitting radionuclides

PURPOSE

Previous studies have shown that the mean absorbed dose to a tissue element may not be a suitable quantity for correlating with the biological response of cells in that tissue element. Cell survival can depend strongly on the distribution of radioactivity at the cellular and multicellular levels. Furthermore, when cellular absorbed doses are examined, the cross-dose from neighbor cells can be less radiotoxic than the self-dose component. To better understand how the nonuniformity of activity among cells can affect the dose response, a computer model of a 3D tissue culture was previously constructed and showed that activity distribution among cells is significantly more relevant than the mean absorbed dose for low-energy-electron emitters. The present work greatly expands upon those findings.

METHODS

In the present study, we used this same computer model but restricted the number of labeled cells to a fraction of the whole cell population (50%, 10%, and 1%, respectively). The labeled cells were randomly distributed among the whole cell population.

RESULTS

While the activity distribution is an important factor in determining the tissue response for low-energy-electron emitters, the fraction of labeled cells has an even more pronounced effect on survival response. For all electron energies studied, reducing the percentage of cells labeled significantly increases the surviving fraction of the whole population.

CONCLUSIONS

This study provides abundant information on killing tumor and normal cells under some conditions relevant to targeted radionuclide therapy of isolated tumor cells and micrometastases. The percentage of cells labeled, activity distribution among the labeled cells, and electron energy play key roles in determining their response. Most importantly, and not previously demonstrated, lognormal activity distributions can have a profound impact on the response of the tumor cells even when the radionuclide emits high-energy electrons.

RadNuc: a graphical user interface to deliver dose rate patterns encountered in nuclear medicine with a 137Cs irradiator

The temporal variations in absorbed dose rates to organs and tissues in the body are very large in diagnostic and therapeutic nuclear medicine. The response of biological endpoints of relevance to radiation safety and therapeutic efficacy is generally modulated by dose rate. Therefore, it is important to understand how the complex dose rate patterns encountered in nuclear medicine impact relevant biological responses. Accordingly, a graphical user interface (GUI) was created to control a cesium-137 irradiator to deliver such dose rate patterns.

METHODS

Visual Basic 6.0 was used to create a user-friendly GUI to control the dose rate by varying the thickness of a mercury attenuator. The GUI facilitates the delivery of a number of dose rate patterns including constant, exponential increase or decrease, and multi-component exponential. Extensive visual feedback is provided by the GUI during both the planning and delivery stages.

RESULTS

The GUI controlled irradiator can achieve a maximum dose rate of 40 cGy/h and a minimum dose rate of 0.01 cGy/h. Addition of machined lead blocks can be used to further reduce the minimum dose rate to 0.0001 cGy/h. Measured dose rate patterns differed from programmed dose rate patterns in total dose by 3.2% to 8.4%.

CONCLUSION

The GUI controlled irradiator is able to accurately create dose rate patterns encountered in nuclear medicine and other related fields. This makes it an invaluable tool for studying the effects of chronic constant and variable low dose rates on biological tissues in the contexts of both radiation protection and clinical administration of internal radionuclides.

Monte Carlo simulation of irradiation and killing in three-dimensional cell populations with lognormal cellular uptake of radioactivity

PURPOSE

The biological response of tissue exposed to radiations emitted by internal radioactivity is often correlated with the mean absorbed dose to a tissue element. However, experimental studies show that even when the mean absorbed dose to the tissue element is constant, the response of the cell population within the tissue element can vary significantly depending on the distribution of radioactivity at the cellular and multicellular levels. The present work develops theoretical models to simulate these observations.

MATERIALS AND METHODS

Two theoretical models were created to simulate experimental three-dimensional cell culture models with homogeneous and inhomogeneous tissue environments. The cells were assigned activities according to lognormal distributions of an alpha particle emitter or a monoenergetic electron emitter. Absorbed doses to the cell nuclei were assessed with point-kernel geometric-factor and Electron Gamma Shower version nrc (EGSnrc) Monte Carlo radiation transport simulations, respectively. The self- and cross-dose to individual cell nuclei were calculated and a Monte Carlo method was used to determine their fate. Survival curves were produced after tallying the live and dead cells.

RESULTS

Both percent cells labeled and breadth of lognormal distribution affected the dose distribution at the cellular level, which in turn, influenced the shape of the cell survival curves.

CONCLUSIONS

Multicellular Monte Carlo dosimetry-models offer improved capacity to predict response to radiopharmaceuticals compared to approaches based on mean absorbed dose to the tissue.

Late normal tissue injury from permanent interstitial implants

PURPOSE

To develop a simple method of calculating biologically effective doses in high-dose regions of permanent interstitial implants.

METHODS AND MATERIALS

The incomplete repair model is used to clarify the relationship between dose, D, and biologically effective dose (BED), for permanent interstitial implants. The relationship is used to ascertain the BED at high-dose regions that may occur in (125)I, (103)Pd, and (198)Au prostate implants.

RESULTS

The relationship between D and BED is nonlinear and is given by BED(D) = D + D(2)/D(lambda), where D(lambda) = [(t(lambda)/t(mu)) + 1](alpha/beta), t(lambda) and t(mu) are the half-lives of the isotope and of sublethal damage repair respectively, and alpha/beta is the alpha:beta ratio. Idealized geometrically identical (125)I, (103)Pd, and (198)Au prostate implants with minimum target dose (MTD) of 160 Gy, 120 Gy, and 64 Gy, respectively, are considered. The BED for (103)Pd and (198)Au will be less than the BED for (125)I, for doses up to about 2.5 times the MTD. For higher doses, the BED for (103)Pd may be significantly higher than for (125)I.

CONCLUSION

Permanent interstitial implants using short-lived isotopes may have regions with very high biologically effective doses.

Optimal scheduling of biologically targeted radiotherapy and total body irradiation with bone marrow rescue for the treatment of systemic malignant disease

A mathematical model analysis is used to address the question of optimal scheduling of combined treatments consisting of biologically targeted radiotherapy (BTR), total body irradiation (TBI), and bone marrow rescue. Radiation effects on normal tissue are described using an extension of the LQ model. Tumor effects are described using a simple model that allows for radiation-induced sterilization and exponential proliferation of tumor cells, a proportion of which completely escapes the effects of targeted radiotherapy. The effect on a tumor cell population of a set of treatment schedules, composed partly of targeted radiotherapy and partly of fractionated external beam irradiation, are calculated. Treatment schedules are chosen to be biologically equivalent, for a "late responding" organ, to a fractionated TBI schedule of 7 fractions of 2 Gy. The tumor effects of the treatment schedules depend on the specificity of targeting, represented by the ratio of initial dose-rate for the tumor cells to that in the dose-limiting organ, and the heterogeneity of targeting, represented by the proportion of tumor cells that escape irradiation by targeted radiotherapy. The main mechanism determining optimal combinations is an overkill of effectively targeted tumor cells. Treatment regiments consisting of targeted radiotherapy alone fail, due to the unimpeded growth of those tumor cells that escape targeted irradiation. Optimal schedules almost invariably consist of elements of both BTR and TBI. Although it is recognized that the model is simplistic in a number of respects, these findings provide support for the clinical use of integrated BTR, TBI, and bone marrow rescue for the treatment of systemic malignant disease.

Dosimetry of iodine 131 metaiodobenzylguanidine for treatment of resistant neuroblastoma: results of a UK study

In 1987, the United Kingdom Children's Cancer Study Group (UKCCSG) set up a multi-centre study to investigate the toxicity of iodine 131 metaiodobenzylguanidine (mIBG) in the treatment of resistant neuroblastoma. Since December 1987, 25 children suffering from neuroblastoma have been treated with 131I-mIBG at six UK centres. All centres followed standardised physics and clinical protocols to provide consistent toxicity and dosimetry data. These protocols describe the methods employed for both the tracer study using 131I-mIBG and the subsequent therapy. Whole-body dosimetry calculations were performed on data from the tracer study. The activity administered for therapy was the amount predicted to deliver a predefined whole-body dose. Estimates of doses delivered to various organs during treatment are given in Table 1.

The radiobiology of targeted radiotherapy

Targeted radiotherapy consists of biologically selective irradiation of malignant cells by means of radionuclides attached to tumour-seeking molecules. A variety of clinical strategies for targeted radiotherapy may be used, for which different normal tissues will be critical. A large number of radionuclides exist, emitting nuclear particles with a range of path lengths from nanometres to millimetres. An important feature of normal-tissue radiobiology is the dose-rate effect, which is especially marked for late-responding tissues. Radiobiological calculations imply that tolerance dose for targeted radiotherapy using low-LET emitters will depend strongly on the effective half-life of the radionuclide, which will be affected by pharmacokinetics and may vary between patients. Some strategies designed to improve the therapeutic radio (e.g. accelerated clearance of radionuclide) may have modulating effects on the tolerance dose. Tumour response will be governed by the 'four Rs' (repair, repopulation, reoxygenation, redistribution) as well as by mechanisms peculiar to targeted radiotherapy. Analysis based on the extended linear quadratic model predicts that dose-rate effects will be of major importance for only a minority of tumours. Most of the radiation dose to tumour will usually be delivered over a time-scale of a few days. This might give insufficient time for tumour reoxygenation, making the use of hypoxic sensitizers appropriate. A special feature of targeted radiotherapy is the complex relationship between tumour curability and tumour size for different radionuclides. For long-range beta-emitters, microscopic tumours may be operationally resistant because of inefficient absorption of radionuclide disintegration energy in small volumes. Short-range emitters will be more efficient in sterilization of micrometastases but sterilization of larger tumours may require an unattainable degree of homogeneity of radionuclide distribution. Optimal use of targeted radiotherapy may require it to be combined with external-beam irradiation or chemotherapy. Experimental studies will be necessary to investigate those features of targeted radiotherapy which differ from external-beam irradiation. Future directions may include targeted radiotherapy of minimal numbers of tumour cells detected by use of molecular probes. Such applications call for use of short-range alpha-emitters and Auger emitters whose radiobiology will become increasingly important.