Radiobiological aspects of continuous low dose-rate irradiation and fractionated high dose-rate irradiation

Technical recommendations for implementation of Volumetric Modulated Arc Therapy and Helical Tomotherapy Total Body Irradiation

As a component of myeloablative conditioning before allogeneic hematopoietic stem cell transplantation (HSCT), Total Body Irradiation (TBI) is employed in radiotherapy centers all over the world. In recent and coming years, many centers are changing their technical setup from a conventional TBI technique to multi-isocenter conformal arc therapy techniques such as Volumetric Modulated Arc Therapy (VMAT) or Helical Tomotherapy (HT). These techniques allow better homogeneity and control of the target prescription dose, and provide more freedom for individualized organ-at-risk sparing. The technical design of multi-isocenter/multi-plan conformal TBI is complex and should be developed carefully. A group of early adopters with conformal TBI experience using different treatment machines and treatment planning systems came together to develop technical recommendations and share experiences, in order to assist departments wishing to implement conformal TBI, and to provide ideas for standardization of practices.

Iodine-125 seed implantation in the treatment of malignant tumors

Malignant tumors are major causes of morbidity and mortality in China. Despite advances in surgical, radiological, chemotherapeutic, molecular targeting, and immunotherapeutic treatments, patients with malignant tumors still have poor prognoses. Low-dose-rate brachytherapy, specifically 125I seed implantation, is beneficial because of its high local delivery dose and minimal damage to surrounding tissues. Consequently, it has gained increasing acceptance as a treatment modality for various malignant tumors. In this study, we explored the fundamental principles, clinical applications, and new technologies associated with 125I radioactive seed implantation.

ESTRO ACROP and SIOPE recommendations for myeloablative Total Body Irradiation in children

BACKGROUND AND PURPOSE

Myeloablative Total Body Irradiation (TBI) is an important modality in conditioning for allogeneic hematopoietic stem cell transplantation (HSCT), especially in children with high-risk acute lymphoblastic leukemia (ALL). TBI practices are heterogeneous and institution-specific. Since TBI is associated with multiple late adverse effects, recommendations may help to standardize practices and improve the outcome versus toxicity ratio for children.

MATERIAL AND METHODS

The European Society for Paediatric Oncology (SIOPE) Radiotherapy TBI Working Group together with ESTRO experts conducted a literature search and evaluation regarding myeloablative TBI techniques and toxicities in children. Findings were discussed in bimonthly virtual meetings and consensus recommendations were established.

RESULTS

Myeloablative TBI in HSCT conditioning is mostly performed for high-risk ALL patients or patients with recurring hematologic malignancies. TBI is discouraged in children <3-4 years old because of increased toxicity risk. Publications regarding TBI are mostly retrospective studies with level III-IV evidence. Preferential TBI dose in children is 12-14.4 Gy in 1.6-2 Gy fractions b.i.d. Dose reduction should be considered for the lungs to <8 Gy, for the kidneys to ≤10 Gy, and for the lenses to <12 Gy, for dose rates ≥6 cGy/min. Highly conformal techniques i.e. TomoTherapy and VMAT TBI or Total Marrow (and/or Lymphoid) Irradiation as implemented in several centers, improve dose homogeneity and organ sparing, and should be evaluated in studies.

CONCLUSIONS

These ESTRO ACROP SIOPE recommendations provide expert consensus for conventional and highly conformal myeloablative TBI in children, as well as a supporting literature overview of TBI techniques and toxicities.

Total Body Irradiation in Haematopoietic Stem Cell Transplantation for Paediatric Acute Lymphoblastic Leukaemia: Review of the Literature and Future Directions

Total body irradiation (TBI) has been a pivotal component of the conditioning regimen for allogeneic myeloablative haematopoietic stem cell transplantation (HSCT) in very-high-risk acute lymphoblastic leukaemia (ALL) for decades, especially in children and young adults. The myeloablative conditioning regimen has two aims: (1) to eradicate leukaemic cells, and (2) to prevent rejection of the graft through suppression of the recipient's immune system. Radiotherapy has the advantage of achieving an adequate dose effect in sanctuary sites and in areas with poor blood supply. However, radiotherapy is subject to radiobiological trade-offs between ALL cell destruction, immune and haematopoietic stem cell survival, and various adverse effects in normal tissue. To diminish toxicity, a shift from single-fraction to fractionated TBI has taken place. However, HSCT and TBI are still associated with multiple late sequelae, leaving room for improvement. This review discusses the past developments of TBI and considerations for dose, fractionation and dose-rate, as well as issues regarding TBI setup performance, limitations and possibilities for improvement. TBI is typically delivered using conventional irradiation techniques and centres have locally developed heterogeneous treatment methods and ways to achieve reduced doses in several organs. There are, however, limitations in options to shield organs at risk without compromising the anti-leukaemic and immunosuppressive effects of conventional TBI. Technological improvements in radiotherapy planning and delivery with highly conformal TBI or total marrow irradiation (TMI), and total marrow and lymphoid irradiation (TMLI) have opened the way to investigate the potential reduction of radiotherapy-related toxicities without jeopardising efficacy. The demonstration of the superiority of TBI compared with chemotherapy-only conditioning regimens for event-free and overall survival in the randomised For Omitting Radiation Under Majority age (FORUM) trial in children with high-risk ALL makes exploration of the optimal use of TBI delivery mandatory. Standardisation and comprehensive reporting of conventional TBI techniques as well as cooperation between radiotherapy centres may help to increase the ratio between treatment outcomes and toxicity, and future studies must determine potential added benefit of innovative conformal techniques to ultimately improve quality of life for paediatric ALL patients receiving TBI-conditioned HSCT.

Thermal limits on MV x-ray production by bremsstrahlung targets in the context of novel linear accelerators

PURPOSE

To study the impact of target geometrical and linac operational parameters, such as target material and thickness, electron beam size, repetition rate, and mean current on the ability of the radiotherapy treatment head to deliver high-dose-rate x-ray irradiation in the context of novel linear accelerators capable of higher repetition rates/duty cycle than conventional clinical linacs.

METHODS

The depth dose in a water phantom without a flattening filter and heat deposition in an x-ray target by 10 MeV pulsed electron beams were calculated using the Monte-Carlo code MCNPX, and the transient temperature behavior of the target was simulated by ANSYS. Several parameters that affect both the dose distribution and temperature behavior were investigated. The target was tungsten with a thickness ranging from 0 to 3 mm and a copper heat remover layer. An electron beam with full width at half maximum (FWHM) between 0 and3 mm and mean current of 0.05-2 mA was used as the primary beam at repetition rates of 100, 200, 400, and 800 Hz.

RESULTS

For a 10 MeV electron beam with FWHM of 1 mm, pulse length of 5 μs, by using a thin tungsten target with thickness of 0.2 mm instead of 1 mm, and by employing a high repetition rate of 800 Hz instead of 100 Hz, the maximum dose rate delivered can increase two times from 0.57 to 1.16 Gy/s. In this simple model, the limiting factor on dose rate is the copper heat remover's softening temperature, which was considered to be 500°C in our study.

CONCLUSIONS

A high dose rate can be obtained by employing thin targets together with high repetition rate electron beams enabled by novel linac designs, whereas the benefit of thin targets is marginal at conventional repetition rates. Next generation linacs used to increase dose rate need different target designs compared to conventional linacs.

Optimal treatment and stochastic modeling of heterogeneous tumors

In this work we review past articles that have mathematically studied cancer heterogeneity and the impact of this heterogeneity on the structure of optimal therapy. We look at past works on modeling how heterogeneous tumors respond to radiotherapy, and take a particularly close look at how the optimal radiotherapy schedule is modified by the presence of heterogeneity. In addition, we review past works on the study of optimal chemotherapy when dealing with heterogeneous tumors.

REVIEWERS

This article was reviewed by Thomas McDonald, David Axelrod, and Leonid Hanin.

In vitro RABiT measurement of dose rate effects on radiation induction of micronuclei in human peripheral blood lymphocytes

Developing new methods for radiation biodosimetry has been identified as a high-priority need in case of a radiological accident or nuclear terrorist attacks. A large-scale radiological incident would result in an immediate critical need to assess the radiation doses received by thousands of individuals. Casualties will be exposed to different doses and dose rates due to their geographical position and sheltering conditions, and dose rate is one of the principal factors that determine the biological consequences of a given absorbed dose. In these scenarios, high-throughput platforms are required to identify the biological dose in a large number of exposed individuals for clinical monitoring and medical treatment. The Rapid Automated Biodosimetry Tool (RABiT) is designed to be completely automated from the input of blood sample into the machine to the output of a dose estimate. The primary goal of this paper was to quantify the dose rate effects for RABiT-measured micronuclei in vitro in human lymphocytes. Blood samples from healthy volunteers were exposed in vitro to different doses of X-rays to acute and protracted doses over a period up to 24 h. The acute dose was delivered at ~1.03 Gy/min and the low dose rate exposure at ~0.31 Gy/min. The results showed that the yield of micronuclei decreases with decreasing dose rate starting at 2 Gy, whereas response was indistinguishable from that of acute exposure in the low dose region, up to 0.5 Gy. The results showed a linear-quadratic dose-response relationship for the occurrence of micronuclei for the acute exposure and a linear dose-response relationship for the low dose rate exposure.

Contrasting Dose-rate Effects of γ-irradiation on Rat Salivary Gland Function

The aim of this study was to investigate the effects of 60Co irradiation delivered at high (HDR) and low (LDR) dose-rates on rat salivary gland function. Total-body irradiation (TBI; total doses 7.5, 10 and 12.5 Gy) was applied from a 60Co source at dose-rates of 1 cGy/min (LDR) and 40 cGy/min (HDR) followed by syngeneic bone marrow rescue. Four days before and 1-30 days after TBI, submandibular and parotid saliva samples were collected in male albino Wistar rats using Lashley cups. Lag phase and flow rate were recorded, and [Na+] and [K+] were measured. The severity of salivary gland dysfunction for each dose-rate was dependent on total TBI dose in all parameters. LDR irradiation significantly enhanced the increase of lag phase, while it tended to further decrease flow rate during days 0-3. At later times the reverse effect was seen with significant LDR sparing in most cases. The changes in [Na+] and [K+] showed similar trends; LDR had an enhancing effect for early damage, while beyond day 3 it consistently produced less damage. From this dose-rate study it is concluded that the early postirradiation changes in salivary gland function are probably predominantly caused by irradiation damage to membrane structures and are less the result of reproductive failure. The later changes in salivary gland function are probably mainly dependent on repopulation of surviving stem cells.

The effect of fraction time in intensity modulated radiotherapy: theoretical and experimental evaluation of an optimisation problem

BACKGROUND AND PURPOSE

In intensity modulated radiotherapy (IMRT), the complexity and the number of treatment fields have expanded. This may imply that the delivery time for each fraction becomes prolonged. In a number of IMRT techniques used in the clinic, the delivery time per fraction is usually 10-15 min, sometimes more than 15 min. In studies on human skin, prolonged delivery time is shown to cause significant reduction of radiation effects compared with acute irradiation. In this paper the effect of changes in fraction delivery time was studied by in vitro irradiation of mammalian cells.

MATERIAL AND METHODS

Chinese hamster fibroblasts (V79-379-A) were used for simulating clinical situations. Most experiments were performed with 2Gy/fraction with 4-h intervals in 40-60 replicates. Each fraction was divided into different subfractions, simulating the delivery of a complicated treatment. The effect of changing the delivery time for each fraction was studied. Parameters for the cell survival curve and repair kinetics were determined experimentally. The same methods were also used for large fraction sizes (8Gy). The validity of the most widely used models in the literature, all derived from linear-quadratic formalism, were tested against the experimental results.

RESULTS

The effect of prolonging the fraction time for 2-Gy fractions was underestimated by the biological models. The experiments showed that 10-min prolonged delivery time gave a ratio between surviving fractions at 2Gy (S-ratio) of 1.054 with a 95% confidence interval (CI) 1.030-1.080, while the models predicted 1.007 and 1.009. Extending the fraction time to 20 min gave an S-ratio of 1.063 with CI of 1.045-1.080, while the models predicted 1.012 and 1.014. For 8-Gy fractions, there was a good agreement between predications and experimental results. The ratio between surviving fractions at 8Gy is 1.370 with CI of 1.300-1.440, while the models predicated 1.37 and 1.35.

CONCLUSIONS

The effect of prolonging fraction time at conventional dose/fraction is underestimated by biological models. Prolonging the fraction time will spare tissues with a fast DNA repair. There is a risk for sparing tumours. This should be considered when IMRT technique is implemented in the clinic.

Treatment planning and dosimetry for the Harvard-MIT Phase I clinical trial of cranial neutron capture therapy

PURPOSE

A Phase I trial of cranial neutron capture therapy (NCT) was conducted at Harvard-MIT. The trial was designed to determine maximum tolerated NCT radiation dose to normal brain.

METHODS AND MATERIALS

Twenty-two patients with brain tumors were treated by infusion of boronophenylalanine-fructose (BPA-f) followed by exposure to epithermal neutrons. The study began with a prescribed biologically weighted dose of 8.8 RBE (relative biologic effectiveness) Gy, escalated in compounding 10% increments, and ended at 14.2 RBE Gy. BPA-f was infused at a dose 250-350 mg/kg body weight. Treatments were planned using MacNCTPlan and MCNP 4B. Irradiations were delivered as one, two, or three fields in one or two fractions.

RESULTS

Peak biologically weighted normal tissue dose ranged from 8.7 to 16.4 RBE Gy. The average dose to brain ranged from 2.7 to 7.4 RBE Gy. Average tumor dose was estimated to range from 14.5 to 43.9 RBE Gy, with a mean of 25.7 RBE Gy.

CONCLUSIONS

We have demonstrated that BPA-f-mediated NCT can be precisely planned and delivered in a carefully controlled manner. Subsequent clinical trials of boron neutron capture therapy at Harvard and MIT will be initiated with a new high-intensity, high-quality epithermal neutron beam.

Single dose irradiation response of pig skin: a comparison of brachytherapy using a single, high dose rate iridium-192 stepping source with 200 kV X-rays

An experimental brachytherapy model has been developed to study acute and late normal tissue reactions as a tool to examine the effects of clinically relevant multifractionation schedules. Pig skin was used as a model since its morphology, structure, cell kinetics and radiation-induced responses are similar to human skin. Brachytherapy was performed using a microSelectron high dose rate (HDR) afterloading machine with a single stepping source and a custom-made template. In this study the acute epidermal reactions of erythema and moist desquamation and the late dermal reactions of dusky mauve erythema and necrosis were evaluated after single doses of irradiation over a follow-up period of 16 weeks. The major aims of this work were: (a) to compare the effects of iridium-192 (192Ir) irradiation with effects after X-irradiation; (b) to compare the skin reactions in Yorkshire and Large White pigs; and (c) to standardize the methodology. For 192Ir irradiation with 100% isodose at the skin surface, the 95% isodose was estimated at the basal membrane, while the 80% isodose covered the dermal fat layers. After HDR 192Ir irradiation of Yorkshire pig skin the ED50 values (95% isodose) for moderate/severe erythema and moist desquamation were 24.8 Gy and 31.9 Gy, respectively. The associated mean latent period (+/- SD) was 39 +/- 7 days for both skin reactions. Late skin responses of dusky mauve erythema and dermal necrosis were characterized by ED50 values (80% isodose) of 16.3 Gy and 19.5 Gy, with latent periods of 58 +/- 7 days and 76 +/- 12 days, respectively. After X-irradiation, the incidence of the various skin reactions and their latent periods were similar. Acute and late reactions were well separated in time. The occurrence of skin reactions and the incidence of effects were comparable in Yorkshire and Large White pigs for both X-irradiation and HDR 192Ir brachytherapy. This pig skin model is feasible for future studies on clinically relevant multifractionation schedules in a brachytherapy setting.

Radiation tolerance of rat spinal cord to pulsed dose rate (PDR-) brachytherapy: the impact of differences in temporal dose distribution

PURPOSE

To investigate the impact of a time-variable dose rate during a high dose rate (HDR-) or pulsed dose rate (PDR-) brachytherapy fraction with the HDR-microSelectron and to compare this with the biological effect of a constant dose rate treatment with the same average dose rate (as in the case of (192)Ir-wires). Moreover, the kinetics of repair in rat spinal cord are investigated using a wide spectrum of temporal dose distributions.

MATERIALS AND METHODS

Two parallel catheters are inserted on each side of the vertebral bodies of the rat spinal column (Th(10)-L(4)) and connected to the HDR-microSelectron. Interstitial irradiation (IRT) is performed with a stepping (192)Ir-point source, varying the activity of the point source between 0.3 and 6.5 Ci. Three different groups of experiments are defined, varying the overall treatment time and average dose rates in the range of 3-8, 28-53 and 82-182 min and 312-489 Gy/h, 32-56 Gy/h and 13-15 Gy/h, respectively. Difference in temporal dose distribution (dose rate variation) during almost the same overall treatment time is obtained by varying the number of pulses per dwell position in either one or ten runs through the implant. For reasons of comparison, previously reported results of continuous irradiation at a constant dose rate obtained with two (192)Ir-wires in a fixed position are reanalyzed. Paralysis of the hindlegs after 5-6 months and histopathological examination of the spinal cord of each animal are used as experimental endpoints.

RESULTS

During one run of the (192)Ir-point source, the peak dose rate is at least 25 times higher as compared with the minimum local dose rate and almost four times higher as compared with the average dose rate. For the three different groups of varying overall treatment times and average dose rates there is a significant difference in biological effect, with an ED(50)-value of 23.1-23.6 Gy (average dose rate 312-489 Gy/h), 25.4-27.9 Gy (average dose rate 312-489 Gy/h) and 29.3-33 Gy (average dose rate 13-15 Gy/h). For these range of single doses, difference in temporal dose distribution with either one or ten runs is only significant for treatment times less then 1 h. For the prolonged treatment times at lower average dose rates, the difference between one or ten run is no longer significant. However, the results with the (192)Ir-point source at an average dose rate/run of 13-15 Gy/h are significantly different from the ED(50)-value of 33 Gy using (192)Ir-wires at the same but constant dose rate. Using different types of analysis to estimate the repair parameters, the best fit of the data is obtained assuming biphasic repair kinetics and a variable dose rate (geometrically dependent) for the (192)Ir-point source. On the basis of the incomplete repair LQ model, two repair processes with an alpha/beta ratio=2.47 Gy and repair halftimes of 0.19 and 2.16 h are detected. The partition coefficient for the longer repair process is 0.98. This results in the proportion of total damage associated with the longer repair halftime being 0.495 for short sharp fractions with complete repair in between.

CONCLUSIONS

Even in the range of high dose rates of 15-500 Gy/h, spinal cord radiation tolerance is significantly increased by a reduction in dose rate. For larger doses per fraction in PDR-brachytherapy dose rate variation is important, especially for tissues with very short repair half times (components). In rat spinal cord the repair of sublethal damage (SLD) is governed by a biphasic repair process with repair halftimes of 0.19 and 2.16 h.

Biological equivalance of low dose rate to multifractionated high dose rate irradiations: investigations in mouse lip mucosa

BACKGROUND AND PURPOSE

The aim of this study was to evaluate the biological equivalence of continuous low dose rate (LDR) irradiations to multifractionated high dose rate (HDR) regimes. The applicability of the LQ model was analysed for fraction sizes and dose rates relevant for the clinic.

MATERIAL AND METHODS

Investigations were performed in mouse lip mucosa. HDR fractions were given in an overall treatment time ranging from 10 h to 3.5 days. The dose rate effect was analysed in the range of 84 to 0.76 Gy/h. For an assessment of biological equivalence in comparison to LDR, HDR irradiations have been performed in the same overall treatment time as the corresponding LDR regimes.

RESULTS

Recovery leads to sparing of radiation damage as the dose rate is reduced from 84 to 0.76 Gy/h (20.0 versus 45.7 Gy ED50). No significant additional sparing from 0.9 to 0.76 Gy/h could be demonstrated (44.9 versus 45.7 Gy ED50). Even 30 HDR fractions in 24 h were not sufficient to match the effect of LDR over the same time period (38.2 versus 41.1 Gy ED50). The present data give evidence for a bi-exponential repair process in mouse lip mucosa (T1/2 fast 27 min, T1/2 slow 150 min). Repair is dominated by the faster component (> 80%).

CONCLUSIONS

LDR is the most efficient way to deliver radiation if recovery is to be maximised and the overall time kept as short as possible. When used with realistic parameters the LQ model is capable of providing quantitative guidelines in areas of clinical interest.

High dose rate (HDR) and low dose rate (LDR) interstitial irradiation (IRT) of the rat spinal cord

PURPOSE

To describe a newly developed technique to study radiation tolerance of rat spinal cord to continuous interstitial irradiation (IRT) at different dose rates.

MATERIAL AND METHODS

Two parallel catheters are inserted just laterally on each side of the vertebral bodies from the level of Th10 to L4. These catheters are afterloaded with two 192Ir wires of 4 cm length each (activity 1-2.3 mCi/cm) for the low dose rate (LDR) IRT or connected to the HDR micro-Selectron for the high dose rate (HDR) IRT. Spinal cord target volume is located at the level of Th12-L2. Due to the rapid dose fall-off around the implanted sources, a dose inhomogeneity across the spinal cord thickness is obtained in the dorso-ventral direction. Using the 100% reference dose (rate) at the ventral side of the spinal cord to prescribe the dose, experiments have been carried out to obtain complete dose response curves at average dose rates of 0.49, 0.96 and 120 Gy/h. Paralysis of the hind-legs after 5-6 months and histopathological examination of the spinal cord of each irradiated rat are used as experimental endpoints.

RESULTS

The histopathological damage seen after irradiation is clearly reflected the inhomogeneous dose distribution around the implanted catheters, with the damage predominantly located in the dorsal tract of the cord or dorsal roots. With each reduction in average dose rate, spinal cord radiation tolerance is significantly increased. When the dose is prescribed at the 100% reference dose rate, the ED50 (induction of paresis in 50% of the animals) for the HDR-IRT is 17.3 Gy. If the average dose rate is reduced from 120 Gy/h to 0.96 or 0.49 Gy/h, a 2.9- or 4.7-fold increase in the ED50 values to 50.3 Gy and 80.9 Gy is observed; for the dose prescribed at the 150% reference dose rate (dorsal side of cord) ED50 values are 26.0, 75.5 and 121.4 Gy, respectively. Using different types of analysis and in dependence of the dose prescription and reference dose rate, the alpha/beta ratio varies between 1.46 (0.06-3.08 CL) and 2.17 Gy (0.08-4.61). The half time of repair during continuous irradiation is 1.76 h (1.33-2.64), while no indication is found for a biphasic pattern of the kinetics of repair.

CONCLUSION

The implantation technique in our study has shown to be a reliable model to compare the effectiveness of HDR- and LDR-interstitial continuous irradiation at different dose rates.

Pulsed dose rate and fractionated high dose rate brachytherapy: choice of brachytherapy schedules to replace low dose rate treatments

PURPOSE

Pulsed dose rate (PDR) brachytherapy is a new type of afterloading brachytherapy (BT) in which a continuous low dose rate (LDR) treatment is simulated by a series of "pulses," i.e., fractions of short duration (less than 0.5 h) with intervals between fractions of 1 to a few hours. At the Dr. Daniel den Hoed Cancer Center, the term "PDR brachytherapy" is used for treatment schedules with a large number of fractions (at least four per day), while the term "fractionated high dose rate (HDR) brachytherapy" is used for treatment schedules with just one or two brachytherapy fractions per day. Both treatments can be applied as alternatives for LDR BT. This article deals with the choice between PDR and fractionated HDR schedules and proposes possible fractionation schedules.

METHODS AND MATERIALS

To calculate HDR and PDR fractionation schedules with the intention of being equivalent to LDR BT, the linear-quadratic (LQ) model has been used in an incomplete repair formulation as given by Brenner and Hall, and by Thames. In contrast to earlier applications of this model, both the total physical dose and the overall time were not kept identical for LDR and HDR/PDR schedules. A range of possible PDR treatment schedules is presented, both for booster applications (in combination with external radiotherapy (ERT) and for BT applications as a single treatment. Because the knowledge of both alpha/beta values and the half time for repair of sublethal damage (T 1/2), which are required for these calculations, is quite limited, calculations regarding the equivalence of LDR and PDR treatments have been performed for a wide range of values of alpha/beta and T 1/2. The results are presented graphically as PDR/LDR dose ratios and as ratios of the PDR/LDR tumor control probabilities.

RESULTS

If the condition that total physical dose and overall time of a PDR treatment must be exactly identical to the values for the corresponding LDR treatment regimen is not applied, there appears to be less need for strong fractionation in PDR schedules. If the overall time is at least as long as that of the LDR schedule and if the total physical dose is (slightly) adapted, PDR schedules can be designed using longer pulse intervals of up to 3 h. Schedules with sufficiently long intervals have significant logistic advantages in terms of patient care and treatment tolerance. However, in general, PDR schedules that apply more fractionation have a lower risk of overdosing normal tissues in comparison to fractionated HDR schedules. Applying probable ranges for the values of alpha/beta and T 1/2, the model calculations indicate that the differences in effects between the proposed fractionated HDR and PDR schedules could be rather small. To detect the magnitude of these differences, (randomized) clinical studies with rather large patient groups might be needed.

CONCLUSIONS

Pulsed dose rate treatment schedules with longer intervals of up to 3 h appear adequate to replace LDR treatment schedules. Whether PDR schedules can, indeed, replace LDR treatment schedules and whether they offer detectable advantages over schedules with less fractionation (fractionated HDR) should be tested in clinical studies.

Twice-per-day fractionated high versus continuous low dose rate intracavitary therapy in the radical treatment of cervical cancer: a nonrandomized comparison of treatment results

PURPOSE

To compare the efficacy of two twice-per-day fractionated high dose rate (HDR) brachytherapies with a historical control group treated with low dose rate (LDR) brachytherapy for cervical cancer patients.

METHODS AND MATERIALS

From 1985 to 1988, 92 patients with cancer of the cervic were treated by remote-controlled, HDR brachytherapy, six fractions of 7 Gy per fraction (42 Gy) at point A (HDR-6). Fifty-seven patients were treated with four fractions of 8 Gy per fraction (32 Gy) at point A (HDR-4). A twice-per-day program was used for all HDR patients by two split courses. As a historical control, treatment results of 259 patients treated with LDR brachytherapy (40 Gy in two split courses) were compared with those of the two HDR regimens. All patients received whole pelvic external irradiation, 36-45 Gy (mostly 40 Gy) before brachytherapy.

RESULTS

Five-year local control rates were not significantly different for the three groups (HDR-6 = 82.0%, HDR-4 = 85.5%, and LDR = 89.5%, respectively). Five-year survival rates were also comparable (67.7%, 77.9%, and 74.1%, respectively). However, late complications were lower in HDR-4 than HDR-6 (11.0% vs. 25.6%).

CONCLUSIONS

Both 5-year local control and survival rates were comparable among the three groups. However, HDR-4, which was more biologically equivalent to our LDR regimen, showed fewer complications compared to HDR-6. In addition, our twice-per-day schedule shortened the hospital stay.

The kinetics of cellular recovery in exponential and plateau growth phase human glioma cells following gamma-irradiation

PURPOSE

In this study the kinetics of recovery following irradiation was examined in a human glioma cell line. Specific objectives were: to determine whether recovery is mono- or biexponential in nature; to determine if recovery half-times are different in exponential and plateau growth phase cells; to compare recovery half-times as a function of dose or recovery levels; and finally, to compare the kinetics of sublethal damage recovery and potentially lethal damage recovery in plateau growth phase cells.

METHODS AND MATERIALS

U-87MG cells were irradiated in exponential and plateau growth phases and then subjected to incubation at 37 degrees C for various periods of time following or between doses prior to assaying for survival. Survival recovery curves were fit to a sum of exponential terms.

RESULTS

Potentially lethal damage recovery was monoexponential in both exponential and plateau growth phase cells and occurred at the same rate when isorecovery values were compared. Recovery half-times increased in an exponential manner within the observed dose range. Recovery between doses of radiation (sublethal damage recovery) proceeded at a slower rate than recovery following a single dose of radiation (potentially lethal damage recovery).

CONCLUSIONS

This study suggests that potentially lethal damage recovery is a saturated process and that the recovery half-time may increase in a linear-quadratic exponential function of dose similar to the absolute recovery level. In addition, if iso-recovery levels are compared, the recovery half-time is similar in rapidly and slowly proliferating cell populations.

Influence of time-dependent stochastic heterogeneity on the radiation response of a cell population

A solid tumor is a cell population with extensive cellular heterogeneity, which severely complicates tumor treatment by therapeutic agents such as ionizing radiation. We model the response to ionizing radiation of a multicellular population whose cells have time-dependent stochastic radiosensitivity. A reaction-diffusion equation, obtained by assuming a random process with the radiation response of a cell partly determined by competition between repair and binary misrepair of DNA double-strand breaks, is used. By a suitable transformation, the equation is reduced to that of an Ornstein-Uhlenbeck process so explicit analytic solutions are available. Three consequences of the model's assumptions are that (1) response diversity within a population increases resistance to radiation, that is, the population surviving is greater than that anticipated from considering an average cell; (2) resistant cell subpopulations preferentially spared by the first part of a prolonged radiation protocol are driven biologically into more radiosensitive states as time increases, that is, resensitization occurs; (3) an inverse dose-rate effect, that is, an increase in cell killing as overall irradiation time is increased, occurs in those situations where resensitization dominates effects due to binary misrepair of repairable damage. The results are consistent with the classic results of Elkind and coworkers on extra cell killing attributed to cell-cycle redistribution and are in agreement with some recent results on in vitro and in vivo population radiosensitivity. They also generalize the therapeutic paradigm that low dose rate or fractionated radiation can help overcome hypoxic radioresistance in tumors.

Analysis of complications in a prospective randomized trial comparing two brachytherapy low dose rates in cervical carcinoma

PURPOSE

The analysis of complications in a prospective randomized trial comparing two preoperative brachytherapy low-dose rates in early stage cervical cancer is presented.

METHODS AND MATERIALS

Between 1985 and 1988, 204 patients with Stage I and limited Stage II cervical cancer were randomized to receive one of two preoperative brachytherapy low-dose rates (0.4 and 0.8 Gy/hr). The objective of this trial was to determine the benefits, if any, of the higher-dose rate within the therapeutic arsenal for this patient population, in terms of survival, local control, and complications. The type and severity of all complications were evaluated according to a common glossary and a strict follow-up schedule was established given that the treatment of cervical cancer is multidisciplinary, involving gynecologists, surgeons, and radiotherapists.

RESULTS

Overall survival: 85% at 2 years and local control: 93% at 2 years, were similarly distributed between the two groups. Regardless of their nature and severity, 139 and 175 complications were observed among 63% and 75% of patients, in the 0.4 and 0.8 Gy/h dose rate groups respectively. Gynecologic and urinary complications were the most frequent (38% and 28% of all complications), followed by vascular (15%), digestive (10%), nervous (5%) and cutaneous (5%). A total of 14 and 17 severe complications (Grade 3) were observed in 7% and 13% of patients, respectively in the 0.4 and 0.8 Gy/h dose rate groups (p = 0.12). Nonparametric survival methods used to compare the time to the first complication did not show a significant difference between the two groups: 62% and 72% at 2 years (p = 0.27). When the first complication and its evolution were considered (early complications), the prevalence of complications was not significantly different between the two groups: 28% vs. 34% at 2 years (p = 0.31). In this prospective trial, patients were regularly followed-up and complications of varying nature and severity were observed in succession during follow-up. When successive complications and their evolution were taken into account, the prevalence of complications was significantly greater in the higher-dose rate group: 30% vs. 45% at 2 years (p = 0.03).

CONCLUSION

The results of this trial showed that long-term effects of treatment, when represented by prevalence of complications over time, were more frequent in the higher dose rate group. This underlines the importance of the regular follow-up of patients and of coding, not only the occurrence of all complications, but also their evolution over time.

A comparison of early effects with two dose rates in brachytherapy of cervix carcinoma in a prospective randomised trial

This phase III randomised trial examined the early effects of two low dose rates (0.38 and 0.73 Gy/h) in brachytherapy of stage I and IIp cervical cancer patients. A total of 204 patients were included between January 1985 and September 1988. Since the main analysis of this paper concerned surgical difficulties, only the 155 patients (76%) on whom surgery was performed at the Institut Gustave-Roussy were retained in this analysis. Treatment consisted of uterovaginal 137Cs irradiation followed by immediate or deferred surgery. The two groups were similar for pretreatment characteristics except for endocervix involvement. Their brachytherapy parameters were also similar (60 Gy pear dimensions, doses to critical organs, total kerma, etc.). The factors with a poor prognosis were, for surgical difficulties, older age, stage II and a small irradiated pear volume; for difficulties with haemostasis, immediate surgery, stage II and previous surgery; and for difficulties in dissection, lymph node involvement. The dose rate significantly influenced surgical difficulties for the stage IIp patients operated on by deferred surgery. Those treated with the higher dose rate showed a 2-fold increase in surgical difficulties compared to those irradiated at the lower dose rate (P = 0.03). The independent prognostic factors for sterilisation of the surgical specimen were small tumour size and absence of lymph node involvement. An inverse dose rate effect was observed for medium size tumours, with significantly more sterilisations observed in stage IIp patients in the lower dose rate group (P < 0.01).

HDR versus LDR gynecological brachytherapy revisited

Despite the obvious breakthrough of high dose-rate (HDR) afterloading systems on the gynecological brachytherapy market, questions still remain regarding the transfer of available expertise gained throughout the last 80 years with low dose-rate (LDR) radium and cesium, especially regarding the conversion of LDR total dose into equivalent HDR dose per fraction and total dose. Calculation of biologically equivalent schedules requires a knowledge of repair capacity and repair kinetics of tumors and normal tissues, both of which influence the biological effect of any radiation dose. The clinical experience with HDR is, however, accumulating and it is acknowledged that the new technique entails an acceptable therapeutic index as compared to the classical LDR. There is thus a state of apparent 'equivalence' between the two treatment modalities. This state is influenced by many factors in which, in contrast to what is frequently claimed, radiobiological factors do not play the most important role. It is probably its high-tech environment which makes HDR an acceptable alternative. Treatment at LDR, indeed, has proven to be quite tolerant to a lack of absolute precision, something that would be disastrous with HDR techniques. Because HDR intracavitary brachytherapy has not been compared in controlled trials with the best existing LDR brachytherapy, but only retrospectively with heterogeneous LDR clinical data, it cannot be claimed to be equivalent, but simply feasible.

Phase III trial comparing two low dose rates in brachytherapy of cervix carcinoma: report at two years

This Phase III randomized trial examined the effect of two low dose rates (0.73 or 0.38 Gy.h-1) on the local control, survival, relapse-free survival, complications, and secondary effects in the treatment of cervical cancers. A total of 204 Stage Ib or II cervical carcinoma patients were included between January 1985 and September 1988. Treatment consisted of uterovaginal 137Cs irradiation followed by surgery. The two groups were similar for age, tumor stage and medical or surgical history. Their brachytherapy parameters were also similar (60 Gy pear dimensions, dose to critical organs, total kerma, etc....) There were no differences in the short-term effects or therapeutic outcome. However, overall complications and side effects observed after 6 months were significantly more frequent (p < 0.01) in the higher dose rate group.

Overview of animal studies comparing radioimmunotherapy with dose equivalent external beam irradiation

As the field of radioimmunotherapy (RIT) continues to develop and looks increasingly promising, there is growing interest in the radiobiology of RIT. Recently, several investigators have conducted studies in animal models comparing the relative efficacy of RIT with dose equivalent external beam irradiation. Although these studies are the first of many to follow, the results are provocative and several patterns are suggested by the available data. The results of the studies are summarized and compared, and preliminary hypotheses that might explain the reported observations are discussed. In summary, results from studies comparing the efficacy of RIT with external beam irradiation have been variable and may be indicative of different underlying mechanisms. While the particular experimental model, design and methodology used to compare the efficacy of RIT with external beam irradiation are probably important influences upon subsequent observations, it appears that for a given tumor type, the size of the survival curve shoulder or alpha/beta ratio, and tumor doubling time are important determinants of the magnitude of the dose rate effect. When this effect is minimal, it is possible that other factors such as reoxygenation, the arrest of cells in G2, and selective targeting of tumor by radiolabelled antibody may explain, in part, the increased efficacy of RIT compared with external beam irradiation that has been observed in some systems.

Changes in the basal cell density in pig skin after single radiation doses with different dose rates

Radiation-induced changes in the basal cell density (BCD) and the labelling index (LI) in the epidermis of pig skin were used to compare the effect of high and low dose rate irradiation. Single doses of 6, 12 and 24 Gy were applied at 1.5 and 0.02 Gy/min with two equal 137Cs sources. Punch biopsies were taken daily for 40 days. The linear BCD was calculated from histological sections. The LI was determined by an in vitro H3-thymidine labelling technique. We found a degenerative phase in the basal cell layer with a linear cell loss of 2% per day, independent of the dose and dose rate. The time and level of the minimum BCD were dose and dose rate dependent. The LI data indicated an increased proliferation rate after a 25% reduction in BCD, i.e. before the nadir was reached. From the BCD data an iso-effective dose factor of 1.8 was estimated for the two dose rates used, which was consistent with that determined from macroscopic scoring. The BCD can be used as an endpoint for comparison of different radiotherapy modalities and gives, together with the LI, further information on the time-course of the proliferation changes in the tissue.