Recovery from sublethal damage during intermittent exposures in cultured tumor cells: implications for dose modification in radiosurgery and imrt

Dynamic conformal arc radiotherapy with rectum hollow-out technique for localized prostate cancer

PURPOSE

To report the feasibility of dynamic conformal arc radiotherapy with rectum hollow-out technique (DCAT-HO) for localized prostate cancer.

METHODS AND MATERIALS

Between October 2000 and April 2007, 204 patients with clinically localized or locally advanced prostate cancer were treated with DCAT-HO. All patients were given neoadjuvant total androgen deprivation (AD) therapy (median 6 months, range 2-27 months). All patients with T3 or T4 stage received post-irradiation AD for 24 months. A total of 128 patients (63%) were treated with 70Gy, and 76 patients (37%) were treated with 74Gy. Acute and late toxicities were scored by the Radiation Therapy Oncology Group morbidity grading scales. PSA relapse was defined as three successive PSA elevations after a post-treatment nadir. The median follow-up was 37 months.

RESULTS

Both the acute Grade 2 rectal and urinary toxicities were 1.0%, and no patients experienced acute Grade 3 or higher symptoms. The 3-year rates of both late Grade 2 rectal and urinary toxicities were 3.4%. The 3-year PSA relapse-free survival for low, intermediate, and high-risk group patients treated with 70 Gy were 54%, 75%, and 87%, respectively.

CONCLUSIONS

These findings demonstrate the feasibility of DCAT-HO in a large number of patients with short follow-up. DCAT-HO reduced the volume of the rectum exposed to higher doses and this led to an overall reduction in late rectal toxicity.

Comparison of the KonRad IMRT and XiO treatment planning systems

Reducing the treatment time for IMRT patients is highly desirable. The objective of this work was to evaluate the new clinical Siemens KonRad inverse treatment planning system (TPS) and compare it to the CMS XiO TPS with special emphasis on the segmentation efficiency. For head and neck, liver and prostate cancer patients, step‐and‐shoot IMRT plans were designed using both CMS XiO and Siemens KonRad TPS. Number, direction and energy of beams used were the same in the plans from both systems for each treatment site. The plans were optimized to achieve the same clinical objectives concerning dose to the target volume and to the relevant organs‐at‐risk (OARs). The number of intensity levels were minimized until the clinical objectives could not be achieved anymore. Dose‐volume histograms (DVHs), mean and maximum doses were compared, as well as the number of beam segments and monitor units (MUs). The beams of each plan were delivered individually on a MapCheck device to verify the agreement between calculations and measurements to be less than 3%–3 mm distance‐to‐agreement. Plans optimized with KonRad resulted in fewer segments and lower number of MUs and therefore reduced delivery time on average by 28% or 3.6 min, while achieving similar dose distributions. CMS XiO plans exhibited a slightly steeper dose fall‐off outside the target volumes; however the difference was not clinically significant. DVHs to OARs were comparable. All calculated dose distributions passed the 3%–3 mm verification check.

PACS numbers: 87.55.D

Effect of radiation protraction in intensity-modulated radiation therapy with direct aperture optimization: a phantom study

The effect of radiation protraction in step-and-shoot IMRT is investigated for treatment plans created with the help of direct aperture optimization. The latter approach can be used during inverse planning for all clinical linear accelerators with conventional MLC. Direct aperture optimization significantly shortens fraction time for IMRT plans as compared to that for plans obtained by using the conventional inverse planning approach. By analyzing several IMRT plans obtained with direct aperture optimization we found that for alpha/beta ratio of 10 Gy (characteristic of fast growing tumors) the protraction effect is probably clinically insignificant for both conventional and large fraction sizes of 1.9 Gy and 5.7 Gy, respectively. For small alpha/beta of 1-1.5 Gy and conventional fraction size the effect of protraction is still small; however, this effect can be significant for hypofractionated treatments. Based on the obtained results it is recommended that, when possible, IMRT for slow growing prostate cancers be performed with small number of beams (e.g., 5) and small number of segments (e.g., 5-7 segments per beam) to reduce delivery time and, as a result, the associated effect of radiation protraction.

Effect of low-dose total-body irradiation on transplantability of tumor cells in syngeneic mice

The effect of pretreatment with various low doses of total-body irradiation (TBI) on tumor cell transplantability in syngeneic mice was investigated. Two cell lines, EMT6 and SCCVII, and two strains of mice, were used. First, Balb/c mice were sham-irradiated or irradiated at 200 mGy, and 6-48 h later, 1000 EMT6 cells were inoculated in the hind legs. Based on the results, 0-1500 mGy of TBI was given 6 h before inoculation of 100 or 1000 cells in the subsequent experiments. All mice were observed for 50 days after transplantation. Tumors were judged as grown when the volume of palpable nodules exceeded 200 mm(3). Tumor transplantability rate was significantly higher in the groups irradiated at 1500 mGy than in the sham-irradiated groups in both Balb/c and C3H/He mice. There were no differences in transplantability rates between the control group and the groups irradiated at various doses of 50-500 mGy. However, the mean time to tumor appearance was significantly elongated in Balb/c mice receiving TBI at 200 mGy and inoculated with 100 or 1000 EMT6 cells 6 h later. This phenomenon was also observed in Balb/c mice receiving 100 mGy TBI and inoculated with 1000 EMT6 cells. The present study might suggest that low-dose TBI to mice may delay tumor growth under certain conditions.

Low-dose whole-body irradiation induced radioadaptive response in C57BL/6 mice

Radioadaptive survival responses after relatively low doses of radiation were investigated in C57BL/6 mice. The 8-week-old mice received whole-body mid-lethal challenging irradiation (5.9 Gy) at various intervals after conditioning whole-body irradiation with 50-400 mGy. Thereafter, survival of the mice was observed for 30 days. The mice receiving 400 mGy at 6 h before the challenging dose had a lower survival rate than the control group, but it was not observed when the conditioning 400-mGy irradiation was given 24 h beforehand. The conditioning doses of 100 and 200 mGy did not influence the survival of mice after the challenging dose. The mice receiving 50 mGy at 1 day, 3 days or 1 week before the challenging dose had a higher survival rate than the control, although this adaptive response was not observed when 50 mGy was given 6 h, 12 h, 3.5 weeks, or 5 weeks beforehand. When 50 mGy was given 2 weeks before the challenging dose, the adaptive response was observed in an experiment in which the mice were caged in our laboratory at the age of 5 weeks, whereas it was not observed in another experiment in which the mice were caged at 3 weeks. This study confirmed the presence of radioadaptive survival responses at the dose of 50 mGy given relatively shortly before the challenging dose.

Biological effect of intermittent radiation exposure in vivo: recovery from sublethal damage versus reoxygenation

PURPOSE

In vivo effects of intermittent irradiation are influenced by recovery from sublethal damage (SLDR) and reoxygenation, so contribution of the two factors were investigated using murine tumors.

METHODS AND MATERIALS

1-cm-diameter SCCVII tumors growing in the legs of C3H/HeN mice were used. First, effects of 5 fractions of 6 Gy given at intervals of 2.5-15 min were compared using an in vivo-in vitro assay, by clamping the tumor-bearing legs to exclude the influence of reoxygenation. In the second and third experiments, changes in the hypoxic fraction at 0-15 min after 13 or 5 Gy were assessed by a paired cell survival method. Fourth, effects of 5 fractions of 5 Gy given at intervals of 3-10 min under conditions of limited reoxygenation were compared using a growth delay assay.

RESULTS

Cell survival from clamped tumors tended to increase with elongation of the intervals, but not significantly. The hypoxic fraction tended to decrease at 5-15 min from the level immediately after irradiation. Effects on tumor growth tended to decrease with elongation of the intervals.

CONCLUSIONS

Reoxygenation occurring within 5-15 min appeared to compensate for SLDR in SCCVII tumors. When reoxygenation was limited, the decrease of radiation effect occurred due to SLDR.

Investigation of the dosimetric effect of respiratory motion using four-dimensional weighted radiotherapy

We have developed a four-dimensional weighted radiotherapy (4DW-RT) technique. This method involves designing the motion of the linear accelerator beam to coincide with the tumour motion determined from 4D-CT imaging while including a weighting factor to account for irregular motion and limitations of the delivery system. Experiments were conducted with a moving phantom to assess limitations of the delivery system when applying this method. Although the multi-leaf collimator motion remains within the tolerance of the linear accelerator, the extent of motion was less than 1 mm larger than the designed one, and there was a net system latency of approximately 0.2 s. The dose distributions were measured and simulated using different weighting factors and motion scenarios. The breathing characteristics (period, extent of motion, drift and standard deviations) of 32 patients were evaluated using the Varian RPM system. Breathing variability was assessed by plotting the average breathing motion as a function of the breathing phase. Simulations were carried out to determine the optimal weighting factor based on typical patient breathing characteristics. These results establish that the 4DW-RT method demonstrates potential for dose escalation without increasing exposure to healthy tissue.

The management of respiratory motion in radiation oncology report of AAPM Task Group 76

This document is the report of a task group of the AAPM and has been prepared primarily to advise medical physicists involved in the external-beam radiation therapy of patients with thoracic, abdominal, and pelvic tumors affected by respiratory motion. This report describes the magnitude of respiratory motion, discusses radiotherapy specific problems caused by respiratory motion, explains techniques that explicitly manage respiratory motion during radiotherapy and gives recommendations in the application of these techniques for patient care, including quality assurance (QA) guidelines for these devices and their use with conformal and intensity modulated radiotherapy. The technologies covered by this report are motion-encompassing methods, respiratory gated techniques, breath-hold techniques, forced shallow-breathing methods, and respiration-synchronized techniques. The main outcome of this report is a clinical process guide for managing respiratory motion. Included in this guide is the recommendation that tumor motion should be measured (when possible) for each patient for whom respiratory motion is a concern. If target motion is greater than 5 mm, a method of respiratory motion management is available, and if the patient can tolerate the procedure, respiratory motion management technology is appropriate. Respiratory motion management is also appropriate when the procedure will increase normal tissue sparing. Respiratory motion management involves further resources, education and the development of and adherence to QA procedures.

Optimization of the temporal pattern of radiation: An IMRT based study

PURPOSE

To investigate how the temporal pattern of dose applied during a single-intensity modulated radiation therapy (IMRT) fraction can be arranged to maximize or minimize cell kill.

METHODS AND MATERIALS

Using the linear-quadratic repair-time model and a simplified IMRT delivery pattern model, the surviving fraction of cells for a single fraction was calculated for all permutations of the dose delivery pattern for an array of clinically based IMRT cases. Maximization of cell kill was achieved by concentrating the highest doses in the middle of a fraction, while minimization was achieved by spreading the highest doses between the beginning and end. The percent difference between maximum and minimum cell kill (%Diff(min/max)) and the difference between maximum and minimum total doses normalized to 2 Gy/fx (deltaNTD(2 Gy)) was calculated for varying fraction durations (T), alpha/beta ratios, and doses/fx.

RESULTS

%Diff(min/max) and deltaNTD(2 Gy) both increased with increasing T and with decreasing alpha/beta. The largest increases occurred with dose/fx. With alpha/beta = 3 Gy and 30 min/fx, %Diff(min/max) ranged from 2.7-5.3% for 2 Gy/fx to 48.6-74.1% for 10 Gy/fx, whereas deltaNTD(2 Gy) ranged from 1.2 Gy-2.4 Gy for 30 fractions of 2 Gy/fx to 2.3-4.8 Gy for 2 fractions of 10.84 Gy/fx. Using alpha/beta = 1.5 Gy, an analysis of prostate hypofractionation schemes yielded differences in clinical outcome based on the pattern of applied dose ranging from 3.2%-6.1% of the treated population.

CONCLUSIONS

Rearrangement of the temporal pattern of dose for a single IMRT fraction could be used to optimize cell kill and to directly, though modestly, affect treatment outcome.

Treatment planning and dosimetry of a multi-axis dynamic arc technique for prostate cancer: A comparison with IMRT

PURPOSE

Intensity-modulated radiation therapy (IMRT) allows greater dose conformity to the tumor target. However, IMRT, especially static delivery, usually requires more time to deliver a dose fraction than conventional external beam radiotherapy (EBRT). The authors have been using a "two-axis dynamic arc therapy" (2A-DAT) technique for prostate cancer treatment to make a concave dose distribution to spare the rectum and bladder while working with limited time and human resources. The objectives of this study were to (1) clinically implement the 2A-DAT technique, (2) evaluate the dosimetry in comparison with IMRT, and (3) analyze the initial treatment outcome.

MATERIALS AND METHODS

The 2A-DAT consists of two dynamic arc therapies (DATs) with half rotation around two isocenters each in two separate symmetrical rhombi. Treatment planning is forward and on a trial-and-error basis. Thirty-four patients received 2A-DAT with a median prescribed dose of 70 Gy.

RESULTS

Although inferior in dose uniformity, the 2A-DAT provided equivalent sparing of normal structures to IMRT. Daily fraction delivery time for the 34 patients ranged from 6.4 to 9.6 minutes, with an average of 7.4 minutes. Five-year survival and five-year prostate specific autigen (PSA) failure-free survival were 89.3% and 79.5%, respectively. Three patients developed grade 2 proctitis.

CONCLUSION

This technique is a possible alternative to IMRT in EBRT of prostate cancer.

A novel four-dimensional radiotherapy method for lung cancer: imaging, treatment planning and delivery

We present treatment planning methods based on four-dimensional computed tomography (4D-CT) to incorporate tumour motion using (1) a static field and (2) a dynamic field. Static 4D fields are determined to include the target in all breathing phases, whereas dynamic 4D fields are determined to follow the shape of the tumour assessed from 4D-CT images with a dynamic weighting factor. The weighting factor selection depends on the reliability of patient breathing and limitations of the delivery system. The static 4D method is compared with our standard protocol for gross tumour volume (GTV) coverage, mean lung dose and V20. It was found that the GTV delineated on helical CT without incorporating breathing motion does not adequately represent the target compared to the GTV delineated from 4D-CT. Dosimetric analysis indicates that the static 4D-CT based technique results in a reduction of the mean lung dose compared with the standard protocol. Measurements on a moving phantom and simulations indicated that 4D radiotherapy (4D-RT) synchronized with respiration-induced motion further reduces mean lung dose and V20, and may allow safe application of dose escalation and CRT/IMRT. The motions of the chest cavity, tumour and thoracic structures of 24 lung cancer patients are also analysed.

Radiobiologic effect of intermittent radiation exposure in murine tumors

PURPOSE

In stereotactic irradiation using a linear accelerator, the effect of radiation may be reduced during intermittent exposures owing to recovery from sublethal damage in tumor cells. After our previous in vitro study suggesting this phenomenon, we investigated the issue in murine tumors.

METHODS AND MATERIALS

We used EMT6 and SCCVII tumors approximately 1 cm in diameter growing in the hind legs of syngeneic mice. Three schedules of intermittent radiation were investigated. First, 2 fractions of 10 Gy were given at an interval of 15-360 min to investigate the pattern of recovery from sublethal damage. Second, 5 fractions of 4 Gy were given with interfraction intervals of 2.5-15 min each. Third, 10 fractions of 2 Gy were given with interfraction intervals of 1-7 min each. Doses of 15-20 Gy were also given without interruption to estimate the dose-modifying factors. Tumors were excised 20 h later, and tumor cell survival was determined by an in vivo-in vitro assay.

RESULTS

In the 2-fraction experiment, the increase in cell survival with elongation of the interval was much less than that observed in our previous in vitro study. In the 5- and 10-fraction experiments, no significant increase in cell survival was observed after the intermittent exposures. Moreover, cell survival decreased at most points of the 5-fraction experiments by interruption of radiation in both EMT6 and SCCVII tumors. In the 10-fraction experiment, cell survival also decreased when the interruption was 3 or 7 min in EMT6 tumors.

CONCLUSION

The results of the present in vivo studies were different from those of our in vitro studies in which cell survival increased significantly when a few minutes or longer intervals were posed between fractions. This suggests that recovery from sublethal damage in vivo may be counterbalanced by other phenomena such as reoxygenation that sensitizes tumor cells to subsequent irradiation.

Absence of radioadaptive responses in four cell-lines in vitro as determined by colony formation assay

The purpose of this study was to investigate radioadaptive response in 4 cell-lines under identical conditions using a colony assay. First, 4 cell-lines (V79, HeLa S3, EMT6 and SCCVII) were exposed to 8 Gy at various intervals after pretreatment with an adapting dose of 50 mGy or without it. Second, V79 cells were exposed to 8 Gy at 4.5 hrs after an adapting dose of 0 to 400 mGy. Third, V79 cells were exposed to 2, 4 or 6 Gy at 6 hrs after an adapting dose of 0 or 50 mGy. In the last experiment, an adapting dose was given either immediately after cell plating or 24 hrs later. Cell survival was assessed by a standard colony assay. Adaptive response was not observed in any of the 4 lines tested. In V79 cells, no adaptive response was seen even by changing the adapting dose, challenging dose, and timing of adapting radiation after cell plating. Although radioadaptive response has been reported for the V79 cell-line, we could not reproduce the result. We also failed to demonstrate the phenomenon in the other 3 tumor cell-lines in culture.

Biological effects of intermittent radiation in cultured tumor cells: influence of fraction number and dose per fraction

In intensity-modulated radiation therapy (IMRT) and stereotactic irradiation using a linear accelerator, radiation is administered intermittently and one treatment session often requires 30 min or a longer time. The purpose of the present study was to investigate the effect of fractionation and dose per fraction on cell killing by irradiation in intermittent exposure. Murine EMT6 and SCCVII cells were used. The cells were irradiated to a total dose of 8 Gy in 2, 5, 10, 20 and 40 fractions over 15, 30 and 46 min. The cells were also given 8 Gy in a single fraction over 15, 30 and 46 min using lower dose rates (continuous prolonged radiation groups). As compared with the control group receiving a single dose of 8 Gy at 1.55 Gy/min, the cell surviving fraction generally increased in groups receiving fractionated or continuous prolonged radiation. There was a general trend for cell survival to increase with the fraction number up to 20 or 40 fractions in both cell lines. The effects of IMRT and linear accelerator radiosurgery given over 15 min or longer may be less than those of 1- or 2-fraction irradiation. There was a trend for radiation effect to decrease with fraction number.

Changes in tumor cell response due to prolonged dose delivery times in fractionated radiation therapy

PURPOSE

Dynamic radiation therapy, such as intensity-modulated radiation therapy, delivers more complex treatment fields than conventional techniques. The increased complexity causes longer dose delivery times for each fraction. The cellular damage after a full treatment may depend on the dose rate, because sublethal radiation damage can be repaired more efficiently during prolonged dose delivery. The goal of this study was to investigate the significance of this effect in fractionated radiation therapy.

METHODS AND MATERIALS

The lethal/potentially lethal model was used to calculate lesion induction rates for repairable and nonrepairable lesions. Dose rate effects were analyzed for 9 different cell lines (8 human tumor xenografts and a C3H10T1/2 cell line). The effects of single-fraction as well as fractionated irradiation for different dose rates were studied.

RESULTS

Significant differences can be seen for dose rates lower than about 0.1 Gy/min for all cell lines considered. For 60 Gy delivered in 30 fractions, the equivalent dose is reduced by between 1.3% and 12% comparing 2 Gy delivery over 30 min per fraction with 2 Gy delivery over 1 min per fraction. The effect is higher for higher doses per fraction. Furthermore, the results show that dose rate effects do not show a simple correlation with the alpha/beta ratio for ratios between 3 Gy and 31 Gy.

CONCLUSIONS

If the total dose delivery time for a treatment fraction in radiation therapy increases to about 20 min, a correction for dose rate effects may have to be considered in treatment planning. Adjustments in effective dose may be necessary when comparing intensity-modulated radiation therapy with conventional treatment plans.

Hypofractionation in non-small cell lung cancer (NSCLC): suggestions from modelling both acute and chronic hypoxia

Based on experimental estimates for acute and chronic tumour hypoxia, a speculative analysis of the therapeutic ratio dependence on the number of once-daily five-days-per-week fractions (n) for non-small cell lung cancer (NSCLC) radiotherapy is proposed. For this purpose an adapted formulation of the linear-quadratic model has been derived, including the effects of tumour repopulation, inter-tumour alpha-heterogeneity and oxygen enhancement ratio dependence on the dose per fraction. The relation between the curative dose D50, assuring 50% tumour control probability, and n has been computed: for (n, D50) fractionation schemes, the therapeutic ratios have been compared in terms of effective normalized total doses to the lungs (NTD(eff)L), estimated by a few supposed fractions of the normalized total dose to the tumour. Results suggest that D50 is dominated by chronic hypoxia for shortly hypofractionated treatments and by acute hypoxia for multifractionated treatments. Furthermore, the optimum number of fractions depends on the rapidity of the reoxygenation from chronically hypoxic cells, almost independently of the extent of both chronic and acute hypoxia. For NSCLC, both the reduction of n until about 20 fractions in hypofractionated dose-escalation trials, and the extension of extra-cranial stereotactic radiotherapy schedules to include at least 5-10 fractions, seem to be supported by this model.