Tolerance of normal tissue to therapeutic irradiation

[Metastatic eye neoplasms. Review of the literature apropos of 4 cases]

Malignant disease metastatic to the eye is a common entity afflicting hundred patients each year. Breast carcinoma is the most common carcinoma responsible for ocular metastases with an estimated range probably closer to 30% and prevalence of 11.000 women by year in United-States. Breast carcinoma is followed by lung cancer and adenocarcinoma of an unknown primary. The diagnosis should be suspected in patient with history of carcinoma and a decreased visual acuity or any other visual symptom. The most useful diagnostic techniques include direct ophthalmoscopy, ultrasonography, computed tomography and magnetic resonance imaging. The cornerstone of treatment of ocular metastases remains radiation therapy. The overall response rates of earlier diagnosis, according to measurement of visual acuity, may be ranged from 72 to 94%. Early diagnosis and treatment of those lesions are a primary concern to maximize the quality of life until death of patients with metastatic disease. We report four cases of women with breast cancer metastatic to the eye.

Volume effects in rhesus monkey spinal cord

PURPOSE

An experiment was conducted to test for the existence of a volume effect in radiation myelopathy using Rhesus monkeys treated with clinically relevant field sizes and fractionation schedules.

METHODS AND MATERIALS

Five groups of Rhesus monkeys were irradiated using 2.2 Gy per fraction to their spinal cords. Three groups were irradiated with 8 cm fields to total doses of 70.4, 77, and 83.6 Gy. Two additional groups were irradiated to 70.4 Gy using 4 and 16 cm fields. The incidence of paresis expressed within 2 years following the completion of treatment was determined for each group. Maximum likelihood estimation was used to determine parameters of a logistic dose response function. The volume effect was modeled using the probability model in which the probability of producing a lesion in an irradiated volume is governed by the probability of the occurrence of independent events. This is a two parameter model requiring only the estimates of the parameters of the dose-response function for the reference volume, but not needing any additional parameters for describing the volume effect.

RESULTS

The probability model using a logistic dose-response function fits the data well with the D50 = 75.8 Gy for the 8-cm field. No evidence was seen for a difference in sensitivities for different anatomical levels of the spinal cord. Most lesions were type 3, combined white matter parenchymal and vascular lesions. Latent periods did not differ significantly from those of type 3 lesions in humans.

CONCLUSION

The spinal cord exhibits a volume effect that is well described by the probability model. Because the dose response function for radiation myelopathy is steep, the volume effect is modest. The Rhesus monkey remains the animal model most similar to humans in dose response, histopathology, and latency for radiation myelopathy.

Dose-volume histogram and 3-D treatment planning evaluation of patients with pneumonitis

PURPOSE

Tolerance of normal lung to inhomogeneous irradiation of partial volumes is not well understood. This retrospective study analyzes three-dimensional (3-D) dose distributions and dose-volume histograms for 63 patients who have had normal lung irradiated in two types of treatment situations.

METHODS AND MATERIALS

3-D treatment plans were examined for 21 patients with Hodgkin's disease and 42 patients with nonsmall-cell lung cancer. All patients were treated with conventional fractionation, with a dose of 67 Gy (corrected) or higher for the lung cancer patients. A normal tissue complication probability description and a dose-volume histogram reduction scheme were used to assess the data. Mean dose to lung was also calculated.

RESULTS

Five Hodgkin's disease patients and nine lung cancer patients developed pneumonitis. Data were analyzed for each individual independent lung and for the total lung tissue (lung as a paired organ). Comparisons of averages of mean lung dose and normal tissue complication probabilities show a difference between patients with and without complications. Averages of calculated normal tissue complication probabilities for groups of patients show that empirical model parameters correlate with actual complication rates for the Hodgkin's patients, but not as well for the individual lungs of the lung cancer patients treated to larger volumes of normal lung and high doses.

CONCLUSION

This retrospective study of the 3-D dose distributions for normal lung for two types of treatment situations for patients with irradiated normal lung gives useful data for the characterization of the dose-volume relationship and the development of pneumonitis. These data can be used to help set up a dose escalation protocol for the treatment of nonsmall-cell lung cancer.

Conformal radiotherapy at the Royal Marsden Hospital (UK)

Conformal radiotherapy seeks to allow increased intensity of radiation by reducing the volume of normal tissues within the treatment volume. Techniques have developed secondary to improvements in three-dimensional imaging and accessible treatment technology is based on computer-controlled multileaf collimators to create an irregular radiation beam shape. Preliminary clinical work in the Royal Marsden Hospital seeks to quantify the toxicity reduction achievable by conformal techniques in the context of a prospective randomized pelvic radiotherapy trial which has now recruited 240 patients. The data accumulated during this trial will allow comparison of conformal and conventional radiotherapy and also analysis of the impact of dose and volume of a particular organ on both acute and late toxicity. Assessments have revealed that conformal techniques reduced significantly the treatment volume of normal tissues, e.g. by a mean of 54% for rectum and 42% for bladder. However, a relationship between volume and acute toxicity has not been established. Late toxicity is currently being analysed. Dose escalation trials in thoracic and in pelvic tumours are planned.

Three-dimensional conformal radiation therapy in locally advanced carcinoma of the prostate: preliminary results of a phase I dose-escalation study

PURPOSE

The acute morbidity of doses of 64.8-75.6 Gy and preliminary observations of late complications and tumor response using 3-dimensional conformal radiation therapy in carcinoma of the prostate are assessed.

METHODS AND MATERIALS

123 patients (Stage A2-12, B1-17, B2-43, C-51) were irradiated to the prostate and seminal vesicles using a 3-dimensional conformal radiation therapy technique. The median follow-up time was 15.2 months. The minimum tumor dose was 64.8-66.6 Gy in 49 patients, 70.2 Gy in 46, and 75.6 Gy in 28. Toxicity was scored according to the Radiation Therapy Oncology Group morbidity grading system.

RESULTS

This technique of 3-dimensional conformal radiation therapy was well-tolerated with minimal acute morbidity. Only 32% of patients had grade 2 or 3 acute morbidity requiring short-term medication for relief of urinary symptoms or diarrhea. Only one patient (0.8%) has so far developed a severe (grade 4) late complication. Serum prostate specific antigen concentrations normalized in 67% of patients (64/96) within 1-14 months (median 4.5 months) after treatment and were progressively decreasing at last measurement in an additional 22% (21/96). Abnormal rising prostate specific antigen levels were observed in 15 patients, 11 of whom have already developed other evidence of relapsing disease.

CONCLUSION

Acute toxicity for the doses tested with this 3-dimensional conformal radiation therapy technique is reduced compared to traditional treatment techniques, and the initial tumor response as assessed by prostate specific antigen measurement is highly encouraging with prostate specific antigen levels returning to normal in the majority of patients. Based on these results, a further increase of the dose to 81 Gy has been implemented in accordance with the schema of an ongoing Phase I dose-escalation study.

[Late post-irradiation cervical spinal cord disease. A case]

A case of delayed progressive radiation myelitis (DPRM) which begin 11 months after naso pharyngeal carcinoma radiation, in a young man, is reported. The initial manifestation is often a Brown-Sequard's syndrome progressing to complete and permanent myelopathy, with notable absence of localized or radicular pain. The parenchymal change of the spinal cord in radiation myelopathy can be easily visualized with magnetic resonance imaging (MRI) however there may be cases in which MRI appearance alone does not distinguish specially between tumor and radiation necrosis with absolute confidence: therefore, DPRM is by necessity a diagnosis of exclusion, based on clinical, paraclinical results and course of disease. Corticosteroid therapy is accompanied by a significant remission of symptoms. The evolution is characterized by a worse prognosis, prevention is absolutely necessary based on perfect radiation technic, knowledge on tolerance of spinal cord to irradiation (time-dose-volume factors) and other risks factors (chemotherapy, age and vascular disease).

Stereotactic radiotherapy of malignancies in the abdomen. Methodological aspects

A method for stereotactic high-dose radiotherapy of malignancies in the abdomen has been developed. A stereotactic frame for the body has been developed and a method for fixation of the patient in the frame is described. The reproducibility in the stereotactic system of tumours in the liver and the lung was found to be within 5-8 mm for 90% of the patient set-ups. The diaphragmatic movements were reduced to 5-10 mm, by applying a pressure on the abdomen. An analytical method is used to calculate dose distributions for a continuum of beams in an isocentric treatment technique. The advantage of a heterogeneous target dose is demonstrated and proposed for the present application. A non-coplanar treatment technique, using eight individually shaped beams is proposed and has been used for patient treatments. The dose distribution for a patient with a metastasis in the liver is shown as well as dose volume histograms for the target and the liver.

The clinical value of different treatment objectives and degrees of freedom in radiation therapy optimization

With inverse radiation therapy planning methods, both biological and physical objective functions can be used to perform the optimization. A biological objective function, namely the probability of achieving tumor control without causing severe complications in normal tissues, P+, has been used to evaluate six different optimization methods. All six methods have been tested on two different clinically relevant treatment geometries. The results show that optimization with a physical objective function which gives the best possible agreement with the desired dose distribution in the least squares sense, may result in severe loss of complication-free tumor control due to insufficient consideration of the organs at risk. It is generally better to use a physical objective function which minimizes the over-dosage when the desired dose distribution can not be exactly reproduced. In all cases the use of physical objective functions results in a lower probability of controlling the tumor without causing severe normal tissue reactions than if the biological objective function, P+, is used. However, the results also show the importance of accurately accounting for beam divergence, dose build-up, beam attenuation, and lateral scatter during the optimization procedure, particularly when the biological objective function is used. The loss in P+ by assuming that all energy deposition kernels are identical and that all the constituent beams have fixed relative weights can be 15% or more. When lateral scatter is not accounted for during the optimization, serious injury to organs at risk may result. This problem is specially severe for organs that are partly or totally encapsulated by the target volume. For superficial target volumes accurate consideration of the dose build-up of the incident pencil beams is fundamental.

Dose-volume effects in the spinal cord

Experimental data on dose-volume relationships for the spinal cord are now available for a variety of animal models (monkey, dog, pig, rat). Most studies show a marginal volume effect for cord lengths longer than 1 cm, but a steep increase in tolerance doses for irradiated lengths of less than 1 cm. From a comparison of several theoretical models with available clinical/experimental data it can be concluded that there is at present no need for a further expansion of models, but a great need of reliable data.

Perspectives of multidimensional conformal radiation treatment

We consider the present technological advancement that underlies the implementation of computer-controlled conformal radiotherapy. We also consider the developments in modern biology that may provide input to therapy planning. The concept of multidimensional conformal radiotherapy is advanced, which integrates geometrical precision and biological conformality, to optimize the treatment planning for individual patients, with a view to improve the overall success of radiotherapy.

Feasibility of patient immobilization for conventional cranial irradiation with a relocatable stereotactic frame

The precision of patient repositioning and the firmness of immobilization are among the major determinants of the accuracy of radiation treatment delivery. A relocatable Gill-Thomas localizer (GTL) developed for neurosurgery and stereotactic radiotherapy achieves highly accurate relocation and immobilization and has been used successfully for fractionated stereotactic radiotherapy of intracranial lesions. The feasibility of using GTL for immobilization in conventional fractionated external beam radiotherapy was assessed by comparison with conventional Cabulite shell head fixation. The GTL was well tolerated at the time of preparation and during a 5 week course of radiotherapy. The principal advantage was superior relocation accuracy maintained throughout the course of treatment, high precision of field set-up and markedly reduced production time. In addition the time taken to position the patient in the treatment room was also marginally shortened. GTL is therefore a feasible method of head fixation for conventional cranial irradiation providing patients have adequate dentition. With further development the relocatable method of immobilization originally designed for stereotactic radiotherapy may become the preferred technique particularly in situations where high accuracy is desirable, such as stereotactically guided conformal radiotherapy.

A new method to determine dose-effect relations for local lung-function changes using correlated SPECT and CT data

PURPOSE

To determine dose-effect relations for regional lung-function changes after radiotherapy.

METHODS

Single Photon Emission Computed Tomography (SPECT) was performed to quantify regional ventilation and perfusion. CT scans were used to calculate the three-dimensional (3-D) dose distribution. Both SPECT and CT scans were performed prior to radiotherapy and 5 months after the start of the treatment. To obtain combined 3-D information on ventilation, perfusion and dose, the SPECT data were correlated with the corresponding CT data. The relative changes in ventilation and perfusion were calculated in each SPECT voxel (voxel size about 6 x 6 x 6 mm) and related to the dose in that voxel. The average relative changes were determined per dose interval of 4 Gy. This procedure was evaluated using the data from five patients treated for Hodgkin's disease with mantle field irradiation with a prescribed total dose of 40-42 Gy.

RESULTS

Dose-effect relations for perfusion were observed in all patients, while in four of the five patients, a dose-effect relation was found for ventilation. The maximal uncertainty of the calculated radiation dose was 11%: a difference between the position of the patient during treatment and during CT scanning caused a maximal dose uncertainty of 6%, while the accuracy of the dose calculation algorithm itself was estimated to be within 5%.

CONCLUSION

The results indicate that the combined use of SPECT and CT information is an effective method for determining dose-effect relations for regional lung function parameters in each individual patient.

Use of Veff and iso-NTCP in the implementation of dose escalation protocols

PURPOSE

This report investigates the use of a normal tissue complication probability (NTCP) model, 3-D dose distributions, and a dose volume histogram reduction scheme in the design and implementation of dose escalation protocols for irradiation of sites that are primarily limited by the dose to a normal tissue which exhibits a strong volume effect (e.g., lung, liver).

METHODS AND MATERIALS

Plots containing iso-NTCP contours are generated as a function of dose and partial volume using a parameterization of a NTCP description. Single step dose volume histograms are generated from 3-D dose distributions using the effective-volume (Veff) reduction scheme. In this scheme, the value of Veff for each dose volume histogram is independent of dose units (Gy, %). Thus, relative dose distributions (%) may be used to segregate patients by Veff into bins containing different ranges of Veff values before the assignment of prescription doses (Gy). The doses for each bin of Veff values can then be independently escalated between estimated complication levels (iso-NTCP contours).

RESULTS AND CONCLUSION

Given that for the site under study, an investigator believes that the NTCP parameterization and the Veff methodology at least describe the general trend of clinical expectations, the concepts discussed allow the use of patient specific 3-D dose/volume information in the design and implementation of dose escalation studies. The result is a scheme with which useful prospective tolerance data may be systematically obtained for testing the different NTCP parameterizations and models.

Results of high-dose thoracic irradiation incorporating beam's eye view display in non-small cell lung cancer: a retrospective multivariate analysis

PURPOSE

To review the University of Michigan clinical experience in nonsmall cell lung cancer using high-dose thoracic irradiation (> or = 60 Gy) so that a starting dose for our prospective dose-escalation study could be determined.

METHODS AND MATERIALS

Eighty-eight consecutive patients diagnosed with medically inoperable or locally advanced, unresectable nonsmall cell lung cancer were identified who were treated with thoracic irradiation alone to a minimum total dose of 60 Gy (uncorrected for lung density). All patients except four (95%) underwent computed tomography scanning for treatment planning that included beam's eye view display for tumor and critical structure localization. All patients were treated with standard fractionation in a continuous course to uncorrected total doses ranging from 60 to 74 Gy (median, 67.6 Gy).

RESULTS

The median follow-up exceeds 24 months for all surviving patients (range, 12 to 78 months). The median survival time was 15 months, and the 2- and 3-year overall actuarial survival rates were 37% and 15%, respectively. Survival was significantly different between stage of disease (p = .004) and N-stage (p = .002) by univariate analysis. In a multivariate analysis, stage becomes the only characteristic significantly associated with outcome. The median time to local progression for 86 evaluable patients was 29 months. Stage (p = .0003), T-stage (p = .0095) and N-stage (p = .027) were significantly different with respect to local progression-free survival by univariate analysis. However, only stage was prognostic for local progression-free survival by multivariate analysis. There was no difference between large volume treatment (inclusion of the contralateral hilar and supraclavicular lymph nodes) and small volume treatment (exclusion of these elective nodal sites) with respect to local progression-free survival (p = .507) or survival (p = .520). With regard to dose, there was no significant difference between patients who received > 67.6 Gy and patients who received < or = 67.6 Gy with respect to local progression-free survival (p = .094) or survival (p = .142). Within the Stage III subgroup, local progression-free survival (p = .018) and survival (p = .061) were longer favoring the high-dose group of patients. Despite these doses, disease progression within the irradiated field was the predominant first site of treatment failure.

CONCLUSION

This retrospective study has shown that it is feasible to deliver uncorrected tumor doses as high as 70 Gy using standard fractionation in NSCLC with acceptable morbidity. Local control remains a significant problem. These data indicate justification for a starting dose in a prospective radiation dose-escalation study.

Renal tolerance to nonhomogenous irradiation: comparison of observed effects to predictions of normal tissue complication probability from different biophysical models

PURPOSE

A patient series was analyzed retrospectively as an example of whole organ kidney irradiation with an inhomogenous dose distribution to test the validity of biophysical models predicting normal tissue tolerance to radiotherapy.

METHODS AND MATERIAL

From 1969 to 1984, 142 patients with seminoma were irradiated to the paraaortic region using predominantly rotational techniques which led to variable but partly substantial exposure of the kidneys. Median follow up was 8.2 (2.1-21) years and actuarial 10-year survival (Kaplan-Meier estimate) 82.8%. For all patients 3-dimensional dose distributions were reconstructed and normal tissue complication probabilities for the kidneys were generated from the individual dose volume histograms. To this respect different published biophysical algorithms had been introduced in a 3-dimensional-treatment planning system.

RESULTS

In seven patients clinical manifest renal impairment was observed (interval 10-84 months). An excellent agreement between predicted and observed effects was seen for two volume-oriented models, whereas complications were overestimated by an algorithm based on critical element assumptions.

CONCLUSIONS

Should these observations be confirmed and extended to different types of organs corresponding algorithms could easily be integrated into 3-dimensional-treatment planning programs and be used for comparing and judging different plans on a more biologically oriented basis.

Stereotactic radiotherapy of irregular targets: a comparison between static conformal beams and non-coplanar arcs

Stereotactic radiotherapy using a linear accelerator is usually equated with the technique of delivery using multiple non-coplanar arcs, which achieves a spherical dose distribution. As the majority of intracranial lesions are not spherical, a range of schematized tumour shapes were planned to assess the role of static conformal beams in the treatment of irregular lesions. A sphere and 2 ellipsoids, ranging from 20 to 50 mm maximum diameter located intracranially were planned using 3, 4, and 6 non-coplanar static beams with conformal blocks and were compared with four 120 degree non-coplanar arcs. Comparison of the plans was made by the relative sparing of normal tissue outside the target volume using three-dimensional dose-volume distributions. Non-coplanar arcs spared more normal tissue at low isodoses and achieved the best high dose sparing for spherical targets. For the majority of irregular targets, 3 and 4 static beams spared more tissue at doses > or = 50% and > or = 80% than the arc technique. For all irregular volumes, maximum sparing of normal tissue to isodoses > or = 50% and > or = 80% of the treatment isodose was obtained with 6 static conformal beams. We conclude that irregularly shaped tumours suitable for stereotactic radiotherapy with a linear accelerator are better treated with conformal static non-coplanar beams rather than with the multiple arc technique.

Three-dimensional conformal radiation therapy may improve the therapeutic ratio of high dose radiation therapy for lung cancer

PURPOSE

The specific aim of 3-dimensional conformal radiation therapy is to improve the target dose distribution while concomitantly reducing normal tissue dose. Such an approach should permit dose escalation until the limits of acceptable normal tissue toxicity are reached. To evaluate the feasibility of tumor dose escalation for nine patients with lung cancer, we determined the dose distribution to the target and normal tissues with 3-dimensional conformal radiation therapy and conventional planning.

METHODS AND MATERIALS

Plans were compared to assess adequacy of dose delivery to target volumes, dose-volume histograms for normal tissue, and normal tissue complication probabilities (NTCP) for nine patients with lung tumors.

RESULTS

The mean percentage of gross disease which received < or = 70.2 Gy with 3-dimensional conformal radiation therapy (3DCRT) was 40% of the mean percentage of gross disease which received < or = 70.2 Gy with conventional treatment planning (CTP). The mean NTCP for lung parenchyma with 3DCRT was 36% of the mean NTCP with CTP. The mean esophageal NTCP with 3DCRT was 88% of the mean NTCP with CTP.

CONCLUSION

This preliminary analysis suggests that three dimensional conformal radiation therapy may provide superior delivery of high dose radiation with reduced risk to normal tissue, suggesting that this approach may have the potential to improve the therapeutic ratio of high dose radiation therapy for lung cancer.

The reliability of optimization under dose-volume limits

PURPOSE

An optimization algorithm improves the distribution of dose among discrete points in tissues, but tolerance depends on the distribution of dose across a continuous volume. This report asks whether an exact algorithm can be completed when enough points are taken to accurately model a dose-volume constraint.

METHODS AND MATERIALS

Trials were performed using a 3-dimensional model of conformal therapy of lung cancer. Trials were repeated with different limits placed on the fraction of lung which could receive > 20 Gy. Bounds were placed on cord dose and target dose inhomogeneity. A mixed integer algorithm was used to find a feasible set of beam weights which would maximize tumor dose. Tests of feasibility and optimality are introduced to check the solution accuracy.

RESULTS

Solutions were optimal for points used to model tissues. An accuracy of 3-4% in a volume condition could be obtained with models of 450-600 points. The error improved to 2% with 800 points to model the lung. Solution times increased six-fold at this level of accuracy.

CONCLUSION

The mixed integer method can find optimum weights which respect dose-volume conditions in usually acceptable times. If constraints are violated by an excessive amount, the optimization model should be rerun with more points.

Muscular fibrosis induced after pig skin irradiation with single doses of 192Ir gamma-rays

Localized irradiation of the skin and subcutaneous tissues with large single doses of gamma-rays can induce delayed effects characterized by fibrosis which invades the irradiated tissues. In this study the depth of penetration of muscle fibrosis was measured in the pig 30 weeks after irradiation of the skin surface with single doses of 192Ir gamma-rays of 16-256 Gy. Irradiation was directed either to the outer side of the thigh or to the back, close to the mid-dorsal line. Fibrosis only developed in irradiated muscle after doses that induced moist desquamation of the skin in the acute phase of the reaction, i.e. after skin surface doses of 48-64 Gy. In skeletal muscles, the limit of fibrotic expansion was reached at a depth dose of 14 +/- 4 Gy (+/- SD) for skin surface doses exceeding 48 Gy.

Objective evaluation of 3-D radiation treatment plans: a decision-analytic tool incorporating treatment preferences of radiation oncologists

PURPOSE

Selecting the optimal radiation treatment plan from a set of competing plans involves making trade-offs among the doses delivered to the target volumes and normal tissues by the competing plans. Evaluation of 3-dimensional radiation treatment plans is difficult because it requires the review of vast amount of graphical and numerical data. We have developed an objective plan-ranking model based on the concepts of decision analysis.

METHODS AND MATERIALS

Our model ranks a set of tentative radiation treatment plans from best to worst. A figure of merit is computed for each plan based on probabilities of possible clinical complications such as non-eradication of the tumor and radiation induced damage to the nearby healthy normal tissues, and weights which indicate their clinical relevance. This figure of merit is used to rank the plans. Key issues addressed by the model include the incorporation of individual treatment preferences of the radiation oncologist and clinical features of the patient.

RESULTS

A methodology has been established for eliciting the treatment preferences of radiation oncologists. Results of this elicitation, and examples of several plan evaluations are presented. An interactive computer-based tool has been developed as one of a set of tools to assist in the evaluation of 3-dimensional radiation treatment plans.

CONCLUSION

The paper presents a decision-analytic model incorporating radiation oncologists' treatment preferences and an interactive computer-based tool for objectively ranking competing radiation treatment plans. The tool can be used by radiation oncologists for the evaluation of competing plans, or as part of a system which tries to automatically generate optimal treatment plans using mathematical or symbolic techniques.

Probability of radiation-induced complications in normal tissues with parallel architecture under conditions of uniform whole or partial organ irradiation

A biologically based model is developed for normal tissue complication probability as a function of dose and irradiated volume fraction for organs such as the kidney and the lung. The organ is assumed to be composed of functional subunits (FSUs) which are arranged in a parallel architecture. The complication is produced only if a sufficiently large fraction of the FSUs are inactivated by radiation and an FSU is inactivated only when all the clonogenic cells within it are killed. The linear-quadratic model is used for the dose-response of individual cells within an FSU. The predictions of this model are compared with those of an empirical power law function for uniform whole and partial organ irradiation.

Modeling of normal tissue response to radiation: the critical volume model

PURPOSE

A model for calculating normal tissue complication probability in response to therapeutic doses of radiation is presented.

METHODS AND MATERIALS

The model which we call the "critical volume model" is based on a concept of functional subunits defined either structurally (e.g., nephrons) or functionally, and an assumption that normal tissue complication probability is fully determined by the number or fraction of surviving functional subunits composing an organ or tissue. The essential features of the model are that it takes into account variations in tissue radiosensitivity and architecture of an organ for a single patient and for a patient population, and predicts the normal tissue complication probability under conditions of 3-dimensional inhomogeneity of the dose distribution. The model can be used for Integral Response, or "parallel," organs (where all functional subunits are performing the same function in parallel and the output of the organ is the sum of the outputs of the functional subunits and for Critical Element, or "serial," organs (where damage to one functional subunit results in an expression of damage for the whole organ). The model combines into one compact scheme new concepts and several ideas and models which have been previously developed by other investigators.

RESULTS

The behavior of the model is presented and discussed for the example of the kidney, with clinical nephritis as the functional endpoint.

CONCLUSIONS

The model has the potential to be a useful tool for evaluation and optimization of 3-dimensional treatment plans for a variety of types of normal tissues.

Artificial pneumothorax can be used to prevent lung toxicity in chest wall radiotherapy

We report two patients in whom an artificial pneumothorax was induced to reduce the risk of radiation pneumonitis and fibrosis after treatment for chest wall tumours. The procedure was well tolerated; the only complication observed was a single episode of syncope following over-inflation. High doses of radiation were given to large chest wall fields with no clinical or radiological evidence of pneumonitis or fibrosis, either during or after treatment. The available literature on the use of artificial pneumothorax with radiation is reviewed, and the technique of induction is described.

Optimal radiation beam profiles considering uncertainties in beam patient alignment

The often large uncertainties that exist in beam patient alignment during radiation therapy may require modification of the incident beams to ensure an optimal delivered dose distribution to the target volume. This problem becomes increasingly severe when the required dose distribution of the incident beams becomes more heterogeneous. A simple analytical formula is derived for the case when the fraction number is high, and the desired relative dose variations are small. This formula adjusts the fluence distribution of the incident beam so that the resultant dose distribution will be as close as possible to the desired one considering the uncertainties in beam patient alignment. When sharp dose gradients are important, for instance at the border of the target volume, the problem is much more difficult. It is shown here that, if the tumor is surrounded by organs at risk, it is generally best to open up the field by about one standard deviation of the positional uncertainty--that is sigma/2 on each side of the target volume. In principle it is simultaneously desirable to increase the prescribed dose by a few per cent compared to the case where the positional uncertainty is negligible, in order to compensate for the rounded shoulders of the delivered dose distribution. When the tissues surrounding the tumor no longer are dose limiting even larger increases in field size may be advantageous. For more critical clinical situations the positional uncertainty may even limit the success of radiotherapy. In such cases one generally wants to create a steeper dose distribution than the underlying random Gaussian displacement process allows. The problem is then best handled by quantifying the treatment outcome under the influence of the stochastic process of patient misalignment. Either the coincidence with the desired dose distribution, or the expectation value of the probability of achieving complication-free tumor control is maximized under the influence of this stochastic process. It is shown that the most advantageous treatment is to apply beams that are either considerably widened or slightly widened and over flattened near the field edges for small and large fraction numbers respectively.

Tumour and normal tissue responses to fractionated non-uniform dose delivery

The dose-volume response of tumours and normal tissues is discussed in terms of 'parallelity' and 'seriality'. The volume dependence of the radiation response of a tumour depends primarily on the eradication of all its clonogenic cells and the tumour has a parallel organization. The response of heterogeneous tumours is examined, and it is shown that a small resistant clonogen population may cause a low dose-response gradient, gamma. Injury to normal tissue is a much more complex and gradual process. It depends on earlier effects induced long before depletion of stem cells or differentiated cells that in addition may have a complex structural and functional organization. The volume dependence of the dose-response relation of normal tissues is therefore described here by a new parameter, the 'relative seriality', s, of the infrastructure of the organ. The model is compared with clinical and experimental data on normal tissue response, and shows good agreement both with regard to the shape of dose-response relation and the volume dependence of the isoeffect dose. For example, the spinal cord has a high and the lung a low 'relative seriality', which is reasonable with regard to the organization of these tissues. The response of tumours and normal tissues to non-uniform dose delivery is quantified for fractionated therapy using the linear quadratic cell survival parameters alpha and beta. The steepness, gamma, and the 50% response dose, D50, of the dose-response relationship are derived both for a constant dose per fraction and a constant number of dose fractions.

Optimization by simulated annealing of three-dimensional, conformal treatment planning for radiation fields defined by a multileaf collimator: II. Inclusion of two-dimensional modulation of the x-ray intensity

Interest is rapidly growing in using multiple x-radiation fields defined by a multileaf collimator to achieve conformal radiotherapy. Three-dimensional treatment planning in such situations is in its infancy and most 3D planning systems provide no tools for optimizing therapy. A previous paper addressed how to calculate optimum beamweights when both the target volume and all or some parts of organs at risk were in the fields-of-view. This work allowed a maximum of two weights per field. The present paper extends this technique to allow each radiation port to be spatially modulated across the geometrically shaped field. An optimization method based on simulated annealing is presented. It is shown that including spatial modulation leads to a wider separation between the dose-volume histograms of the target volume and organs at risk. The improvement is quantified in terms of the tumour control probability at constant normal tissue complication probability. Possible limitations of the a posteriori applied biological model are discussed in detail.

Radiotherapy update

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An algorithm for maximizing the probability of complication-free tumour control in radiation therapy

New radiobiological models are used to describe tumour and normal tissue reactions and to account for their dependence on the irradiated volume and inhomogeneities of the delivered dose distribution and cell sensitivity. The probability of accomplishing complication-free tumour control is maximized by an iterative algorithm. The algorithm is demonstrated by applying it to a one-dimensional (1D) tumour model but also to a more clinically relevant 2D case. The new algorithm is n-dimensional so it could simultaneously optimize the dose delivery in a 3D volume and in principle also select the ideal beam orientations, beam modalities (photons, electrons, neutrons, etc) and optimal spectral distributions of the corresponding modalities. To make calculation time reasonable, 2D-3D problems are most practical, and suitable beam orientations are preselected by the choice of irradiation kernel. The energy deposition kernel should therefore be selected in order to avoid irradiation through organs at risk. Clinically established dose response parameters for the tissues of interest are used to make the optimization as relevant as possible to the clinical problems at hand. The algorithm can be used even with a poorly selected kernel because it will always, as far as possible, avoid irradiating organs at risk. The generated dose distribution will be optimal with respect to the spatial distribution and assumed radiobiological properties of the tumour and normal tissues at risk for the kernel chosen. More specifically the probability of achieving tumour control without fatal complications in normal tissues is maximized. In the clinical examples a reduced tumour dose is seen at the border to sensitive organs at risk, but instead an increased dose just inside the tumour border is generated. The increased tumour dose has the effect that the dose fall-off is as steep as possible at the border to organs at risk.

Normal tissue complication probabilities: variable dose per fraction

Because of the large amount of data generated by 3D treatment planning, new tools are being developed for the evaluation and optimization of the plans. Estimates of the probability of local control of the tumor and for the probability of specific normal tissue complications are among the new tools. The normal tissue complication probability (NTCP) is based on clinical estimates of the tolerance doses for specific tissues/organs. These tolerance doses are assumed to apply for uniform partial and full volume irradiations delivered at 2 Gy per fraction and 5 fractions per week. A different tolerance dose may apply when the dose is delivered at a different dose per fraction and over a different period of time. This study evaluates the maximum change expected in the NTCP when the normal structure receives the dose at a different dose per fraction than the target volume due to different choices in the delivery of the daily fraction.

The use of 3-D dose volume analysis to predict radiation hepatitis

Although it is well known that the tolerance of the liver to external beam irradiation depends on the volume of liver irradiated, few data exist which quantify this dependence. Therefore, a review was carried out of our clinical trial for the treatment of intrahepatic malignancies in which the dose of radiation delivered depended on the volume of normal liver treated. Three dimensional treatment planning using dose-volume histogram analysis of the normal liver was used for all patients. Nine of the 79 patients treated developed clinical radiation hepatitis. None of the patient related variables assessed were associated with radiation hepatitis. All patients who developed radiation hepatitis received whole liver irradiation, as all or part of their treatment, which produced a mean dose greater than or equal to 37 Gy. Dose volume histograms were used to calculate normal tissue complication probabilities based on parameters derived from the literature. The risk of complication was greatly overestimated among patients receiving a high dose of radiation to part of the liver without whole liver treatment. An estimation of model parameters based on the clinical results indicated a larger magnitude for the "volume effect parameter" than the literature estimate (n = 0.69 +/- 0.05 vs 0.32; p less than 0.001). Computation of the normal tissue complication probabilities using the larger value of n produced a good description of the observed risk of radiation hepatitis. These findings suggest that dose volume histogram analysis can be used to quantify the tolerance of the liver to radiation. The predictive value of this parameterization of the normal tissue complication probability model will need to be tested with liver tolerance and dose volume histogram data from an independent clinical trial.

Three-dimensional photon treatment planning for Hodgkin's disease

A multi-institutional study was undertaken using computerized planning systems to develop three-dimensional (3-D) radiotherapy plans for Hodgkin's disease (H.D.). Two patients, the first afflicted with bulky stage II disease and another one with early stage I H.D., were studied. Three main categories of plan were produced for each patient: a) a traditional plan which modelled a conventional mantle treatment on the 3-D system, b) a 3-D standard plan where anterior and posterior fields were designed to cover 3-D target volumes, and c) a 3-D unconstrained plan where innovational techniques were employed. Three-dimensional planning provides information about the dose distribution throughout the large volume irradiated in patients with H.D. that is not available with conventional mantle planning. The use of 3-D techniques resulted in improved tumor coverage, but by allowing for uncertainties such as motion, the doses to normal tissues tended to be higher. The use of unorthodox beam arrangements introduced added complexities, and further increased the lung doses. The most even dose distributions were obtained by incorporating compensating filters into anterior fields. Clinicians showed wide variations in their assessment of the plans, possible reasons for which are addressed in this paper. In addition, calculated probabilities from models of tumor control and normal tissue damage are also presented.

Histogram reduction method for calculating complication probabilities for three-dimensional treatment planning evaluations

New tools are needed to help in evaluating 3-D treatment plans because of the large volume of data. One technique which may prove useful is the application of complication probability calculations. A method of calculating complication probabilities for inhomogeneously irradiated normal tissues is presented in this paper. The method uses clinical estimates of tolerance doses for a few discreet conditions of uniform partial organ irradiation, an empirical fit of a continuous function to these data, and a technique (the effective volume method) for transforming nonuniform dose-volume histograms into equivalent uniform histograms. The behavior of the effective volume histogram reduction method for various boundary conditions is reviewed. The use of complication probabilities in evaluating treatment plans is presented, using examples from an NCI 3-D treatment planning contract.

Three-dimensional treatment planning for lung cancer

The experience of four institutions involved in a three-dimensional treatment planning contract (NCI) for lung cancer is described. It was found that three-dimensional treatment planning has a significant potential for optimization of treatment plans for radiotherapy of lung cancer both for tumor coverage and sparing of critical normal tissues within the complex anatomy of the human thorax. Evaluation tools, such as dose-volume histograms, and three-dimensional isodose displays, such as multiple plane views, surface dose displays, etc., were found to be extremely valuable in evaluation and comparison of these complex plans. It is anticipated that with further developments in three-dimensional simulation and treatment delivery systems, major progress towards uncomplicated local regional control of lung cancer may be forthcoming.

Fitting of normal tissue tolerance data to an analytic function

During external beam radiotherapy, normal tissues are irradiated along with the tumor. Radiation therapists try to minimize the dose of normal tissues while delivering a high dose to the target volume. Often this is difficult and complications arise due to irradiation of normal tissues. These complications depend not only on the dose but also on volume of the organ irradiated. Lyman has suggested a four-parameter empirical model which can be used to represent normal tissue response under conditions of uniform irradiation to whole and partial volumes as a function of the dose and volume irradiated. In this paper, Lyman's model has been applied to a compilation of clinical tolerance data developed by Emami et al. The four parameters to characterize the tissue response have been determined and graphical representations of the derived probability distributions are presented. The model may, therefore, be used to interpolate clinical data to provide estimated normal tissue complication probabilities for any combination of dose and irradiated volume for the normal tissues and end points considered.

Three-dimensional photon treatment planning in carcinoma of the larynx

The role of three-dimensional (3-D) treatment planning in the definitive treatment of carcinoma of the larynx with radiation was evaluated at four institutions as part of an NCI contract. A total of 30 different treatment approaches were devised for two patients with larynx cancer. CT scans were obtained for both patients and various treatment planning tools were employed to optimize beam arrangements and to evaluate the resulting dose distribution. The effect on dose distribution of a number of factors was also examined: 1) the use of dose calculation algorithms which correct for tissue inhomogeneities, 2) the variation of the CT numbers used for inhomogeneity corrections to simulate inaccuracies in the knowledge of the CT numbers, and 3) the modification of beam energy. A multitude of data was used in plan evaluation and a numerical score was given to each plan to estimate the tumor control probability and the normal tissue complication probability. We found 3-D treatment planning to be of potential value in optimizing treatment plans in larynx cancer. Improved target coverage was achieved when complete information describing 3-D geometry of the anatomy was utilized. In some cases, the treatment planning tools employed, such as the beam's eye view, helped devise novel beam arrangements which were useful alternatives to standard techniques. We found little effect of change in CT number on dose distributions. A comparison between dose distributions calculated with tissue inhomogeneity corrections to those calculated without this correction showed little difference. We did find some improvement in the dose to the primary tumor volume at lower beam energies, but with an increased larynx volume potentially receiving doses above tolerance.

Three-dimensional photon treatment planning for carcinoma of the nasopharynx

The role of 3-D treatment planning for carcinoma of the nasopharynx was assessed in a four institution study. Two patients were worked up and had an extensive number of CT scans on which target volumes and normal tissues were defined. Treatment planning was then performed using state of the art dose planning systems for these patients to assess the value of the new technology. In general, it was demonstrated that multi-field conformal plans could achieve good tumor dose coverage, while at the same time reducing normal tissue doses, compared to standard treatment planning techniques. The role of inhomogeneity corrections, beam energy, and the use of CT vs. simulation films for defining target volumes were also discussed. In addition, techniques to evaluate 3-D plans for the nasopharynx were considered, and some analysis of this problem is presented in this paper.

Three-dimensional treatment planning for para-aortic node irradiation in patients with cervical cancer

Three-dimensional treatment planning has been used by four cooperating centers to prepare and analyze multiple treatment plans on two cervix cancer patients. One patient had biopsy-proven and CT-demonstrable metastasis to the para-aortic nodes, while the other was at high risk for metastatic involvement of para-aortic nodes. Volume dose distributions were analyzed, and an attempt was made to define the role of 3-D treatment planning to the para-aortic region, where moderate to high doses (50-66 Gy) are required to sterilize microscopic and gross metastasis. Plans were prepared using the 3-D capabilities for tailoring fields to the target volumes, but using standard field arrangements (3-D standard), and with full utilization of the 3-D capabilities (3-D unconstrained). In some but not all 3-D unconstrained plans, higher doses were delivered to the large nodal volume and to the volume containing gross nodal disease than in plans analyzed but not prepared with full 3-D capability (3-D standard). The small bowel was the major dose limiting organ. Its tolerance would have been exceeded in all plans which prescribed 66 Gy to the gross nodal mass, although some reduction in small bowel near-maximum dose was achieved in the 3-D unconstrained plans. All plans were able to limit doses to other normal organs to tolerance levels or less, with significant reductions seen in doses to spinal cord, kidneys, and large bowel in the 3-D unconstrained plans, as compared to the 3-D standard plans. A high probability of small bowel injury was detected in one of four 3-D standard plans prescribed to receive 50 Gy to the large para-aortic nodal volume; the small bowel dose was reduced to an acceptable level in the corresponding 3-D unconstrained plan. An optimum beam energy for treating this site was not identified, with plans using 4, 6, 10, 15, 18, and 25 MV photons all being equally acceptable. Attempts to deliver moderate or high doses (50-66 Gy) to this region should be made only after careful analysis of the plan with techniques similar to those employed in this study.

Numerical scoring of treatment plans

This is a report on numerical scoring techniques developed for the evaluation of treatment plans as part of a four-institution study of the role of 3-D planning in high energy external beam photon therapy. A formal evaluation process was developed in which plans were assessed by a clinician who displayed dose distributions in transverse, sagittal, coronal, and arbitrary oblique planes, viewed dose-volume histograms which summarized dose distributions to target volumes and the normal tissues of interest, and reviewed dose statistics which characterized the volume dose distribution for each plan. In addition, tumor control probabilities were calculated for each biological target volume and normal tissue complication probabilities were calculated for each normal tissue defined in the agreed-upon protocols. To score a plan, the physician assigned a score for each normal tissue to reflect possible complications; for each target volume two separate scores were assigned, one representing the adequacy of tumor coverage, the second the likelihood of a complication. After scoring each target and normal tissue individually, two summary scores were given, one for target coverage, the second reflecting the impact on all normal tissues. Finally, each plan was given an overall rating (which could include a downgrading of the plan if the treatment was judged to be overly complex).

Three-dimensional photon treatment planning of the intact breast

Three-dimensional treatment planning for the intact breast was performed on two patients who had undergone CT scanning. A total of 38 treatment plans were evaluated. Multiple plans were evaluated for each patient including plans with and without inhomogeneity corrections, plans using varying photon energies of 60Co, 4 MV, 6 MV, 10 MV, and 15 MV, and three-dimensionally unconstrained plans. Increased hot spots were appreciated in the central axis plane when lung inhomogeneity corrections were used. Additional hot spots were appreciated in off-axis planes towards the cephalad and caudad aspects of the target volume because of lung inhomogeneity corrections and changes in the breast contour. The use of 60Co was associated with an increase in the magnitude and volume of hot spots, whereas the use of higher energy photons such as 10 MV and 15 MV was associated with an unacceptable target coverage at shallow depths. Therefore, for the two patients studied, the use of a medium energy photon beam (such as from a 6 MV linear accelerator) appeared to be the energy of choice for treatment of the intact breast. The three-dimensionally unconstrained plans were able to improve slightly upon the standard plans, particularly with relationship of dose to normal tissue structures. Areas for future research were identified, including the use of tissue compensators.

Natural history of hepatocellular carcinoma and prognosis in relation to treatment. Study of 850 patients

A total of 850 patients with hepatocellular carcinoma seen during the last 8 years were analyzed retrospectively for survival in relation to treatment and disease stage. A new staging scheme based on tumor size, ascites, jaundice and serum albumin was used. Clearly, the prognosis depended on disease stage. The median survival of 229 patients who received no specific treatment was 1.6 months, 0.7 month for Stage III patients, 2.0 months for Stage II, and 8.3 months for Stage I. The median survival of Stage I patients who had hepatic resection (n = 115) was 25.6 months and Stage II patients with resection (n = 42) was 12.2 months. In patients who had a small cancer (less than or equal to 25% of liver area in size) the median survival was 29.0 months. Survival of the surgically treated patients, which represented a highly selected group, was better than that of medically treated patients of a comparable stage. Median survival of Stage I medically treated patients (n = 124) was 9.4 months, for Stage II (n = 290) 3.5 months, and for Stage III (n = 50) 1.6 months. Medical treatment prolonged survival in Stage II and III patients, but not in Stage I. Transcatheter arterial embolization gave a better survival compared with chemotherapy, whether intra-arterial bolus administration of mitomycin C, systemic mitomycin C, or oral/rectal tegafur, in Stage II. Among various chemotherapeutic modalities, intra-arterial bolus injection was superior to systemic chemotherapy in survival in Stage II. In Stage III, chemotherapy improved survival as compared with no specific treatment. The major causes of death were hepatic failure and gastrointestinal bleeding, probably due to the coexistent advanced cirrhosis. These results in survival are much improved over the past reports, and the differences are probably a result of earlier diagnosis and frequent hepatic resections.