Reporting and analyzing dose distributions: a concept of equivalent uniform dose

Intensity modulated radiation therapy versus three-dimensional conformal radiation therapy for the treatment of high grade glioma: a dosimetric comparison

The present study compared the dosimetry of intensity-modulated radiation therapy (IMRT) and three-dimensional conformal radiation therapy (3D-CRT) techniques in patients treated for high-grade glioma. A total of 20 patients underwent computed tomography treatment planning in conjunction with magnetic resonance imaging fusion. Prescription dose and normal-tissue constraints were identical for the 3D-CRT and IMRT plans. The prescribed dose was 59.4 Gy delivered at 1.8 Gy per fraction using 4-10 MV photons. Normal-tissue dose constraints were 50-54 Gy for the optic chiasm and nerves, and 55-60 Gy for the brainstem. The IMRT plan yielded superior target coverage as compared with the 3D-CRT plan. Specifically, minimum and mean planning target volume cone down doses were 54.52 Gy and 61.74 Gy for IMRT and 50.56 Gy and 60.06 Gy for 3D-CRT (p < or = 0.01). The IMRT plan reduced the percent volume of brainstem receiving a dose greater than 45 Gy by 31% (p = 0.004) and the percent volume of brain receiving a dose greater than 18 Gy, 24 Gy, and 45 Gy by 10% (p = 0.059), 14% (p = 0.015), and 40% (p < or = 0.0001) respectively. With IMRT, the percent volume of optic chiasm receiving more than 45 Gy was also reduced by 30.40% (p = 0.047). As compared with 3D-CRT, IMRT significantly increased the tumor control probability (p < or = 0.005) and lowered the normal-tissue complication probability for brain and brainstem (p < 0.033). Intensity-modulated radiation therapy improved target coverage and reduced radiation dose to the brain, brainstem, and optic chiasm. With the availability of new cancer imaging tools and more effective systemic agents, IMRT may be used to intensify tumor doses while minimizing toxicity, therefore potentially improving outcomes in patients with high-grade glioma.

Full-dose gemcitabine and concurrent radiotherapy for unresectable pancreatic cancer

PURPOSE

Full-dose gemcitabine and concurrent radiotherapy is a promising treatment approach in unresectable pancreatic cancer. This study was conducted to assess the pattern of failure and toxicity associated with the use of conformal treatment volumes, omitting prophylactic lymph node irradiation.

METHODS AND MATERIALS

Seventy-four patients with locally advanced pancreatic cancer were treated between 1997 and 2005 with full-dose (1000 mg/m(2), Days 1, 8, and 15) gemcitabine and concurrent radiotherapy (36 Gy [median] in 15 daily fractions). The planning target volume (PTV) was limited to the gross tumor volume (GTV) plus 1-cm margin. Patient computed tomography (CT) scans were systematically reviewed to determine the pattern of failure. Kaplan-Meier and Cox-regression models were used to analyze freedom from local progression (FFLP), distant failure, overall survival (OS), and toxicity.

RESULTS

With a median follow-up of 10.6 months (20.6 months in living patients), the 1-year and 2-year FFLP rates were 64% and 38%, respectively. Four patients (5%) failed in the peripancreatic lymph nodes (3 in-field and 1 marginal failure). Median OS was 11.2 months. Analyzed as a time-dependent covariate, local failure was a significant predictor of OS (p = 0.0074). Sixteen patients (22%) had significant gastrointestinal (GI) toxicity (> or = Grade 3). PTV correlated with significant GI toxicity (p = 0.007).

CONCLUSIONS

Freedom from local progression in unresectable pancreatic cancer is suboptimal. In conjunction with full-dose gemcitabine, the use of conformal fields encompassing only the GTV helps reduce toxicity and does not result in marginal failures. Our findings provide rationale for intensification of local therapy in conjunction with more effective systemic therapy.

A new homogeneity index based on statistical analysis of the dose-volume histogram

The goal of the present study was to develop a new dose–volume histogram (DVH)– based homogeneity index for effectively evaluating the dose homogeneity of intensity‐modulated radiotherapy plans. The new index, called the sigma‐index (“S‐index”) is defined as the standard deviation of the normalized differential DVH curve. In a study of 16 patients with brain tumors at our institution, the S‐index was found to vary from 0.80 to 3.15. Our results showed that the S‐index provides a more reliable and accurate measure of dose homogeneity than that given by conventional methods. A guideline for evaluating the dose homogeneity of treatment plans based on the S‐index and its relation to equivalent uniform dose is discussed.

PACS numbers: 87.53.Xd, 87.53.Tf

Quantification of the impact of MLC modeling and tissue heterogeneities on dynamic IMRT dose calculations

This study quantifies the dose prediction errors (DPEs) in dynamic IMRT dose calculations resulting from (a) use of an intensity matrix to estimate the multi-leaf collimator (MLC) modulated photon fluence (DPE(IGfluence) instead of an explicit MLC particle transport, and (b) handling of tissue heterogeneities (DPE(hetero)) by superposition/convolution (SC) and pencil beam (PB) dose calculation algorithms. Monte Carlo (MC) computed doses are used as reference standards. Eighteen head-and-neck dynamic MLC IMRT treatment plans are investigated. DPEs are evaluated via comparing the dose received by 98% of the GTV (GTV D 98%), the CTV D 95%, the nodal D 90%, the cord and the brainstem D 02%, the parotid D 50%, the parotid mean dose (D (Mean)), and generalized equivalent uniform doses (gEUDs) for the above structures. For the MC-generated intensity grids, DPE(IGfluence) is within +/- 2.1% for all targets and critical structures. The SC algorithm DPE(hetero) is within +/- 3% for 98.3% of the indices tallied, and within +/- 3.4% for all of the tallied indices. The PB algorithm DPE(hetero) is within +/- 3% for 92% of the tallied indices. Statistical equivalence tests indicate that PB DPE(hetero) requires a +/- 3.6% interval to state equivalence with the MC standard, while the intervals are < 1.5% for SC DPE(hetero) and DPE(IGfluence). Overall, these results indicate that SC and MC IMRT dose calculations which use MC-derived intensity matrices for fluence prediction do not introduce significant dose errors compared with full Monte Carlo dose computations; however, PB algorithms may result in clinically significant dose deviations.

Development of an inverse optimization package to plan nonuniform dose distributions based on spatially inhomogeneous radiosensitivity extracted from biological images

An inverse optimization package which is capable of generating nonuniform dose distribution with subregional dose escalation is developed to achieve maximum equivalent uniform dose (EUD) for target while keeping the critical structure doses as low as possible. Relative cerebral blood volume (rCBV) maps obtained with a dynamic susceptibility contrast-enhanced MRI technique were used to delineate spatial radiosensitivity distributions. The voxel rCBV was converted to voxel radiosensitivity parameters (e.g., alpha and alpha/beta) based on previously reported correlations between rCBV, tumor grade, and radiosensitivity. A software package, DOSEPAINT, developed using MATLAB, optimizes the beamlet weights to achieve maximum EUD for target while limiting doses to critical structures. Using DOSEPAINT, we have generated nonuniform 3D-dose distributions for selected patient cases. Depending on the variation of the pixel radiosensitivity, the subregional dose escalation can be as high as 35% of the uniform dose as planned conventionally. The target dose escalation comes from both the inhomogeneous radiosensitivities and the elimination of integral target dose constraint. The target EUDs are found to be higher than those for the uniform dose planned ignoring the spatial inhomogeneous radiosensitivity. The EUDs for organs at risk are found to be approximately equal to or lower than those for the uniform dose plans. In conclusion, we have developed a package that is capable of generating nonuniform dose distributions optimized for spatially inhomogeneous radiosensitivity. Subregional dose escalation may lead to increased treatment effectiveness as indicated by higher EUDs. The current development will impact biological image guided radiotherapy.

The effect of positional realignment on dose delivery to the prostate and organs-at-risk for 3DCRT

In this study, we evaluate the impact of daily image-guided patient repositioning on dose delivery to prostate and sensitive organs in the treatment of prostate carcinoma with 3-dimensional conformal radiation therapy (3DCRT). Five patients with substantial ultrasound-documented interfractional prostate motion during their 3DCRT treatment course were selected. Starting with the original treatment plan, 2 additional plans were retrospectively generated for each patient. In one set, organ contours were moved for each fraction, thus simulating positioning with misalignment caused by organ motion if ultrasound guidance were not used. In a second set of plans, the isocenter was shifted, as were the organ contours, simulating realignment based on the ultrasound image. In all cases, the number of planned monitor units was set to those of the original plan. For a given patient, isodose distributions, dose-volume histograms (DVHs), equivalent uniform dose (EUD) for prostate, and generalized equivalent uniform dose (gEUDs) for bladder and rectum were calculated for each fraction and then combined for each shift condition. In all reconstructed plans, the results show no substantial changes in dose coverage of the prostate <0.21% change in EUD) compared to the original plan. However, in some cases with no realignment, a larger volume of the bladder or rectum gets higher dose, with the consequent gEUD for each organ significantly greater compared to the original plan.

Equivalent uniform dose concept evaluated by theoretical dose volume histograms for thoracic irradiation

BACKGROUND AND PURPOSE

The goal of our study was to quantify the limits of the EUD models for use in score functions in inverse planning software, and for clinical application.

MATERIALS AND METHODS

We focused on oesophagus cancer irradiation. Our evaluation was based on theoretical dose volume histograms (DVH), and we analyzed them using volumetric and linear quadratic EUD models, average and maximum dose concepts, the linear quadratic model and the differential area between each DVH.

RESULTS

We evaluated our models using theoretical and more complex DVHs for the above regions of interest. We studied three types of DVH for the target volume: the first followed the ICRU dose homogeneity recommendations; the second was built out of the first requirements and the same average dose was built in for all cases; the third was truncated by a small dose hole. We also built theoretical DVHs for the organs at risk, in order to evaluate the limits of, and the ways to use both EUD(1) and EUD/LQ models, comparing them to the traditional ways of scoring a treatment plan. For each volume of interest we built theoretical treatment plans with differences in the fractionation.

CONCLUSION

We concluded that both volumetric and linear quadratic EUDs should be used. Volumetric EUD(1) takes into account neither hot-cold spot compensation nor the differences in fractionation, but it is more sensitive to the increase of the irradiated volume. With linear quadratic EUD/LQ, a volumetric analysis of fractionation variation effort can be performed.

Evaluation of clinical margins via simulation of patient setup errors in prostate IMRT treatment plans

This work evaluates: (i) the size of random and systematic setup errors that can be absorbed by 5 mm clinical target volume (CTV) to planning target volume (PTV) margins in prostate intensity modulated radiation therapy (IMRT); (ii) agreement between simulation results and published margin recipes; and (iii) whether shifting contours with respect to a static dose distribution accurately predicts dose coverage due to setup errors. In 27 IMRT treatment plans created with 5 mm CTV-to-PTV margins, random setup errors with standard deviations (SDs) of 1.5, 3, 5 and 10 mm were simulated by fluence convolution. Systematic errors with identical SDs were simulated using two methods: (a) shifting the isocenter and recomputing dose (isocenter shift), and (b) shifting patient contours with respect to the static dose distribution (contour shift). Maximum tolerated setup errors were evaluated such that 90% of plans had target coverage equal to the planned PTV coverage. For coverage criteria consistent with published margin formulas, plans with 5 mm margins were found to absorb combined random and systematic SDs = 3 mm. Published recipes require margins of 8-10 mm for 3 mm SDs. For the prostate IMRT cases presented here a 5 mm margin would suffice, indicating that published recipes may be pessimistic. We found significant errors in individual plan doses given by the contour shift method. However, dose population plots (DPPs) given by the contour shift method agreed with the isocenter shift method for all structures except the nodal CTV and small bowel. For the nodal CTV, contour shift DPP differences were due to the structure moving outside the patient. Small bowel DPP errors were an artifact of large relative differences at low doses. Estimating individual plan doses by shifting contours with respect to a static dose distribution is not recommended. However, approximating DPPs is acceptable, provided care is taken with structures such as the nodal CTV which lie close to the surface.

EUCLID: an outcome analysis tool for high-dimensional clinical studies

Treatment management decisions in three-dimensional conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT) are usually made based on the dose distributions in the target and surrounding normal tissue. These decisions may include, for example, the choice of one treatment over another and the level of tumour dose escalation. Furthermore, biological predictors such as tumour control probability (TCP) and normal tissue complication probability (NTCP), whose parameters available in the literature are only population-based estimates, are often used to assess and compare plans. However, a number of other clinical, biological and physiological factors also affect the outcome of radiotherapy treatment and are often not considered in the treatment planning and evaluation process. A statistical outcome analysis tool, EUCLID, for direct use by radiation oncologists and medical physicists was developed. The tool builds a mathematical model to predict an outcome probability based on a large number of clinical, biological, physiological and dosimetric factors. EUCLID can first analyse a large set of patients, such as from a clinical trial, to derive regression correlation coefficients between these factors and a given outcome. It can then apply such a model to an individual patient at the time of treatment to derive the probability of that outcome, allowing the physician to individualize the treatment based on medical evidence that encompasses a wide range of factors. The software's flexibility allows the clinicians to explore several avenues to select the best predictors of a given outcome. Its link to record-and-verify systems and data spreadsheets allows for a rapid and practical data collection and manipulation. A wide range of statistical information about the study population, including demographics and correlations between different factors, is available. A large number of one- and two-dimensional plots, histograms and survival curves allow for an easy visual analysis of the population. Several visual and analytical methods are available to quantify the predictive power of the multivariate regression model. The EUCLID tool can be readily integrated with treatment planning and record-and-verify systems.

How does performance of ultrasound tissue typing affect design of prostate IMRT dose-painting protocols?

PURPOSE

To investigate how the performance characteristics of ultrasound tissue typing (UTT) affect the design of a population-based prostate dose-painting protocol.

METHODS AND MATERIALS

The performance of UTT is evaluated using the receiver operating characteristic curve. As the imager's sensitivity increases, more tumors are detected, but the specificity worsens, causing more false-positive results. The UTT tumor map, obtained with a specific sensitivity and specificity setup, was used with the patient's CT image to guide intensity-modulated radiotherapy (IMRT) planning. The optimal escalation dose to the UTT positive region, as well as the safe dose to the negative background, was obtained by maximizing the uncomplicated control (i.e., a combination of tumor control probability and weighted normal tissue complication probability). For high- and low-risk tumors, IMRT plans guided by conventional ultrasound or UTT with a one-dimensional or two-dimensional spectrum analysis technique were compared with an IMRT plan in which the whole prostate was dose escalated.

RESULTS

For all imaging modalities, the specificity of 0.9 was chosen to reduce complications resulting from high false-positive results. If the primary tumors were low risk, the IMRT plans guided by all imaging modalities achieved high tumor control probability and reduced the normal tissue complication probability significantly compared with the plan with whole gland dose escalation. However, if the primary tumors were high risk, the accuracy of the imaging modality was critical to maintain the tumor control probability and normal tissue complication probability at acceptable levels.

CONCLUSION

The performance characteristics of an imager have important implications in dose painting and should be considered in the design of dose-painting protocols.

3D dose reconstruction for clinical evaluation of IMRT pretreatment verification with an EPID

BACKGROUND AND PURPOSE

Pretreatment verification with an electronic portal imaging device is an important part of our patient-specific quality assurance program for advanced treatment techniques. Up to now, this verification has been performed for over 400 IMRT patient plans. For every treatment field, a 2D portal dose image (PDI) is measured and compared with a predicted PDI. Often it is not straightforward to interpret dose deviations found in these 2D comparisons in terms of clinical implications for the patient. Therefore, a method to derive the 3D patient dose based on the measured PDIs was implemented.

METHODS AND MATERIALS

For reconstruction of the 3D patient dose, the actual fluences delivered by the accelerator are derived from measured portal dose images using an iterative method. The derived fluence map for each beam direction is then used as input for the treatment planning system to generate an adapted 3D patient dose distribution. The accuracy of this method was assessed by measurements in a water phantom. Clinical evaluation of the 3D dose reconstruction was performed for 17 IMRT patients with different tumor sites. Dose differences with respect to the original treatment plan were evaluated in individual CT slices using dose difference maps and a 3D gamma analysis and by comparing dose-volume histograms (DVHs).

RESULTS

The measurements indicated that the accuracy of the 3D dose reconstruction was within 2%/2mm. For the patients observed dose differences with respect to the original plan were generally within 2%, except at the field edges and in the sharp dose gradients around the planning target volume (PTV). Gamma analysis showed that the dose differences were within 2%/2mm for more than 95% of the points in all cases. Differences in DVH parameters for the PTV and organs at risk were also within 2% in nearly all cases.

CONCLUSION

A method to derive actual delivered fluence maps from measured PDIs and to use them to reconstruct the 3D patient dose was implemented. The reconstruction eases the estimation of the clinical relevance of observed dose differences in the pretreatment measurements.

Is There an Optimum Overall Time for Head and Neck Radiotherapy? A Review, with New Modelling

AIMS

To test by modelling whether a non-standard fractionated schedule giving optimum log cell kill could be expected, between short (accelerated) and longer multiple fraction/day schedules.

MATERIALS AND METHODS

Linear quadratic modelling was carried out for many schedules, with biologically effective doses converted to normalised total doses (NTDs; in 2 Gy fractions). Late complication and acute mucosal NTDs were calculated as constraint doses for each schedule, and the highest tumour NTDs and log cell kill values within both constraints were calculated. This modelling is robust and agrees with conclusions in a very recent meta-analysis (Bourhis J, Overgaard J, Audry H, et al. Hyperfractionated or accelerated radiotherapy in head and neck cancer: a meta-analysis. www.thelancet.com. Published online August 17, 2006).

RESULTS

The six schedules that gave the highest tumour log cell kill deliver a narrow range of 11.1-11.2 log10 cell kill in the present parameters. Other regularly used schedules give closer to 10 log10. Using one fraction/day fails to achieve the highest therapeutic ratios. Suggestions are made for escalating certain UK schedules. Fractionated radiotherapy results in a nearly constant tumour cell kill if the acute mucosal NTD is held constant. However, a small (3%) gain in tumour cell kill occurs from 3 weeks to 73 fractions of 1.15 Gy in 7 weeks. That is how fractionation works, within both acute and late constraints. Short accelerated schedules enable fewer late complications, but do not do as well for the minority of head and neck tumours that repopulate slowly.

CONCLUSIONS

Schedules of 4-6 weeks overall time could be chosen to give at least 11 log10 cell kill, which are safe. Most tumours would require two fractions/day, until routine monitoring of repopulation rates becomes feasible to select individual tumours. There is no 'optimum schedule', but each chosen schedule can be balanced against its own risk of excessive acute or late complications, as shown in these examples.

Three-dimensional conformal radiation may deliver considerable dose of incidental nodal irradiation in patients with early stage node-negative non-small cell lung cancer when the tumor is large and centrally located

BACKGROUND AND PURPOSE

To determine the dose to regional nodal stations in patients with T1-3N0M0 non-small cell lung cancer (NSCLC) treated with three-dimensional conformal radiation therapy (3DCRT) without intentional elective nodal irradiation (ENI).

MATERIALS AND METHODS

Twenty-three patients with medically inoperable T1-3N0M0 NSCLC were treated with 3DCRT without ENI. Hilar and mediastinal nodal regions were contoured on planning CT. The prescription dose was normalized to 70 Gy. Equivalent uniform dose (EUD) and other dosimetric parameters (e.g., V40) were calculated for each nodal station.

RESULTS

The median EUD for the whole group ranged from 0.4 to 4.4 Gy for all elective nodal regions. Gross tumor volume (GTV) and the relationship between GTV and hilum were significantly correlated with irradiation dose to ipsilateral hilar nodal regions (P<.05). For patients with GTV>or=30.2 cm3 (diameter approximately 4 cm) and or having any overlap with hilum, the median EUDs were 9.6, 22.6, and 62.9 Gy for ipsilateral lower paratracheal, subcarinal, and ipsilateral hilar regions, respectively. The corresponding median V40 were 32.5%, 39.3%, and 97.6%, respectively.

CONCLUSIONS

Although incidental nodal irradiation dose is low in the whole group, the dose to high-risk nodal regions is considerable in patients with T1-3N0 NSCLC when the primary is large and/or centrally located.

NTCP modelling and pulmonary function tests evaluation for the prediction of radiation induced pneumonitis in non-small-cell lung cancer radiotherapy

This work aims to evaluate the predictive strength of the relative seriality, parallel and Lyman-Kutcher-Burman (LKB) normal tissue complication probability (NTCP) models regarding the incidence of radiation pneumonitis (RP), in a group of patients following lung cancer radiotherapy and also to examine their correlation with pulmonary function tests (PFTs). The study was based on 47 patients who received radiation therapy for stage III non-small-cell lung cancer. For each patient, lung dose volume histograms (DVHs) and the clinical treatment outcome were available. Clinical symptoms, radiological findings and pulmonary function tests incorporated in a post-treatment follow-up period of 18 months were used to assess the manifestation of radiation induced complications. Thirteen of the 47 patients were scored as having radiation induced pneumonitis, with RTOG criteria grade 3 and 28 of the 47 with RTOG criteria grade 2. Using this material, different methods of estimating the likelihood of radiation effects were evaluated, by analysing patient data based on their full dose distributions and associating the calculated complication rates with the clinical follow-up records. Lungs were evaluated as a paired organ as well as individual lungs. Of the NTCP models examined in the overall group considering the dose distribution in the ipsilateral lung, all models were able to predict radiation induced pneumonitis only in the case of grade 2 radiation pneumonitis score, with the LKB model giving the best results (chi2-test: probability of agreement between the observed and predicted results Pchi(chi2)=0.524 using the 0.05 significance level). The NTCP modelling considering lungs as a paired organ did not give statistically acceptable results. In the case of lung cancer radiotherapy, the application of different published radiobiological parameters alters the NTCP results, but not excessively as in the case of breast cancer radiotherapy. In this relatively small group of lung cancer patients, no positive statistical correlation could be established between the incidence of radiation pneumonitis as estimated by NTCP models and the pulmonary function test evaluation. However, the use of PFTs as markers or predictors for the incidence or severity of radiation induced pneumonitis must be investigated further.

Reduced-order parameter optimization for simplifying prostate IMRT planning

Intensity-modulated radiotherapy (IMRT) has become an effective tool for cancer treatment with radiation. However, even expert radiation planners still need to spend a substantial amount of time manually adjusting IMRT optimization parameters such as dose limits and costlet weights in order to obtain a clinically acceptable plan. In this paper, we describe two main advances that simplify the parameter adjustment process for five-field prostate IMRT planning. First, we report the results of a sensitivity analysis that quantifies the effect of each hand-tunable parameter of the IMRT cost function on each clinical objective and the overall quality of the resulting plan. Second, we show that a recursive random search over the six most sensitive parameters as an outer loop in IMRT planning can quickly and automatically determine parameters for the cost function that lead to a plan meeting the clinical requirements. Our experiments on a ten-patient dataset show that for 70% of the cases, we can automatically determine a plan in 10 min (on the average) that is either clinically acceptable or requires only minor adjustment by the planner. The outer-loop optimization can be easily integrated into a traditional IMRT planning system.

The atlas of complication incidence: a proposal for a new standard for reporting the results of radiotherapy protocols

We present a new method of reporting the results of radiotherapy protocols. The dose-volume atlas of complication incidence is a comprehensive and unbiased summary of the dose-volume exposures and complications occurring in patients after treatment. This new tool provides clear and systematic information about the safety of regions of dose-volume exposure previously treated that can be used when considering new treatments. Actuarial and model-dependent versions of the atlas are described. By using the raw data in the appropriate forms of the atlas, logistic regression, Kaplan-Meier, and Cox proportional hazards analysis can be performed, allowing for the independent calculation of dose-volume response. The data required are simple enough that provided compatible definitions of dose, volume, and complications are used, atlases from different protocols are potentially additive, facilitating the meta-analysis of inter-interinstitutional data. If this method were adopted as a standard for reporting the outcome of treatment protocols, a potentially synergistic increase in the utility of each protocol could result.

Optimum parameters in a model for tumour control probability, including interpatient heterogeneity: evaluation of the log-normal distribution

The heterogeneity of human tumour radiation response is well known. Researchers have used the normal distribution to describe interpatient tumour radiosensitivity. However, many natural phenomena show a log-normal distribution. Log-normal distributions are common when mean values are low, variances are large and values cannot be negative. These conditions apply to radiosensitivity. The aim of this work was to evaluate the log-normal distribution to predict clinical tumour control probability (TCP) data and to compare the results with the homogeneous (delta-function with single alpha-value) and normal distributions. The clinically derived TCP data for four tumour types-melanoma, breast, squamous cell carcinoma and nodes-were used to fit the TCP models. Three forms of interpatient tumour radiosensitivity were considered: the log-normal, normal and delta-function. The free parameters in the models were the radiosensitivity mean, standard deviation and clonogenic cell density. The evaluation metric was the deviance of the maximum likelihood estimation of the fit of the TCP calculated using the predicted parameters to the clinical data. We conclude that (1) the log-normal and normal distributions of interpatient tumour radiosensitivity heterogeneity more closely describe clinical TCP data than a single radiosensitivity value and (2) the log-normal distribution has some theoretical and practical advantages over the normal distribution. Further work is needed to test these models on higher quality clinical outcome datasets.

On the visualization of universal degeneracy in the IMRT problem

BACKGROUND

In general, the IMRT optimisation problem possesses many equivalent solutions. This makes it difficult to decide whether a result produced by an IMRT planning algorithm can be further improved, e.g. by adding more beams, or whether it is close to the globally best solution.

RESULTS

It is conjectured that the curvature properties of the objective function around any globally optimum dose distribution are universal. This allows an assessment of optimality of dose distributions that are generated by different beam arrangements in a complementary manner to the objective function value alone. A tool to visualize the curvature structure of the objective function is devised.

CONCLUSION

In an example case, it is demonstrated how the assessment of the curvature space can indicate the equivalence of rival beam configurations and their proximity to the global optimum.

Dose distribution in 3-dimensional conformal radiotherapy for prostate cancer: Comparison of two treatment techniques (six coplanar fields and two dynamic arcs)

PURPOSE

To compare dose distribution for two techniques of 3-dimensional conformal radiotherapy (RT): 6-field technique (6F) and 2-dynamic arc therapy (2DA).

METHODS AND MATERIALS

Thirty nonmetastatic prostate cancer patients were included. In each patient, two treatment plans were prepared: with six coplanar fields (45 degrees , 90 degrees , 135 degrees , 225 degrees , 270 degrees , 315 degrees ) and with two dynamic lateral 100 degrees -wide arcs (40-140 degrees , 220-320 degrees ). Dose-volume histograms (DVHs) were computed and mean area under curve (AUC) values were calculated for the DVHs of Planning Target Volume (PTV), rectum, urinary bladder and femoral heads. Doses given to 30% of rectum (DR(30)), to 60% of rectum (DR(60)), to 50% of bladder (DB(50)), to 50% of femoral head (DF(50)) and to 95% of PTV (DPTV(95)) were reported as a percentage of the total dose.

RESULTS

Mean DR(30) and DR(60) for 6F and 2DA were 75.8%, 51.5% and 72.2%, 37.2%, respectively. Mean DB(50) for 6F and 2DA were 68% and 64.2%, respectively. Mean right DF(50) for 6F and 2DA were 35.4% and 45.5%, respectively. Mean DPTV(95) for 6F and 2DA were 99% and 99.2%, respectively. Mean AUCs of DVHs of rectum and urinary bladder were significantly higher for 6F (this was more evident for small PTV and in the intermediate dose range). Mean AUC of DVHs of PTV and femoral heads were significantly higher for 2DA.

CONCLUSIONS

Both 6F and 2DA offer good dose distribution for PTV. 2DA allows for significantly better sparing of rectum and urinary bladder with slightly worse femoral head dose distribution. Further study is warranted in order to establish the clinical relevance of these differences.

Multiobjective optimization with a modified simulated annealing algorithm for external beam radiotherapy treatment planning

Inverse planning in external beam radiotherapy often requires a scalar objective function that incorporates importance factors to mimic the planner's preferences between conflicting objectives. Defining those importance factors is not straightforward, and frequently leads to an iterative process in which the importance factors become variables of the optimization problem. In order to avoid this drawback of inverse planning, optimization using algorithms more suited to multiobjective optimization, such as evolutionary algorithms, has been suggested. However, much inverse planning software, including one based on simulated annealing developed at our institution, does not include multiobjective-oriented algorithms. This work investigates the performance of a modified simulated annealing algorithm used to drive aperture-based intensity-modulated radiotherapy inverse planning software in a multiobjective optimization framework. For a few test cases involving gastric cancer patients, the use of this new algorithm leads to an increase in optimization speed of a little more than a factor of 2 over a conventional simulated annealing algorithm, while giving a close approximation of the solutions produced by a standard simulated annealing. A simple graphical user interface designed to facilitate the decision-making process that follows an optimization is also presented.

On relating the generalized equivalent uniform dose formalism to the linear-quadratic model

Two main approaches are commonly used in the literature for computing the equivalent uniform dose (EUD) in radiotherapy. The first approach is based on the cell-survival curve as defined in the linear-quadratic model. The second approach assumes that EUD can be computed as the generalized mean of the dose distribution with an appropriate fitting parameter. We have analyzed the connection between these two formalisms by deriving explicit formulas for the EUD which are applicable to normal distributions. From these formulas we have established an explicit connection between the two formalisms. We found that the EUD parameter has strong dependence on the parameters that characterize the distribution, namely the mean dose and the standard deviation around the mean. By computing the corresponding parameters for clinical dose distributions, which in general do not follow the normal distribution, we have shown that our results are also applicable to actual dose distributions. Our analysis suggests that caution should be used in using generalized EUD approach for reporting and analyzing dose distributions.

Dosimetric comparison of four target alignment methods for prostate cancer radiotherapy

PURPOSE

The aim of this study was to compare the dosimetric consequences of 4 treatment delivery techniques for prostate cancer patients treated with intensity-modulated radiotherapy (IMRT).

METHODS AND MATERIALS

During an 8-week course of radiotherapy, 10 patients underwent computed tomography (CT) scans 3 times per week (243 total) before daily treatment with a CT-linear accelerator. Treatment delivery was simulated by realigning a fixed-margin treatment plan on each CT scan and calculating doses. The alignment methods were those based on the following: skin marks, bony registration, ultrasonography (US), and in-room CT. For the last two methods, prostate was the alignment target. The dosimetric effects of these alignment methods on the prostate, seminal vesicles, rectum, and bladder were compared. The average daily minimum dose to 0.1 cm3 was used as the metric for target coverage.

RESULTS

Skin and bone alignments provided acceptable prostate coverage for only 70% of patients, US alignment for 90%, and CT alignment for 100%. CT-based alignment of the prostate provided seminal vesicle (SV) coverage of > or = 69 Gy for all patients; US and bone alignments provided SV coverage of > or = 60 Gy. This SV coverage may be acceptable for early-stage cancer (equivalent SV dose = 55.8 Gy at 1.8 Gy per fraction), but unacceptable for late-stage cancer (SV dose = 75.6 Gy). At 75.6 Gy, the acceptable rate for SV coverage was 40% for skin and bone alignments, 70% for US, and 80% for CT.

CONCLUSIONS

Direct target alignment methods (US and CT) provided better target coverage. CT-guided alignment provided the best and most consistent dosimetric coverage. A larger planning target volume margin is needed for SV coverage when the alignment target is the prostate.

Dynamic intensity-modulated non-coplanar arc radiotherapy (INCA) for head and neck cancer

BACKGROUND AND PURPOSE

To define the potential advantages of intensity-modulated radiotherapy (IMRT) applied using a non-coplanar dynamic arc technique for the treatment of head and neck cancer.

MATERIALS AND METHODS

External beam radiotherapy (EBRT) was planned in ten patients with head and neck cancer using coplanar IMRT and non-coplanar arc techniques, termed intensity modulated non-coplanar arc EBRT (INCA). Planning target volumes (PTV1) of first order covered the gross tumor volume and surrounding clinical target volume treated with 68-70 Gy, whereas PTV2 covered the elective lymph nodes with 54-55 Gy using a simultaneous internal boost. Treatment plan comparison between IMRT and INCA was carried out using dose-volume histogram and "equivalent uniform dose" (EUD).

RESULTS

INCA resulted in better dose coverage and homogeneity of the PTV1, PTV2, and reduced dose delivered to most of the organs at risk (OAR). For the parotid glands, a reduction of the mean dose of 2.9 (+/- 2.0) Gy was observed (p = 0.002), the mean dose to the larynx was reduced by 6.9 (+/- 2.9) Gy (p = 0.003), the oral mucosa by 2.4 (+/- 1.1) Gy (p < 0.001), and the maximal dose to the spinal cord by 3.2 (+/- 1.7) Gy (p = 0.004). The mean dose to the brain was increased by 3.0 (+/- 1.4) Gy (p = 0.002) and the mean lung dose increased by 0.2 (+/- 0.4) Gy (p = 0.87). The EUD suggested better avoidance of the OAR, except for the lung, and better coverage and dose uniformity were achieved with INCA compared to IMRT.

CONCLUSION

Dose delivery accuracy with IMRT using a non-coplanar dynamic arc beam geometry potentially improves treatment of head and neck cancer.

Assessing the quality of conformal treatment planning: a new tool for quantitative comparison

We develop a novel radiotherapy plan comparison index, critical organ scoring index (COSI), which is a measure of both target coverage and critical organ overdose. COSI is defined as COSI=1-(V(OAR)>tol/TC), where V(OAR)>tol is the fraction of volume of organ at risk receiving more than tolerance dose, and TC is the target coverage, VT,PI/VT, where VT,PI is the target volume receiving at a least prescription dose and VT is the total target volume. COSI approaches unity when the critical structure is completely spared and the target coverage is unity. We propose a two-dimensional, graphical representation of COSI versus conformity index (CI), where CI is a measure of a normal tissue overdose. We show that this 2D representation is a reliable, visual quantitative tool for evaluating competing plans. We generate COSI-CI plots for three sites: head and neck, cavernous sinus, and pancreas, and evaluate competing non-coplanar 3D and IMRT treatment plans. For all three sites this novel 2D representation assisted the physician in choosing the optimal plan, both in terms of target coverage and in terms of critical organ sparing. We verified each choice by analysing individual DVHs and isodose lines. Comparing our results to the widely used conformation number, we found that in all cases where there were discrepancies in the choice of the best treatment plan, the COSI-CI choice was considered the correct one, in several cases indicating that a non-coplanar 3D plan was superior to the IMRT plans. The choice of plan was quick, simple and accurate using the new graphical representation.

Three-Dimensional Conformal External Beam Radiotherapy (3D-CRT) for Accelerated Partial Breast Irradiation (APBI)

OBJECTIVE

This study is an evaluation of the biologic equivalence of the dose prescriptions for brachytherapy and 3-dimensional conformal external beam radiotherapy (3D-CRT) accelerated partial breast irradiation (APBI), using actual patient dose matrix data, and is based on the concept of equivalent uniform biologically effective dose (EUBED). This formalism allows a nonuniform dose distribution to be reduced to an equivalent uniform dose, while also accounting for fraction size.

MATERIALS AND METHODS

Five computed tomography scans were selected from a group of patients treated with multicatheter interstitial APBI. Dose matrices for the brachytherapy plans were computed and analyzed with in-house software. For each patient, the EUBED for the brachytherapy dose matrix was generated based on calculations performed at the voxel-level. These EUBED values were then used to calculate the biologically equivalent fraction size for 3D-CRT (eud).

RESULTS

The mean equivalent fraction size (eudmean) and maximum equivalent fraction size (eudmax) were calculated for each patient using 100 different values of the alpha/beta ratio. The eudmean ranged from 3.67 to 3.69 Gy, while the eudmax ranged from 3.79 to 3.82 Gy. For all values of the alpha/beta ratio, the maximum fraction size calculated to deliver a biologically equivalent dose with 3D-CRT was 3.82 Gy, with an equivalent total prescription dose of 38.2 Gy.

CONCLUSION

Utilizing a wide range of established radiobiological parameters, this study suggests that the maximum fraction size needed to deliver a biologically equivalent dose using 3D-CRT is 3.82 Gy, supporting the continued use of 3.85Gy BID in the current national cooperative trial.

Comparison of non-coplanar and coplanar irradiation techniques to treat cancer of the pancreas

We compared two different techniques of pancreatic irradiation using measures associated with normal tissue complications. Seven consecutive patients with pancreatic cancer were planned for both coplanar and non-coplanar (NCP) external beam radiation treatments, using the same defined anatomical volumes for each patient, in each case. Each pair of plans was then compared using a range of objective criteria. Individual normal tissues were assessed against traditional tolerance limits. Selected dose-points, normal tissue complication probability (NTCP) and equivalent uniform doses (EUD) were also compared, as were indices combining information from individual tissues - total NTCP and total weighted EUD. All individual normal tissues doses were within established tolerance limits. For NCP relative to coplanar planning, NTCP and EUD were lower for all individual tissues in four cases and one case, respectively, i.e. in most cases a benefit to one tissue was offset by detriment to others. Summary measures demonstrated overall benefits for NCP techniques, with the total NTCP in six patients, and with the total weighted EUD in all patients. NCP techniques show potentially useful benefits. We present a new objective measure, the total weighted EUD, which may be particularly useful comparing plans where there are multiple critical tissues.

Assessing the Difference between Planned and Delivered Intensity-modulated Radiotherapy Dose Distributions based on Radiobiological Measures

AIMS

Because of the highly conformal distributions that can be obtained with intensity-modulated radiotherapy (IMRT), any discrepancy between the intended and delivered distributions would probably affect the clinical outcome. Consequently, there is a need for a measure that would quantify those differences in terms of a change in the expected clinical outcome.

MATERIALS AND METHODS

To evaluate such a measure, cancer of the cervix was used, where the bladder and rectum are proximal and partially overlapping with the internal target volume. A solid phantom simulating the pelvic anatomy was fabricated and a treatment plan was developed to deliver the prescribed dose to the phantom. The phantom was then irradiated with films positioned in several transverse planes. The racetrack microtron at 50 MV was used in the treatment planning and delivery processes. The dose distribution delivered was analysed based on the film measurements and compared against the treatment plan. The differences in the measurements were evaluated using both physical and biological criteria. Whereas the physical comparison of dose distributions can assess the geometric accuracy of delivery, it does not reflect the clinical effect of any measured dose discrepancies.

RESULTS

It is shown how small inaccuracies in delivered dose can affect the treatment outcome in terms of complication-free tumour cure.

CONCLUSIONS

With highly conformal IMRT, the accuracy of the patient set-up and treatment delivery are critical for the success of the treatment. A method is proposed to evaluate the precision of the delivered plan based on changes in complication and control rates as they relate to uncertainties in dose delivery.

A theoretical approach to the problem of dose-volume constraint estimation and their impact on the dose-volume histogram selection

This paper outlines a theoretical approach to the problem of estimating and choosing dose-volume constraints. Following this approach, a method of choosing dose-volume constraints based on biological criteria is proposed. This method is called "reverse normal tissue complication probability (NTCP) mapping into dose-volume space" and may be used as a general guidance to the problem of dose-volume constraint estimation. Dose-volume histograms (DVHs) are randomly simulated, and those resulting in clinically acceptable levels of complication, such as NTCP of 5 +/- 0.5%, are selected and averaged producing a mean DVH that is proven to result in the same level of NTCP. The points from the averaged DVH are proposed to serve as physical dose-volume constraints. The population-based critical volume and Lyman NTCP models with parameter sets taken from literature sources were used for the NTCP estimation. The impact of the prescribed value of the maximum dose to the organ, D(max), on the averaged DVH and the dose-volume constraint points is investigated. Constraint points for 16 organs are calculated. The impact of the number of constraints to be fulfilled based on the likelihood that a DVH satisfying them will result in an acceptable NTCP is also investigated. It is theoretically proven that the radiation treatment optimization based on physical objective functions can sufficiently well restrict the dose to the organs at risk, resulting in sufficiently low NTCP values through the employment of several appropriate dose-volume constraints. At the same time, the pure physical approach to optimization is self-restrictive due to the preassignment of acceptable NTCP levels thus excluding possible better solutions to the problem.

Dosimetric characteristics of a newly designed grid block for megavoltage photon radiation and its therapeutic advantage using a linear quadratic model

Grid radiation therapy with megavoltage x-ray beam has been proven to be an effective technique for management of large, bulky malignant tumors. The clinical advantage of GRID therapy, combined with conventional radiation therapy, has been demonstrated using a prototype GRID block [Mohiuddin, Curtis, Grizos, and Komarnicky, Cancer 66, 114-118 (1990)]. Recently, a new GRID block design with improved dosimetric properties has become commercially available from Radiation Product Design, Inc. (Albertive, MN). This GRID collimator consists of an array of focused apertures in a cerrobend block arranged in a hexagonal pattern having a circular cross-section with a diameter and center-to-center spacing of 14.3 and 21.1 mm, respectively, in the plane of isocenter. In this project, dosimetric characteristics of the newly redesigned GRID block have been investigated for a Varian 21EX linear accelerator (Varian Associates, Palo Alto, CA). These determinations were performed using radiographic films, thermoluminescent dosimeters in Solid Water phantom materials, and an ionization chamber in water. The output factor, percentage depth dose, beam profiles, and isodose distributions of the GRID radiation as a function of field size and beam energy have been measured using both 6 and 18 MV x-ray beams. In addition, the therapeutic advantage obtained from this treatment modality with the new GRID block design for a high, single fraction of dose has been calculated using the linear quadratic model with alpha/beta ratios for typical tumor and normal cells. These biological characteristics of the new GRID block design will also be presented.

Radiobiological modelling of dose-gradient effects in low dose rate, high dose rate and pulsed brachytherapy

This paper presents a generalization of a previously published methodology which quantified the radiobiological consequences of dose-gradient effects in brachytherapy applications. The methodology uses the linear-quadratic (LQ) formulation to identify an equivalent biologically effective dose (BED(eq)) which, if applied uniformly to a specified tissue volume, would produce the same net cell survival as that achieved by a given non-uniform brachytherapy application. Multiplying factors (MFs), which enable the equivalent BED for an enclosed volume to be estimated from the BED calculated at the dose reference surface, have been calculated and tabulated for both spherical and cylindrical geometries. The main types of brachytherapy (high dose rate (HDR), low dose rate (LDR) and pulsed (PB)) have been examined for a range of radiobiological parameters/dimensions. Equivalent BEDs are consistently higher than the BEDs calculated at the reference surface by an amount which depends on the treatment prescription (magnitude of the prescribed dose) at the reference point. MFs are closely related to the numerical BED values, irrespective of how the original BED was attained (e.g., via HDR, LDR or PB). Thus, an average MF can be used for a given prescribed BED as it will be largely independent of the assumed radiobiological parameters (radiosensitivity and alpha/beta) and standardized look-up tables may be applicable to all types of brachytherapy treatment. This analysis opens the way to more systematic approaches for correlating physical and biological effects in several types of brachytherapy and for the improved quantitative assessment and ranking of clinical treatments which involve a brachytherapy component.

Halftime for repair of sublethal damage in normal bladder and rectum: an analysis of clinical data from cervix brachytherapy

The halftime for repair of sub-lethal damage is an important radiobiological parameter in analysing radiation responses and in designing new treatments involving different dose rates. This work is to resolve an inconsistency existing in the repair halftime for the bladder and rectum, two of the most dose limiting critical structures for pelvic irradiation. Both long (1.5-2 h) and short (0.3-1 h) repair halftimes have been reported previously. In this work, by reconciling clinical data from cervical brachytherapy of different dose rates and by introducing a sparing factor to consider the dose sparing occurring for critical structures, we have estimated that the most likely value of the repair halftime for bladder and rectum is short, 0.2-0.4 h if assuming alpha/beta = 2-4 Gy. The present analysis does not support the long repair halftimes reported previously for the bladder and rectum and for other normal structures.

Dosimetric and radiobiological impact of dose fractionation on respiratory motion induced IMRT delivery errors: a volumetric dose measurement study

Respiratory motion can introduce substantial dose errors during IMRT delivery. These errors are difficult to predict because of the nonsynchronous interplay between radiation beams and tissues. The present study investigates the impact of dose fractionation on respiratory motion induced dosimetric errors during IMRT delivery and their radiobiological implications by using measured 3D dose. We focused on IMRT delivery with dynamic multileaf collimation (DMLC-IMRT). IMRT plans using several beam arrangements were optimized for and delivered to a polystyrene phantom containing a simulated target and critical organs. The phantom was set in linear sinusoidal motion at a frequency of 15 cycles/min (0.25 Hz). The amplitude of the motion was +/- 0.75 cm in the longitudinal direction and +/- 0.25 cm in the lateral direction. Absolute doses were measured with a 0.125 cc ionization chamber while dose distributions were measured with transverse films spaced 6 mm apart. Measurements were performed for varying number of fractions with motion, with respiratory-gated motion, and without motion. A tumor control probability (TCP) model for an inhomogeneously irradiated tumor was used to calculate and compare TCPs for the measurements and the treatment plans. Equivalent uniform doses (EUD) were also computed. For individual fields, point measurements using an ionization chamber showed substantial dose deviations (-11.7% to 47.8%) for the moving phantom as compared to the stationary phantom. However, much smaller deviations (-1.7% to 3.5%) were observed for the composite dose of all fields. The dose distributions and DVHs of stationary and gated deliveries were in good agreement with those of treatment plans, while those of the nongated moving phantom showed substantial differences. Compared to the stationary phantom, the largest differences observed for the minimum and maximum target doses were -18.8% and +19.7%, respectively. Due to their random nature, these dose errors tended to average out over fractionated treatments. The results of five-fraction measurements showed significantly improved agreement between the moving and stationary phantom. The changes in TCP were less than 4.3% for a single fraction, and less than 2.3% for two or more fractions. Variation of average EUD per fraction was small (< 3.1 cGy for a fraction size of 200 cGy), even when the DVHs were noticeably different from that of the stationary tumor. In conclusion, IMRT treatment of sites affected by respiratory motion can introduce significant dose errors in individual field doses; however, these errors tend to cancel out between fields and average out over dose fractionation. 3D dose distributions, DVHs, TCPs, and EUDs for stationary and moving cases showed good agreement after two or more fractions, suggesting that tumors affected by respiration motion may be treated using IMRT without significant dosimetric and biological consequences.

IMRT: a review and preview

The very first cornerstone paper on intensity-modulated radiation therapy (IMRT) was published in Physics in Medicine and Biology, and many seminal IMRT works have since appeared in this journal. Today IMRT is a widely used clinical treatment modality in many countries. This contribution to the 50th anniversary issue reviews the physical, mathematical, and technological milestones that have facilitated the clinical implementation and success of IMRT. In particular, the basic concepts and developments of both IMRT treatment planning ('inverse planning') and the delivery of cone-beam IMRT with a multileaf collimator from a fixed number of static beam directions are discussed. An outlook into the future of IMRT concludes the paper.

The IMRT information process—mastering the degrees of freedom in external beam therapy

The techniques and procedures for intensity-modulated radiation therapy (IMRT) are reviewed in the context of the information process central to treatment planning and delivery of IMRT. A presentation is given of the evolution of the information based radiotherapy workflow and dose delivery techniques, as well as the volume and planning concepts for relating the dose information to image based patient representations. The formulation of the dose shaping process as an optimization problem is described. The different steps in the calculation flow for determination of machine parameters for dose delivery are described starting from the formulation of optimization objectives over dose calculation to optimization procedures. Finally, the main elements of the quality assurance procedure necessary for implementing IMRT clinically are reviewed.

Development of radiobiology for oncology-a personal view

When I came into radiotherapy in 1950, I was puzzled that some patients were treated to 3000 rads (cGy) in 3 weeks but others received 4000 in 5 or 6000 in 6 weeks. When I asked why, there were no convincing answers given, except 'this is what we usually do'. It wasn't until I went to a course on 'Radiobiology for Radiotherapy' in Cambridge that I learnt about the basic theories of Douglas Lea and the very considerable history of research into radiobiology and clinical radiotherapy. And there were still some questions outstanding, such as the relative importance of intracellular repair between 'daily' fractions, whether a 2 day gap each week was a good or a bad idea, and the role of proliferation, if any, during irradiation. I thought that a few simple animal experiments might help to give answers! That led me to a continuing interest in these questions and answers, which has taken me more than 50 years to pursue. This is the very personal story of what I saw happening in the subject, decade by decade. I was happy to experience all this together with scientists in many other countries, and our own, along the way.

192Ir low dose rate brachytherapy for boosting locally advanced prostate cancers after external beam radiotherapy: A phase II trial

BACKGROUND AND PURPOSE

To evaluate on 201 locally advanced prostatic cancers prospectively treated in a phase II trial, the efficacy of a combination of external beam radiotherapy (39.6 Gy) and (192)Ir low dose rate brachytherapy (Bt) (40-45 Gy).

PATIENTS AND METHODS

Sixty-four patients were included in the intermediate prognosis group with only one of the following adverse factors (PSA > 10 ng/ml, Gleason score > or = 7 or clinical stage > or =T2b) and 137 in the unfavourable group when at least two of these factors were present.

RESULTS

The actuarial 4 years biochemical no evidence of disease is 82.8% for the entire population. It is, respectively, 97 and 76% in the intermediate and unfavourable prognosis groups (P < 0.0001). Grade > or =3 late urinary complications occurred in 13 patients (6.5%). Eight patients (4%) presented late grade 2 rectal complications but no grades 3-5 was observed.

CONCLUSIONS

Even if an alpha/beta of 1.5-3 Gy theoretically favours the use of a high dose rate mode of irradiation, the early results presented here are as good as those reported for similar groups of patients with high dose rate treatments. Late toxicity is identical but our urinary toxicity is within the less favourable and rectal toxicity within the most favourable results. We can postulate that while inducing very high hyperdosage regions (V150) mainly focused on the peripheral zone, most of the Bt techniques consist of a more ablative treatment. Many of the radiobiological studies on Bt did not in fact take into account the heterogeneity of irradiation inside the CTV. This study highlights the need to explore pulsed dose rate therapies, permanent implant and new available radioisotopes such as (169)Ytterbium that will offer the safety of low and lower dose rates. The actual late toxicity of the different Bt techniques is not yet inexistent indeed.

The dosimetric effect of inhomogeneity correction in dynamic conformal arc stereotactic body radiation therapy for lung tumors

For patients treated with lung stereotactic body radiation therapy (SBRT) using dynamic conformal arcs, the influence of inhomogeneity correction (IC) on normal tissue and tumor dosimetry was studied. For the same numbers of monitor units, the planning target volume equivalent uniform doses calculated without path-length IC were lower than those calculated with IC (mean difference 18%, range 1-34%; p < 0.0001). Normal lung dose differences were of the same magnitude in opposite direction. In reports of SBRT, it will be helpful to maintain clear communication about the type of IC used to avoid future uncertainties about true normal tissue tolerance and tumor dose-response relationships.

Modeling radiation pneumonitis risk with clinical, dosimetric, and spatial parameters

PURPOSE

To determine the clinical, dosimetric, and spatial parameters that correlate with radiation pneumonitis.

METHODS AND MATERIALS

Patients treated with high-dose radiation for non-small-cell lung cancer with three-dimensional treatment planning were reviewed for clinical information and radiation pneumonitis (RP) events. Three-dimensional treatment plans for 219 eligible patients were recovered. Treatment plan information, including parameters defining tumor position and dose-volume parameters, was extracted from non-heterogeneity-corrected dose distributions. Correlation to RP events was assessed by Spearman's rank correlation coefficient (R). Mathematical models were generated that correlate with RP.

RESULTS

Of 219 patients, 52 required treatment for RP (median interval, 142 days). Tumor location was the most highly correlated parameter on univariate analysis (R = 0.24). Multiple dose-volume parameters were correlated with RP. Models most frequently selected by bootstrap resampling included tumor position, maximum dose, and D35 (minimum dose to the 35% volume receiving the highest doses) (R = 0.28). The most frequently selected two- or three-parameter models outperformed commonly used metrics, including V20 (fractional volume of normal lung receiving >20 Gy) and mean lung dose (R = 0.18).

CONCLUSIONS

Inferior tumor position was highly correlated with pneumonitis events within our population. Models that account for inferior tumor position and dosimetric information, including both high- and low-dose regions (D(35), International Commission on Radiation Units and Measurements maximum dose), risk-stratify patients more accurately than any single dosimetric or clinical parameter.

Estimation of the delivered patient dose in lung IMRT treatment based on deformable registration of 4D-CT data and Monte Carlo simulations

The purpose of this study is to accurately estimate the difference between the planned and the delivered dose due to respiratory motion and free breathing helical CT artefacts for lung IMRT treatments, and to estimate the impact of this difference on clinical outcome. Six patients with representative tumour motion, size and position were selected for this retrospective study. For each patient, we had acquired both a free breathing helical CT and a ten-phase 4D-CT scan. A commercial treatment planning system was used to create four IMRT plans for each patient. The first two plans were based on the GTV as contoured on the free breathing helical CT set, with a GTV to PTV expansion of 1.5 cm and 2.0 cm, respectively. The third plan was based on the ITV, a composite volume formed by the union of the CTV volumes contoured on free breathing helical CT, end-of-inhale (EOI) and end-of-exhale (EOE) 4D-CT. The fourth plan was based on GTV contoured on the EOE 4D-CT. The prescribed dose was 60 Gy for all four plans. Fluence maps and beam setup parameters of the IMRT plans were used by the Monte Carlo dose calculation engine MCSIM for absolute dose calculation on both the free breathing CT and 4D-CT data. CT deformable registration between the breathing phases was performed to estimate the motion trajectory for both the tumour and healthy tissue. Then, a composite dose distribution over the whole breathing cycle was calculated as a final estimate of the delivered dose. EUD values were computed on the basis of the composite dose for all four plans. For the patient with the largest motion effect, the difference in the EUD of CTV between the planed and the delivered doses was 33, 11, 1 and 0 Gy for the first, second, third and fourth plan, respectively. The number of breathing phases required for accurate dose prediction was also investigated. With the advent of 4D-CT, deformable registration and Monte Carlo simulations, it is feasible to perform an accurate calculation of the delivered dose, and compare our delivered dose with doses estimated using prior techniques.

Efficient on-line setup correction strategies using plan-intent functions

With the introduction of image-guided radiation therapy (IGRT) delivery systems on-line set-up correction strategies have gained in popularity. Usually, the correction workload of these strategies is high compared to off-line strategies as daily setup corrections have to be performed based on a predefined action level. In this work, it is proposed that on-line strategies must not only be judged in terms of workload but also in terms of efficacy. While workload can be easily predicted for such strategies, the efficacy must ultimately reflect the efficiency with which the original treatment plan intent is met. The purpose of this work is to investigate the tradeoff between workload and efficacy of three different on-line set-up correction strategies: The common fixed action level strategy and two novel on-line setup correction strategies, i.e., a dose-volume histogram (DVH) constraint and an equivalent uniform dose (EUD) score strategy that aim directly for better compliance with original treatment plan intent. All strategies were reformulated in terms of a score function that reflected treatment plan intent. A retrospective study was conducted on 5 prostate patients (7-field conformal, 79.8 Gy, 42 fractions). PTV margins were 10 mm except in the posterior direction (7 mm). The original treatment plan intent for these patients was defined using a set of DVH constraints. The results show that the on-line setup correction strategy based on a fixed action level of 3 mm resulted in a considerable correction workload. For larger action levels, a dose benefit (in terms of EUD) in the rectum and bladder was observed for all patients which is clinically "fortuitous" but difficult to take advantage of. In contrast, the application of the two novel strategies generally resulted in a controlled decrease of the dose to the rectum and the bladder with a smaller workload. It is concluded that using information about target anatomy and the planned dose distribution allows the design of specific correction strategies that are better tailored to the individual patient and that comply effectively with initial treatment plan intent.

A unified approach for inversion problems in intensity-modulated radiation therapy

We propose and study a unified model for handling dose constraints (physical dose, equivalent uniform dose (EUD), etc) and radiation source constraints in a single mathematical framework based on the split feasibility problem. The model does not impose on the constraints an exogenous objective (merit) function. The optimization algorithm minimizes a weighted proximity function that measures the sum of the squares of the distances to the constraint sets. This guarantees convergence to a feasible solution point if the split feasibility problem is consistent (i.e., has a solution), or, otherwise, convergence to a solution that minimally violates the physical dose constraints and EUD constraints. We present computational results that demonstrate the validity of the model and the power of the proposed algorithmic scheme.

IMRT: Improvement in treatment planning efficiency using NTCP calculation independent of the dose-volume-histogram

The normal tissue complication probability (NTCP) is a predictor of radiobiological effect for organs at risk (OAR). The calculation of the NTCP is based on the dose-volume-histogram (DVH) which is generated by the treatment planning system after calculation of the 3D dose distribution. Including the NTCP in the objective function for intensity modulated radiation therapy (IMRT) plan optimization would make the planning more effective in reducing the postradiation effects. However, doing so would lengthen the total planning time. The purpose of this work is to establish a method for NTCP determination, independent of a DVH calculation, as a quality assurance check and also as a mean of improving the treatment planning efficiency. In the study, the CTs of ten randomly selected prostate patients were used. IMRT optimization was performed with a PINNACLE3 V 6.2b planning system, using planning target volume (PTV) with margins in the range of 2 to 10 mm. The DVH control points of the PTV and OAR were adapted from the prescriptions of Radiation Therapy Oncology Group protocol P-0126 for an escalated prescribed dose of 82 Gy. This paper presents a new model for the determination of the rectal NTCP (R(NTCP)). The method uses a special function, named GVN (from Gy, Volume, NTCP), which describes the R(NTCP) if 1 cm3 of the volume of intersection of the PTV and rectum (R(int)) is irradiated uniformly by a dose of 1 Gy. The function was "geometrically" normalized using a prostate-prostate ratio (PPR) of the patients' prostates. A correction of the R(NTCP) for different prescribed doses, ranging from 70 to 82 Gy, was employed in our model. The argument of the normalized function is the R(int), and parameters are the prescribed dose, prostate volume, PTV margin, and PPR. The R(NTCPs) of another group of patients were calculated by the new method and the resulting difference was < +/- 5% in comparison to the NTCP calculated by the PINNACLE3 software where Kutcher's dose-response model for NTCP calculation is adopted.

Effect of bladder filling on doses to prostate and organs at risk: a treatment planning study

In the present study, we aimed to evaluate effects of bladder filling on dose–volume distributions for bladder, rectum, planning target volume (PTV), and prostate in radiation therapy of prostate cancer. Patients (n=21) were scanned with a full bladder, and after 1 hour, having been allowed to void, with an empty bladder. Radiotherapy plans were generated using a four‐field box technique and dose of 70 Gy in 35 fractions. First, plans obtained for full‐ and empty‐bladder scans were compared. Second, situations in which a patient was planned on full bladder but was treated on empty bladder, and vice versa, were simulated, assuming that patients were aligned to external tattoos. Doses to the prostate [equivalent uniform dose (EUD)], bladder and rectum [effective dose (Deff)], and normal tissue complication probability (NTCP) were compared. Dose to the small bowel was examined. Mean bladder volume was 354.3 cm³ when full and 118.2 cm³ when empty. Median prostate EUD was 70 Gy for plans based on full‐ and empty‐bladder scans alike. The median rectal Deff was 55.6 Gy for full‐bladder anatomy and 56.8 Gy for empty‐bladder anatomy, and the corresponding bladder Deff was 29.0 Gy and 49.3 Gy respectively. In 1 patient, part of the small bowel (7.5 cm³) received more than 50 Gy with full‐bladder anatomy, and in 6 patients, part (2.5 cm3−30 cm3) received more than 50 Gy with empty‐bladder anatomy. Bladder filling had no significant impact on prostate EUD or rectal Deff. A minimal volume of the small bowel received more than 50 Gy in both groups, which is below dose tolerance. The bladder Deff was higher with empty‐bladder anatomy; however, the predicted complication rates were clinically insignificant. When the multileaf collimator pattern was applied in reverse, substantial underdosing of the planning target volume (PTV) was observed, particularly for patients with prostate shifts in excess of 0.5 cm in any one direction. However, the prostate shifts showed no correlation with bladder filling, and therefore the PTV underdosing also cannot be related to bladder filling. For some patients, bladder dose–volume constraints were not fulfilled in the worst‐case scenario—that is, when a patient planned with full bladder consistently arrived for treatment with an empty bladder.

PACS numbers: 87.53.‐j, 87.53.Kn, 87.53.Tf

More optimal dose distributions for moving lung tumours: A planning study

BACKGROUND AND PURPOSE

Target volumes for moving lung tumours encompass the full range of respiratory motion, increasing the risk of lung complications. Intensity modulated radiotherapy (IMRT) allows for more precise dose distributions. Distributions corresponding to the probability density function (PDF) of tumour motion may better spare lung yet deliver adequate target dose. The planning study purpose is to compare and evaluate different dose distributions on a moving lung tumour: (A) conformal RT (CRT) encompassing the full range of tumour motion, (B) CRT encompassing the modal tumour position only, and (C) an IMRT technique where the dose delivered corresponds to the tumour PDF.

MATERIALS AND METHODS

A 5 cm diameter spherical target within a rectangular lung equivalent phantom was treated using a parallel-opposed pair technique with a 1.5 cm margin around the tumour (in the beam's eye view). Asymmetrical sinusoidal (superior-inferior) target movement (peak-trough = 3 cm) was simulated for different dose distributions (prescription dose = 60 Gy). Equivalent uniform dose (EUD) for the tumour and normal tissue complication probabilities (NTCPs) for radiation pneumonitis were evaluated.

RESULTS

The EUDs were 60.0, 48.5, and 57.9 Gy while the NTCPs were 5, 1, and 3% for cases A, B, and C, respectively (assuming survival fraction, SF(2)(Gy) = 0.5).

CONCLUSIONS

Since these results rely on unvalidated radiobiologic models, they must be interpreted cautiously. However, more optimized dose distributions for moving lung targets appear feasible and can reduce lung complications with only a negligible impact on the expected EUD and, thus, deserve further study.

Geometric and dosimetric evaluations of an online image-guidance strategy for 3D-CRT of prostate cancer

PURPOSE

To evaluate an online image-guidance strategy for conformal treatment of prostate cancer and to estimate margin-reduction benefits.

METHODS AND MATERIALS

Twenty-eight patients with at least 16 helical computed tomography scans were each used in this study. Two prostate soft-tissue registration methods, including sagittal rotation, were evaluated. Setup errors and rigid organ motion were corrected online; non-rigid and intrafraction motion were included in offline analysis. Various clinical target volume-planning target volume (CTV-PTV) margins were applied. Geometrical evaluations included analyses of isocenter shifts and rotations and overlap index. Dosimetric evaluations included minimum dose and equivalent uniform dose (EUD) for prostate and gEUD for rectum.

RESULTS

Average isocenter shift and rotation were (dX,dY,dZ,theta) = (0.0 +/- 0.7,-1.1 +/- 4.0,-0.1 +/- 2.5,0.7 degrees +/- 2.0 degrees ) mm. Prostate motion in anterior-posterior (AP) direction was significantly higher than superior-inferior and left-right (LR) directions. This observation was confirmed by isocenter shift in perspectives AP (1.8 +/- 1.8 mm) and RL (3.7 +/- 3.0 mm). Organ motion degrades target coverage and reduces doses to rectum. If 2% dose reduction on prostate D(99) is allowed for 90% patients, then minimum 3 mm margins are necessary with ideal image registration.

CONCLUSIONS

Significant margin reduction can be achieved through online image guidance. Certain margins are still required for nonrigid and intrafraction motion. To further reduce margin, a strategy that combines online geometric intervention and offline dose replanning is necessary.

An estimation of radiobiologic parameters from clinical outcomes for radiation treatment planning of brain tumor

PURPOSE

To estimate a plausible set of radiobiologic parameters such as alpha, alpha/beta values, from clinical outcomes for biologically based radiation treatment planning of brain tumors.

METHODS AND MATERIALS

Linear-quadratic (LQ) formalism and the concept of equivalent uniform dose were used to analyze a series of published clinical data for malignant gliomas involving different forms of radiation therapy.

RESULTS

A plausible set of LQ parameters was obtained for gliomas: alpha = 0.06 +/- 0.05 Gy(-1), alpha/beta = 10.0 +/- 15.1 Gy, the tumor cell doubling time T(d) = 50 +/- 30 days, with the repair half-time of 0.5 h. The present estimated biologic parameters can reasonably predict the effectiveness of most of the recently reported clinical results employing either single or combined radiation therapy modalities. Different LQ parameters between Grade 3 and Grade 4 astrocytomas were found, implying the radiosensitivity for different grade tumors may be different. Smaller alpha, beta from in vivo was observed, indicating lower radiosensitivity occurred in vivo as compared with in vitro.

CONCLUSIONS

A plausible set of radiobiologic parameters for gliomas was estimated based on clinical data. These parameters can reasonably predict most of the clinical results. They may be used to design new treatment fractionation schemes and to evaluate and optimize treatment plans.

Comparison of linac based fractionated stereotactic radiotherapy and tomotherapy treatment plans for skull-base tumors

BACKGROUND AND PURPOSE

To compare and evaluate helical tomotherapy and linac based fractionated stereotactic radiotherapy (FSRT) techniques in the treatment of skull-base tumors.

PATIENTS AND METHODS

Ten patients diagnosed with skull-base tumors, originally planned for optically guided FSRT to prescribed doses of 50.4-54 Gy were replanned for treatment with clinically deliverable helical tomotherapy. All original CT scans, MR-CT fusion defined target and normal structure contours, and PTV margins were used for helical tomotherapy planning. Linac based plans utilized one of the following FSRT planning techniques: non-coplanar or coplanar intensity modulated radiation therapy (IMRT), multiple non-coplanar conformal arcs, and non-coplanar conformal radiation therapy (CRT). These plans were used as the standard to which the subsequent tomotherapy plans were compared, using the following criteria: prescription isodose to target volume (PITV) ratios, an inhomogeneity index (II), equivalent uniform dose (EUD) for PTV volumes, mean normalized total doses (NTDmean) for critical structures, and size of 10, 20, and 30 Gy isodose volumes.

RESULTS

Use of both linac based FSRT techniques and helical tomotherapy generated highly conformal treatment plans. Tomotherapy plans, which are predominantly coplanar in nature, compared to non-coplanar linac based plans exhibited increased PITV ratios, variable change in II, similar EUD values, and generally comparable NTD(mean) values for organs at risk. When compared to non-coplanar field arrangements, deliverable (as opposed to idealized) tomotherapy plans also resulted in 13-540% increases in low dose isodose volumes. All criteria except for the II, which was generally improved with tomotherapy, were found to be similar when coplanar linac based plans were compared to helical tomotherapy plans.

CONCLUSIONS

Results show a distinct advantage in using non-coplanar beam arrangements for treatment of skull-base tumors. In the case where disease spreads far inferiorly, limiting the ability to use non-coplanar arrangements, helical tomotherapy can be used to generate a comparable treatment plan, with potentially superior homogeneity.

Monte Carlo–based dosimetry of head-and-neck patients treated with SIB-IMRT

PURPOSE

To evaluate the accuracy of previously reported superposition/convolution (SC) dosimetric results by comparing with Monte Carlo (MC) dose calculations for head-and-neck intensity-modulated radiation therapy (IMRT) patients treated with the simultaneous integrated boost technique.

METHODS AND MATERIALS

Thirty-one plans from 24 patients previously treated on a phase I/II head-and-neck squamous cell carcinoma simultaneous integrated boost IMRT protocol were used. Clinical dose distributions, computed with an SC algorithm, were recomputed using an EGS4-based MC algorithm. Phantom-based dosimetry quantified the fluence prediction accuracy of each algorithm. Dose-volume indices were used to compare patient dose distributions.

RESULTS AND DISCUSSION

The MC algorithm predicts flat-phantom measurements better than the SC algorithm. Average patient dose indices agreed within 2.5% of the local dose for targets; 5.0% for parotids; and 1.9% for cord and brainstem. However, only 1 of 31 plans agreed within 3% for all indices; 4 of 31 agreed within 5%. In terms of the prescription dose, 4 of 31 plans agreed within 3% for all indices, whereas 28 of 31 agreed within 5%.

CONCLUSIONS

Average SC-computed doses agreed with MC results in the patient geometry; however deviations >5% were common. The fluence modulation prediction is likely the major source of the dose discrepancy. The observed dose deviations can impact dose escalation protocols, because they would result in shifting patients to higher dose levels.