Specification of Dose Delivery in Radiation Therapy. Recommendations by the Nordic Association of Clinical Physics (NACP)

TP53 and the Ultimate Biological Optimization Steps of Curative Radiation Oncology

The new biological interaction cross-section-based repairable-homologically repairable (RHR) damage formulation for radiation-induced cellular inactivation, repair, misrepair, and apoptosis was applied to optimize radiation therapy. This new formulation implies renewed thinking about biologically optimized radiation therapy, suggesting that most TP53 intact normal tissues are low-dose hypersensitive (LDHS) and low-dose apoptotic (LDA). This generates a fractionation window in LDHS normal tissues, indicating that the maximum dose to organs at risk should be ≤2.3 Gy/Fr, preferably of low LET. This calls for biologically optimized treatments using a few high tumor dose-intensity-modulated light ion beams, thereby avoiding secondary cancer risks and generating a real tumor cure without a caspase-3-induced accelerated tumor cell repopulation. Light ions with the lowest possible LET in normal tissues and high LET only in the tumor imply the use of the lightest ions, from lithium to boron. The high microscopic heterogeneity in the tumor will cause local microscopic cold spots; thus, in the last week of curative ion therapy, when there are few remaining viable tumor clonogens randomly spread in the target volume, the patient should preferably receive the last 10 GyE via low LET, ensuring perfect tumor coverage, a high cure probability, and a reduced risk for adverse normal tissue reactions. Interestingly, such an approach would also ensure a steeper rise in tumor cure probability and a higher complication-free cure, as the few remaining clonogens are often fairly well oxygenated, eliminating a shallower tumor response due to inherent ion beam heterogeneity. With the improved fractionation proposal, these approaches may improve the complication-free cure probability by about 10-25% or even more.

Treatment planning and 4D robust evaluation strategy for proton therapy of lung tumors with large motion amplitude

Purpose

Intensity‐modulated proton therapy (IMPT) for lung tumors with a large tumor movement is challenging due to loss of robustness in the target coverage. Often an upper cut‐off at 5‐mm tumor movement is used for proton patient selection. In this study, we propose (1) a robust and easily implementable treatment planning strategy for lung tumors with a movement larger than 5 mm, and (2) a four‐dimensional computed tomography (4DCT) robust evaluation strategy for evaluating the dose distribution on the breathing phases.

Materials and methods

We created a treatment planning strategy based on the internal target volume (ITV) concept (aim 1). The ITV was created as a union of the clinical target volumes (CTVs) on the eight 4DCT phases. The ITV expanded by 2 mm was the target during robust optimization on the average CT (avgCT). The clinical plan acceptability was judged based on a robust evaluation, computing the voxel‐wise min and max (VWmin/max) doses over 28 error scenarios (range and setup errors) on the avgCT. The plans were created in RayStation (RaySearch Laboratories, Stockholm, Sweden) using a Monte Carlo dose engine, commissioned for our Mevion S250i Hyperscan system (Mevion Medical Systems, Littleton, MA, USA). We developed a new 4D robust evaluation approach (4DRobAvg; aim 2). The 28 scenario doses were computed on each individual 4DCT phase. For each scenario, the dose distributions on the individual phases were deformed to the reference phase and combined to a weighted sum, resulting in 28 weighted sum scenario dose distributions. From these 28 scenario doses, VWmin/max doses were computed. This new 4D robust evaluation was compared to two simpler 4D evaluation strategies: re‐computing the nominal plan on each individual 4DCT phase (4DNom) and computing the robust VWmin/max doses on each individual phase (4DRobInd). The treatment planning and dose evaluation strategies were evaluated for 16 lung cancer patients with tumor movement of 4–26 mm.

Results

The ratio of the ITV and CTV volumes increased linearly with the tumor amplitude, with an average ratio of 1.4. Despite large ITV volumes, a clinically acceptable plan fulfilling all target and organ at risk (OAR) constraints was feasible for all patients. The 4DNom and 4DRobInd evaluation strategies were found to under‐ or overestimate the dosimetric effect of the tumor movement, respectively. 4DRobInd showed target underdosage for five patients, not observed in the robust evaluation on the avgCT or in 4DRobAvg. The accuracy of dose deformation used in 4DRobAvg was quantified and found acceptable, with differences for the dose‐volume parameters below 1 Gy in most cases.

Conclusion

The proposed ITV‐based planning strategy on the avgCT was found to be a clinically feasible approach with adequate tumor coverage and no OAR overdosage even for large tumor movement. The new proposed 4D robust evaluation, 4DRobAvg, was shown to give an easily interpretable understanding of the effect of respiratory motion dose distribution, and to give an accurate estimate of the dose delivered in the different breathing phases.

A DNA Repair-Based Model of Cell Survival with Important Clinical Consequences

This work provides a description of a new interaction, cross-section-based model for radiation-induced cellular inactivation, sublethal damage, DNA repair and cell survival, with the ability to more accurately elucidate different radiation-response phenomena. The principal goal of this work is to describe the damage-induction cross sections, as well as repair and survival, as Poisson processes with two main types of damage: mild damage that can be rapidly handled by the most basic repair processes; and more complex damage requiring longer repair times and the high-fidelity homologous recombination (HR) repair process to ensure accuracy and safety in the survival. This work is unique in its use of Poisson statistics to quantify the main repairable cell compartments that are exposed to simple and more complex sublethal hits, the cross section of which determines what is homologically and non-homologically repairable. The new method is applied to central radiation damage and survival data, such as in vitro cellular repair and survival with key DNA repair genes knocked out, low-dose hypersensitivity (LDHS), change in survival over the cell cycle, and variation with linear energy transfer (LET) for densely ionizing ions, all results supporting our basic assumptions. Among the results, it was shown that less than 1% of the simple DSBs are lethal at approximately 2 Gy and below for sparsely ionizing radiations, but their δ-electron track ends of between 1.5 and 0.5 keV can deliver 0.5 MGy to a few hundred nm3 volumes, mainly due to multiple scatter detours and multiple secondary electrons. They can cause dual double-strand breaks (DSBs) on the periphery of nucleosomes that are the most common multiply damaged sites, with an average of 1-2 δ-electron track ends per cell nucleus at 2 Gy. LDHS is most likely due to the normal lack of fast, efficient repair of sublethal damage below approximately 0.5 Gy, and requires largely intact key DNA repair genes to achieve significant repair recovery at higher doses. The new repair model describes this phenomenon quite accurately. Cells with key non-homologous end joining (NHEJ) genes knocked-out, lose LDHS but provoke HR repair, and cells with HR genes knocked out may lose some LDHS, but provoke NHEJ repair. The DNA duplication during the S phase results in a direct doubling as well of the total and sublethal hit cross sections. For the lowest LET carbon ions, NHEJ is reduced to where it is almost eliminated at maximum relative biological effectiveness (RBE), while HR is induced more than by X rays, due to complex damage and misrepair of DSBs produced by numerous δ electrons. The use of a lower LET such as electrons or photons during the final week of radiation treatment may potentially maximize complication-free cure. Optimally-designed weekly fractionation schedules are proposed to maximize the DNA repair potential in normal tissues. Additionally, the optimal therapeutic ion species, LET, apoptosis and permanent growth arrest/senescence window is identified with helium, lithium and boron ions and LETs at approximately 15-55 eV/nm, to maximize these quantities in the tumor and minimize them in the normal tissues, resulting in a very high probability of complication-free cure.

Image-guided Radiotherapy to Manage Respiratory Motion: Lung and Liver

Organ motion as a result of respiratory and cardiac motion poses significant challenges for the accurate delivery of radiotherapy to both the thorax and the upper abdomen. Modern imaging techniques during radiotherapy simulation and delivery now permit better quantification of organ motion, which in turn reduces tumour and organ at risk position uncertainty. These imaging advances, coupled with respiratory correlated radiotherapy delivery techniques, have led to the development of a range of approaches to manage respiratory motion. This review summarises the key strategies of image-guided respiratory motion management with a focus on lung and liver radiotherapy.

Integral dose based inverse optimization objective function promises lower toxicity in head-and-neck

PURPOSE

The voxels in a CT data sets contain density information. Besides its use in dose calculation density has no other application in modern radiotherapy treatment planning. This work introduces the use of density information by integral dose minimization in radiotherapy treatment planning for head-and-neck squamous cell carcinoma (HNSCC).

MATERIALS AND METHODS

Eighteen HNSCC cases were studied. For each case two intensity modulated radiotherapy (IMRT) plans were created: one based on dose-volume (DV) optimization, and one based on integral dose minimization (Energy hereafter) inverse optimization. The target objective functions in both optimization schemes were specified in terms of minimum, maximum, and uniform doses, while the organs at risk (OAR) objectives were specified in terms of DV- and Energy-objectives respectively. Commonly used dosimetric measures were applied to assess the performance of Energy-based optimization. In addition, generalized equivalent uniform doses (gEUDs) were evaluated. Statistical analyses were performed to estimate the performance of this novel inverse optimization paradigm.

RESULTS

Energy-based inverse optimization resulted in lower OAR doses for equivalent target doses and isodose coverage. The statistical tests showed dose reduction to the OARs with Energy-based optimization ranging from ∼2% to ∼15%.

CONCLUSIONS

Integral dose minimization based inverse optimization for HNSCC promises lower doses to nearby OARs. For comparable therapeutic effect the incorporation of density information into the optimization cost function allows reduction in the normal tissue doses and possibly in the risk and the severity of treatment related toxicities.

Feasibility of Intensity-Modulated Radiation Therapy in the Treatment of Advanced Cervical Chordoma

Aims and Background

Postoperative radiation is often given in cases of cervical chordoma because of the high incidence of local recurrence. The tumor mass usually surrounds the spinal cord and infiltrates vertebral bone. A combined technique using protons or electrons to boost the initial photon fields is generally applied. We evaluated the use of dynamic intensity-modulated radiation therapy as an alternative technique for treating advanced cervical chordoma.

Methods and Study Design

A female patient with incomplete resection of a vertebral chordoma surrounding C2-C3 was irradiated with a total dose of 58 Gy (ICRU point) in 2 Gy daily fractions for 29 days between December 2001 and January 2002. Beam arrangement consisted of seven 6 MV non-opposed coplanar fields. Pretreatment quality assurance included checking of the absolute dose at reference points and 2D dose map analysis. Treatment was delivered with a 120-leaf collimator in sliding window mode. To verify the daily setup, portal images at 0° and 90° were compared with the simulation images before treatment delivery (manual matching) and after treatment delivery (automatic anatomy matching).

Results and Conclusions

The mean dose to the planning target volume (PTV) was 57.6 ± 2.1 Gy covering 95% of the PTV per 95% isodose. The minimum dose to the PTV (D99) was 53.6 Gy in the overlapping area between the PTV and the spinal cord planning organ at risk volume (PRV). The maximum dose to the spinal cord was 42.2 Gy and to the spinal cord PRV (8 mm margin) 53.7 Gy. The mean dose to the parotid glands was 37.4 Gy (homolateral gland) and 19.5 Gy (contralateral gland). Average deviation in setup was -1.1 ± 2.5 mm (anterior-posterior), 2.4 ±1.3 mm (latero-lateral), 0.7 ± 0.9 mm (craniocaudal) and -0.43 ± 1° (rotation).

Conclusions

In the treatment of chordomas surrounding the spinal cord, intensity-modulated radiotherapy can provide high dose homogeneity and PTV coverage. Frequent digital portal image-based setup control is able to reduce random positioning errors for head and neck cancer patients immobilized with conventional thermoplastic masks.

Dosimetric and Radiobiological Evaluation of Patient Setup Accuracy in Head-and-neck Radiotherapy Using Daily Computed Tomography-on-rails-based Corrections

Introduction:

This study evaluates treatment plans aiming at determining the expected impact of daily patient setup corrections on the delivered dose distribution and plan parameters in head-and-neck radiotherapy.

Materials and Methods:

In this study, 10 head-and-neck cancer patients are evaluated. For the evaluation of daily changes of the patient internal anatomy, image-guided radiation therapy based on computed tomography (CT)-on-rails was used. The daily-acquired CT-on-rails images were deformedly registered to the CT scan that was used during treatment planning. Two approaches were used during data analysis (“cascade” and “one-to-all”). The dosimetric and radiobiological differences of the dose distributions with and without patient setup correction were calculated. The evaluation is performed using dose–volume histograms; the biologically effective uniform dose () and the complication-free tumor control probability (P₊) were also calculated. The dose–response curves of each target and organ at risk (OAR), as well as the corresponding P₊ curves, were calculated.

Results:

The average difference for the “one-to-all” case is 0.6 ± 1.8 Gy and for the “cascade” case is 0.5 ± 1.8 Gy. The value of P₊ was lowest for the cascade case (in 80% of the patients).

Discussion:

Overall, the lowest P_I is observed in the one-to-all cases. Dosimetrically, CT-on-rails data are not worse or better than the planned data.

Conclusions:

The differences between the evaluated “one-to-all” and “cascade” dose distributions were small. Although the differences of those doses against the “planned” dose distributions were small for the majority of the patients, they were large for given patients at risk and OAR.

Automated inverse optimization facilitates lower doses to normal tissue in pancreatic stereotactic body radiotherapy

Purpose

Inverse planning is trial-and-error iterative process. This work introduces a fully automated inverse optimization approach, where the treatment plan is closely tailored to the unique patient anatomy. The auto-optimization is applied to pancreatic stereotactic body radiotherapy (SBRT).

Materials and methods

The automation is based on stepwise reduction of dose-volume histograms (DVHs). Five uniformly spaced points, from 1% to 70% of the organ at risk (OAR) volumes, are used. Doses to those DVH points are iteratively decreased through multiple optimization runs. With each optimization run the doses to the OARs are decreased, while the dose homogeneity over the target is increased. The iterative process is terminated when a pre-specified dose heterogeneity over the target is reached. Twelve pancreatic cases were retrospectively studied. Doses to the target, maximum doses to duodenum, bowel, stomach, and spinal cord were evaluated. In addition, mean doses to liver and kidneys were tallied. The auto-optimized plans were compared to the actual treatment plans, which are based on national protocols.

Results

The prescription dose to 95% of the planning target volume (PTV) is the same for the treatment and the auto-optimized plans. The average difference for maximum doses to duodenum, bowel, stomach, and spinal cord are -4.6 Gy, -1.8 Gy, -1.6 Gy, and -2.4 Gy respectively. The negative sign indicates lower doses with the auto-optimization. The average differences in the mean doses to liver and kidneys are -0.6 Gy, and -1.1 Gy to -1.5 Gy respectively.

Conclusions

Automated inverse optimization holds great potential for personalization and tailoring of radiotherapy to particular patient anatomies. It can be utilized for normal tissue sparing or for an isotoxic dose escalation.

Integral Dose-Based Inverse Optimization May Reduce Side Effects in Radiotherapy of Prostate Carcinoma

PURPOSE

The purpose of this work is to apply a novel inverse optimization approach, based on utilization of quantitative imaging information in the optimization function, to prostate carcinoma.

MATERIALS AND METHODS

This new inverse optimization algorithm relies upon quantitative information derived from computed tomography (CT) imaging studies. The Hounsfield numbers of the CT voxels are converted to physical density, which in turn is used to calculate voxel mass and the corresponding integral dose, by summation over the product of dose and mass in each dose voxel. This integral dose is used for plan optimization through its global minimization. The optimization results are compared to the optimization results derived from most commonly used dose-volume-based inverse optimization, where objective functions are formed as summation over all dose voxels of the squared differences between voxel doses and user specified doses. The data from 25 prostate plans were optimized with dose-volume histogram (DVH) and integral dose (energy) minimization objective functions. The results obtained with the energy- and DVH-based optimization schemes were studied through commonly used dosimetric indices (DIs). Statistical equivalence tests were further performed to establish population-based significance results.

RESULTS

Both DVH- and energy-based plans for each case were normalized so that 95% of the planning target volume receives the prescription dose. The average differences for the rectum and bladder DIs ranged from 1.6 to 25%, where the energy-based quantities were lower. For both femoral heads, the energy-based optimization-derived doses were lower on average by 32%. The statistical tests demonstrated that the significant differences in the tallied dose indices range from 2.7% to more than 50% for rectum, bladder, and femoral heads.

CONCLUSION

For majority of the clinically relevant dosimetric quantities, energy-based inverse optimization performs better than the standard of care DVH-based optimization in prostate carcinoma. The population averaged statistically significant differences range from ~3 to ~50%. Therefore, this newly proposed optimization approach, incorporating explicitly quantitative imaging information in the inverse optimization function, holds potential for further reduction of complication rates in prostate cancer.

Toward a better prescription method for external radiotherapy

Aim: The aim of the study was to compare several methods of dose prescription, the mean dose, the median dose, the effective dose and the generalized Equivalent Uniform Dose (gEUD).

Background: The dose distribution in the planning target volume is never fully homogenous. Depending on the dose prescription method for the same prescribed dose different biologically equivalent doses are delivered. The latest ICRU Report 83 proposes to prescribe the dose to the median dose in the PTV. Several other methods are also in common use. It is important to know what are differences of doses actually delivered depending on the dose prescription method.

Materials and methods: The study was performed for three groups of patients treated radically with external beams in Brzozow, over the 2012-2013 period. The groups were of patients with breast, lung and prostate cancer. There were 10 patients in each group. For each patient all metrics, i.e. the mean dose, the median dose, the effective dose and the generalized Equivalent Uniform Dose, were calculated. The influence of the dose homogeneity in the PTV on the results is also evaluated. The gEUD was used as a reference dose prescription method.

Results: For all patients, an almost perfect correlation between the median dose and the gEUD was obtained. Worse correlation was obtained between other metrics and the gEUD. The median dose is almost always a little higher than the gEUD, but the ratio of these two values never exceeded 1.013.

Conclusion: The median dose seems to be a good and simple method of dose prescription.

Impact of incorporating visual biofeedback in 4D MRI

Precise radiation therapy (RT) for abdominal lesions is complicated by respiratory motion and suboptimal soft tissue contrast in 4D CT. 4D MRI offers improved con-trast although long scan times and irregular breathing patterns can be limiting. To address this, visual biofeedback (VBF) was introduced into 4D MRI. Ten volunteers were consented to an IRB-approved protocol. Prospective respiratory-triggered, T2-weighted, coronal 4D MRIs were acquired on an open 1.0T MR-SIM. VBF was integrated using an MR-compatible interactive breath-hold control system. Subjects visually monitored their breathing patterns to stay within predetermined tolerances. 4D MRIs were acquired with and without VBF for 2- and 8-phase acquisitions. Normalized respiratory waveforms were evaluated for scan time, duty cycle (programmed/acquisition time), breathing period, and breathing regularity (end-inhale coefficient of variation, EI-COV). Three reviewers performed image quality assessment to compare artifacts with and without VBF. Respiration-induced liver motion was calculated via centroid difference analysis of end-exhale (EE) and EI liver contours. Incorporating VBF reduced 2-phase acquisition time (4.7 ± 1.0 and 5.4 ± 1.5 min with and without VBF, respectively) while reducing EI-COV by 43.8% ± 16.6%. For 8-phase acquisitions, VBF reduced acquisition time by 1.9 ± 1.6 min and EI-COVs by 38.8% ± 25.7% despite breathing rate remaining similar (11.1 ± 3.8 breaths/min with vs. 10.5 ± 2.9 without). Using VBF yielded higher duty cycles than unguided free breathing (34.4% ± 5.8% vs. 28.1% ± 6.6%, respectively). Image grading showed that out of 40 paired evaluations, 20 cases had equivalent and 17 had improved image quality scores with VBF, particularly for mid-exhale and EI. Increased liver excursion was observed with VBF, where superior-inferior, anterior-posterior, and left-right EE-EI displacements were 14.1± 5.8, 4.9 ± 2.1, and 1.5 ± 1.0 mm, respectively, with VBF compared to 11.9 ± 4.5, 3.7 ± 2.1, and 1.2 ± 1.4 mm without. Incorporating VBF into 4D MRI substantially reduced acquisition time, breathing irregularity, and image artifacts. However, differences in excursion were observed, thus implementation will be required throughout the RT workflow.

Relationships between various indices of doses distribution homogeneity

AIM

In this study we compared three different methods of evaluation of dose distribution.

BACKGROUND

The aim of treatment planning is to prepare the treatment plan which the criteria are defined according to the international recommendations.

MATERIALS AND METHODS

For three groups of patients, for lung, breast and prostate, treated radically in Brzozow with external beams the treatment plans were prepared. For each patient the metrics of dose distribution in the PTV defined according to the ICRU Reports 50, 83 and according to the Nordic Association of Clinical were calculated. Also Homogeneity Index defined by Yoon was used in this work. Additionally for similar group of patients treated in Warsaw the same calculations were performed. Correlations between the standard deviations and: (1) the differences between the maximum and minimum doses, and (2) the differences between near maximum and near minimum doses normalized to median dose and (3) to prescribed dose were calculated.

RESULTS

There was a very strong correlation between the standard deviation and the difference between the near-maximum and near-minimum doses for all locations regardless the prescription. Also good correlation was observed for the standard deviation and the difference between the maximum and minimum doses for patients treated in Brzozow.

CONCLUSIONS

The standard deviation may be estimated by the Homogeneity Index, however the relationship should be established for each location and each center separately.

Comparing conformal, arc radiotherapy and helical tomotherapy in craniospinal irradiation planning

Currently, radiotherapy treatment plan acceptance is based primarily on dosimetric performance measures. However, use of radiobiological analysis to assess benefit in terms of tumor control and harm in terms of injury to normal tissues can be advantageous. For pediatric craniospinal axis irradiation (CSI) patients, in particular, knowing the technique that will optimize the probabilities of benefit versus injury can lead to better long-term outcomes. Twenty-four CSI pediatric patients (median age 10) were retrospectively planned with three techniques: three-dimensional conformal radiation therapy (3D CRT), volumetric-modulated arc therapy (VMAT), and helical tomotherapy (HT). VMAT plans consisted of one superior and one inferior full arc, and tomotherapy plans were created using a 5.02cm field width and helical pitch of 0.287. Each plan was normalized to 95% of target volume (whole brain and spinal cord) receiving prescription dose 23.4Gy in 13 fractions. Using an in-house MATLAB code and DVH data from each plan, the three techniques were evaluated based on biologically effective uniform dose (D=), the complication-free tumor control probability (P+), and the width of the therapeutically beneficial range. Overall, 3D CRT and VMAT plans had similar values of D= (24.1 and 24.2 Gy), while HT had a D= slightly lower (23.6 Gy). The average values of the P+ index were 64.6, 67.4, and 56.6% for 3D CRT, VMAT, and HT plans, respectively, with the VMAT plans having a statistically significant increase in P+. Optimal values of D= were 28.4, 33.0, and 31.9 Gy for 3D CRT, VMAT, and HT plans, respectively. Although P+ values that correspond to the initial dose prescription were lower for HT, after optimizing the D= prescription level, the optimal P+ became 94.1, 99.5, and 99.6% for 3D CRT, VMAT, and HT, respectively, with the VMAT and HT plans having statistically significant increases in P+. If the optimal dose level is prescribed using a radiobiological evaluation method, as opposed to a purely dosimetric one, the two IMRT techniques, VMAT and HT, will yield largest overall benefit to CSI patients by maximizing tumor control and limiting normal tissue injury. Using VMAT or HT may provide these pediatric patients with better long-term outcomes after radiotherapy.

Quality assurance in radiotherapy on a national level; experience from Norway: the KVIST initiative

Background and purpose

In radiotherapy (RT), there are high requirements for quality assurance (QA) in all the steps of the process. Development of QA systems are demanding in terms of financial and human resources. A national QA programme (KVIST) has been established in Norway to facilitate implementation of QA activity on hospital level.

Method

The KVIST organisation comprises the KVIST team, the reference group (RG) and the working groups (WGs). The KVIST team is multidisciplinary and are employed in permanent positions. The RG acts as an advisory body for the KVIST team in defining and ranking the priority of projects. Relevant national QA projects are identified in collaboration with the RG, and WGs are established to carry out the various projects.

Result

Several national consensus documents have been prepared by the various WGs. Systems for incident handling and activity reporting have been established and clinical audits have been implemented in Norwegian RT. Guidelines for RT of various diagnoses have also been prepared in collaboration with National Cancer groups.

Conclusion

The KVIST programme has been very well acknowledged in the Norwegian RT community. It has succeeded in creating a positive attitude towards QA and improved the communication between centres and the various professions.

The effect of gantry spacing resolution on plan quality in a single modulated arc optimization

Volumetric‐modulated arc technique (VMAT) is an efficient form of IMRT delivery. It is advantageous over conventional IMRT in terms of treatment delivery time. This study investigates the relation between the number of segments and plan quality in VMAT optimization for a single modulated arc. Five prostate, five lung, and five head‐and‐neck (HN) patient plans were studied retrospectively. For each case, four VMAT plans were generated. The plans differed only in the number of control points used in the optimization process. The control points were spaced 2°, 3°, 4°, and 6° apart, respectively. All of the optimization parameters were the same among the four schemes. The 2° spacing plan was used as a reference to which the other three plans were compared. The plan quality was assessed by comparison of dose indices (DIs) and generalized equivalent uniform doses (gEUDs) for targets and critical structures. All optimization schemes generated clinically acceptable plans. The differences between the majority of reference and compared DIs and gEUDs were within 3%. DIs and gEUDs which differed in excess of 3% corresponded to dose levels well below the organ tolerances. The DI and the gEUD differences increased with an increase in plan complexity from prostates to HNs. Optimization with gantry spacing resolution of 4° seems to be a very balanced alternative between plan quality and plan complexity.

PACS number: 87.55.de

Biological optimization in volumetric modulated arc radiotherapy for prostate carcinoma

PURPOSE

To investigate the potential benefits achievable with biological optimization for modulated volumetric arc (VMAT) treatments of prostate carcinoma.

METHODS AND MATERIALS

Fifteen prostate patient plans were studied retrospectively. For each case, planning target volume, rectum, and bladder were considered. Three optimization schemes were used: dose-volume histogram (DVH) based, generalized equivalent uniform dose (gEUD) based, and mixed DVH/gEUD based. For each scheme, a single or dual 6-MV, 356° VMAT arc was used. The plans were optimized with Pinnacle(3) (v. 9.0 beta) treatment planning system. For each patient, the optimized dose distributions were normalized to deliver the same prescription dose. The quality of the plans was evaluated by dose indices (DIs) and gEUDs for rectum and bladder. The tallied DIs were D(1%), D(15%), D(25%), and D(40%), and the tallied gEUDs were for a values of 1 and 6. Statistical tests were used to quantify the magnitude and the significance of the observed differences. Monitor units and treatment times for each optimization scheme were also assessed.

RESULTS

All optimization schemes generated clinically acceptable plans. The statistical tests indicated that biological optimization yielded increased organs-at-risk sparing, ranging from ~1% to more than ~27% depending on the tallied DI, gEUD, and anatomical structure. The increased sparing was at the expense of longer treatment times and increased number of monitor units.

CONCLUSIONS

Biological optimization can significantly increase the organs-at-risk sparing in VMAT optimization for prostate carcinoma. In some particular cases, however, the DVH-based optimization resulted in superior treatment plans.

Dose-response relations for stricture in the proximal oesophagus from head and neck radiotherapy

BACKGROUND AND PURPOSE

Determination of the dose-response relations for oesophageal stricture after radiotherapy of the head and neck.

MATERIAL AND METHODS

In this study 33 patients who developed oesophageal stricture and 39 patients as controls are included. The patients received radiation therapy for head and neck cancer at Karolinska University Hospital, Stockholm, Sweden. For each patient the 3D dose distribution delivered to the upper 5 cm of the oesophagus was analysed. The analysis was conducted for two periods, 1992-2000 and 2001-2005, due to the different irradiation techniques used. The fitting has been done using the relative seriality model.

RESULTS

For the treatment period 1992-2005, the mean doses were 49.8 and 33.4 Gy, respectively, for the cases and the controls. For the period 1992-2000, the mean doses for the cases and the controls were 49.9 and 45.9 Gy and for the period 2001-2005 were 49.8 and 21.4 Gy. For the period 2001-2005 the best estimates of the dose-response parameters are D(50)=61.5 Gy (52.9-84.9 Gy), γ=1.4 (0.8-2.6) and s=0.1 (0.01-0.3).

CONCLUSIONS

Radiation-induced strictures were found to have a dose response relation and volume dependence (low relative seriality) for the treatment period 2001-2005. However, no dose response relation was found for the complete material.

Lung dose for minimally moving thoracic lesions treated with respiration gating

PURPOSE

To evaluate incidental doses to benign lung tissue for patients with minimally moving lung lesions treated with respiratory gating.

METHODS AND MATERIALS

Seventeen lung patient plans were studied retrospectively. Tumor motion was less than 5 mm in all cases. For each patient, mid-ventilation (MidVen) and mid-inhalation (MidInh) breathing phases were reconstructed. The MidInh phase was centered on the end-of-inhale (EOI) phase within a 30% gating window. Planning target volumes, heart, and spinal cord were delineated on the MidVen phase and transferred to the MidInh phase. Lungs were contoured separately on each phase. Intensity-modulated radiotherapy plans were generated on the MidVen phases. The plans were transferred to the MidInh phase, and doses were recomputed. The evaluation metric was based on dose indices, volume indices, generalized equivalent uniform doses, and mass indices for targets and critical structures. Statistical tests were used to establish the significance of the differences between the reference (MidVen) and compared (MidInh) dose distributions.

RESULTS

Statistical tests demonstrated that the indices evaluated for targets, cord, and heart differed by within 2.3%. The index differences in the lungs, however, are in excess of 6%, indicating the potentially achievable lung sparing and/or dose escalation.

CONCLUSIONS

Respiratory gating is a clinical option for patients with minimally moving lung lesions treated at EOI. Gating will be more beneficial for larger tumors, since dose escalation in those cases will result in a larger increase in the tumor control probability.

Expected clinical impact of the differences between planned and delivered dose distributions in helical tomotherapy for treating head and neck cancer using helical megavoltage CT images

Helical Tomotherapy (HT) has become increasingly popular over the past few years. However, its clinical efficacy and effectiveness continues to be investigated. Pre-treatment patient repositioning in highly conformal image-guided radiation therapy modalities is a prerequisite for reducing setup uncertainties. A MVCT image set has to be acquired to account for daily changes in the patient's internal anatomy and setup position. Furthermore, a comparison should be performed to the kVCT study used for dosimetric planning, by a registration process which results in repositioning the patient according to specific transitional and rotational shifts. Different image registration techniques may lead to different repositioning of the patient and, as a result, to varying delivered doses. This study aims to investigate the expected effect of patient setup correction using the Hi-Art tomotherapy system by employing radiobiological measures such as the biologically effective uniform dose (BEUD) and the complication-free tumor control probability (P+). In this study, a typical case of lung cancer with metastatic head & neck disease was investigated by developing a Helical Tomotherapy plan. For the Tomotherapy HiArt plan, the dedicated Tomotherapy treatment planning station was used. Three dose distributions (planned and delivered with and without patient setup correction) were compared based on radiobiological measures by using the P+ index and the BEUD concept as the common prescription point of the plans and plotting the tissue response probabilities against the mean target dose for a range of prescription doses. The applied plan evaluation method shows that in this cancer case the planned and delivered dose distributions with and without patient setup correction give a P+ of 81.6%, 80.9% and 72.2%, for a BEUD to the planning target volume (PTV) of 78.0Gy, 77.7Gy and 75.4Gy, respectively. The corresponding tumor control probabilities are 86.3%, 85.1% and 75.1%, whereas the total complication probabilities are 4.64%, 4.20% and 2.89%, respectively. HT can encompass the often large PTV required while minimizing the volume of the organs at risk receiving high dose. However, the effectiveness of a HT treatment plan can be considerably deteriorated if an accurate patient setup system is not available. Taking into account the dose-response relations of the irradiated tumors and normal tissues, a radiobiological treatment plan evaluation can be performed, which may provide a closer association of the delivered treatment with the clinical outcome. In such situations, for effective evaluation and comparison of different treatment plans, traditional dose based evaluation tools can be complemented by the use of P+,BEUD diagrams.

Feasibility of concurrent treatment with the scanning ultrasound reflector linear array system (SURLAS) and the helical tomotherapy system

PURPOSE

To evaluate the feasibility of concurrent treatment with the scanning ultrasound reflector linear array system (SURLAS) and helical tomotherapy (HT) intensity modulated radiation therapy (IMRT).

METHODS

The SURLAS was placed on a RANDO phantom simulating a patient with superficial or deep recurrent breast cancer. A megavoltage CT (MVCT) of the phantom with and without the SURLAS was obtained in the HT system. MVCT images with the SURLAS were obtained for two configurations: (1) with the SURLAS's long axis parallel and (2) perpendicular to the longitudinal axis of the phantom. The MVCT simulation data set was then transferred to a radiation therapy planning station. Organs at risk (OAR) were contoured including the lungs, heart, abdomen and spinal cord. The metallic parts of the SURLAS were contoured as well and constraints were assigned to completely or directionally block radiation through them. The MVCT simulation data set and regions of interest (ROI) files were subsequently transferred to the HT planning station. Several HT plans were obtained with optimization parameters that are usually used in the clinic. For comparison purposes, planning was also performed without the SURLAS on the phantom.

RESULTS

All plans with the SURLAS on the phantom showed adequate dose covering 95% of the planning target volume (PTV D95%), average dose and coefficient of variation of the planning target volume (PTV) dose distribution regardless of the SURLAS's orientation with respect to the RANDO phantom. Likewise, all OAR showed clinically acceptable dose values. Spatial dose distributions and dose-volume histogram (DVH) evaluation showed negligible plan degradation due to the presence of the SURLAS. Beam-on time varied depending on the selected optimization parameters.

CONCLUSION

From the perspective of the radiation dosage, concurrent treatment with the SURLAS and HT IMRT is feasible as demonstrated by the obtained clinically acceptable treatment plans. In addition, proper orientation of the SURLAS may be of benefit in reducing dose to organs at risk in some cases.

Comparison of IMRT treatment plans between linac and helical tomotherapy based on integral dose and inhomogeneity index

Intensity modulated radiotherapy (IMRT) is an advanced treatment technology for radiation therapy. There are several treatment planning systems (TPS) that can generate IMRT plans. These plans may show different inhomogeneity indices to the planning target volume (PTV) and integral dose to organs at risk (OAR). In this study, we compared clinical cases covering different anatomical treatment sites, including head and neck, brain, lung, prostate, pelvis, and cranio-spinal axis. Two treatment plans were developed for each case using Pinnacle(3) and helical tomotherapy (HT) TPS. The inhomogeneity index of the PTV and the non-tumor integral dose (NTID) were calculated and compared for each case. Despite the difference in the number of effective beams, in several cases, NTID did not increase from HT as compared to the step-and-shoot delivery method. Six helical tomotherapy treatment plans for different treatment sites have been analyzed and compared against corresponding step-and-shoot plans generated with the Pinnacle(3) planning system. Results show that HT may produce plans with smaller integral doses to healthy organs, and fairly homogeneous doses to the target as compared to linac-based step-and-shoot IMRT planning in special treatment site such as cranio-spinal.

Effective beam directions using radiobiologically optimized IMRT of node positive breast cancer

The purpose of this study was to investigate the optimal coplanar beam directions when treating an early breast cancer with locoregional lymphatic spread with a few radiobiologically optimized intensity modulated beams. Also to determine the increase in the probability of complication-free cure with the number of beam portals and the smallest number required to perform a close to optimal treatment for this tumour site. Four test patients with stage II left-sided breast cancer were studied with heart, lung and contralateral breast as principal organs at risk. The clinical target volume consisted of the breast tissue remaining after surgery, the axillary, the internal mammary as well as the supraclavicular lymph nodes. Through an exhaustive search of all possible beam directions the most effective coplanar beams with one to four intensity modulated photon beam portals were investigated. Comparisons with uniform beam treatment techniques and up to 12 intensity modulated beams were also made. The different plans were optimized using the probability of complication-free tumour cure, P(+), as biological objective function. When using two intensity modulated beam directions three major sets of suitable directions were identified denoted by A, P and T. A corresponds to an anterior oblique pair of beams around 25 degrees and 325 degrees , P is a perpendicular lateral pair at around 50 degrees and 130 degrees whereas T is a more conventional tangential pair at around 155 degrees and 300 degrees . Interestingly, these configurations identify simply three major effective beam directions namely at 30 degrees +/-20 degrees , 145 degrees +/-20 degrees and 310 degrees +/-15 degrees . For the three intensity modulated beam technique a combination of these three effective beam directions generally covered the global maximum of the probability of complication-free tumour control. The improvement in complication-free cure probability with two optimally selected intensity modulated beams is around 10% when compared to a uniform beam technique with three to four beam portals. This increase is mainly due to a reduction by almost 1% in the probability of injury to the heart and an increase of 6% in the probability of local tumour control. When three or four biologically optimized beam portals are used a further increase in the probability of complication-free cure of about 6% can often be obtained. This improvement is caused by a small decrease in the probability of injury to the heart, left lung and other surrounding normal tissue, as well as a slight further increase in the probability of tumour control. The increase in the treatment outcome is minimal when more than four intensity modulated beams are employed. A small increase in dose homogeneity in the target volume and a slight decrease in the normal tissue volume receiving high dose may be seen, but without appreciably improving the complication-free cure probability. For a stage II breast cancer, three and in more complex cases four optimally oriented beams are sufficient to reach close to the maximum probability of complication-free tumour control when biologically optimized intensity modulated dose delivery is used. Angle of incidence optimization may then be advantageous starting from the given most effective three beam directions.

Treatment plan comparison between helical tomotherapy and MLC-based IMRT using radiobiological measures

The rapid implementation of advanced treatment planning and delivery technologies for radiation therapy has brought new challenges in evaluating the most effective treatment modality. Intensity-modulated radiotherapy (IMRT) using multi-leaf collimators (MLC) and helical tomotherapy (HT) are becoming popular modes of treatment delivery and their application and effectiveness continues to be investigated. Presently, there are several treatment planning systems (TPS) that can generate and optimize IMRT plans based on user-defined objective functions for the internal target volume (ITV) and organs at risk (OAR). However, the radiobiological parameters of the different tumours and normal tissues are typically not taken into account during dose prescription and optimization of a treatment plan or during plan evaluation. The suitability of a treatment plan is typically decided based on dosimetric criteria such as dose-volume histograms (DVH), maximum, minimum, mean and standard deviation of the dose distribution. For a more comprehensive treatment plan evaluation, the biologically effective uniform dose (D) is applied together with the complication-free tumour control probability (P(+)). Its utilization is demonstrated using three clinical cases that were planned with two different forms of IMRT. In this study, three different cancer types at different anatomical sites were investigated: head and neck, lung and prostate cancers. For each cancer type, a linac MLC-based step-and-shoot IMRT plan and a HT plan were developed. The MLC-based IMRT treatment plans were developed on the Philips treatment-planning platform, using the Pinnacle 7.6 software release. For the tomotherapy HiArt plans, the dedicated tomotherapy treatment planning station was used, running version 2.1.2. By using D as the common prescription point of the treatment plans and plotting the tissue response probabilities versus D for a range of prescription doses, a number of plan trials can be compared based on radiobiological measures. The applied plan evaluation method shows that in the head and neck cancer case the HT treatment gives better results than MLC-based IMRT in terms of expected clinical outcome P(+) of 62.2% and 46.0%, D to the ITV of 72.3 Gy and 70.7 Gy, respectively). In the lung cancer and prostate cancer cases, the MLC-based IMRT plans are better over the clinically useful dose prescription range. For the lung cancer case, the HT and MLC-based IMRT plans give a P(+) of 66.9% and 72.9%, D to the ITV of 64.0 Gy and 66.9 Gy, respectively. Similarly, for the prostate cancer case, the two radiation modalities give a P(+) of 68.7% and 72.2%, D to the ITV of 86.0 Gy and 85.9 Gy, respectively. If a higher risk of complications (higher than 5%) could be allowed, the complication-free tumour control could increase by over 40%, 2% and 30% compared to the initial dose prescription for the three cancer cases, respectively. Both MLC-based IMRT and HT can encompass the often-large ITV required while they minimize the volume of the organs at risk receiving high doses. Radiobiological evaluation of treatment plans may provide an improved correlation of the delivered treatment with the clinical outcome by taking into account the dose-response characteristics of the irradiated targets and normal tissues. There may exist clinical cases, which may look dosimetrically similar but in radiobiological terms may be quite different. In such situations, traditional dose-based evaluation tools can be complemented by the use of P(+)--D diagrams to effectively evaluate and compare treatment plans.

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.

On probabilistically defined margins in radiation therapy

Margins about a target volume subject to external beam radiation therapy are designed to assure that the target volume of tissue to be sterilized by treatment is adequately covered by a lethal dose. Thus, margins are meant to guarantee that all potential variation in tumour position relative to beams allows the tumour to stay within the margin. Variation in tumour position can be broken into two types of dislocations, reducible and irreducible. Reducible variations in tumour position are those that can be accommodated with the use of modern image-guided techniques that derive parameters for compensating motions of patient bodies and/or motions of beams relative to patient bodies. Irreducible variations in tumour position are those random dislocations of a target that are related to errors intrinsic in the design and performance limitations of the software and hardware, as well as limitations of human perception and decision making. Thus, margins in the era of image-guided treatments will need to accommodate only random errors residual in patient setup accuracy (after image-guided setup corrections) and in the accuracy of systems designed to track moving and deforming tissues of the targeted regions of the patient's body. Therefore, construction of these margins will have to be based on purely statistical data. The characteristics of these data have to be determined through the central limit theorem and Gaussian properties of limiting error distributions. In this paper, we show how statistically determined margins are to be designed in the general case of correlated distributions of position errors in three-dimensional space. In particular, we show how the minimal margins for a given level of statistical confidence are found. Then, how they are to be used to determine geometrically minimal PTV that provides coverage of GTV at the assumed level of statistical confidence. Our results generalize earlier recommendations for statistical, central limit theorem-based recommendations for margin construction that were derived for uncorrelated distributions of errors (van Herk, Remeijer, Rasch and Lebesque 2000 Int. J. Radiat. Oncol. Biol. Phys. 47 1121-35; Stroom, De Boer, Huizenga and Visser 1999 Int. J. Radiat. Oncol. Biol. Phys. 43 905-19).

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.

Three-dimensional atlas of lymph node topography based on the visible human data set

Comprehensive atlases of lymph node topography are necessary tools to provide a detailed description of the lymphatic distribution in relation to other organs and structures. Despite the recent developments of atlases and guidelines focusing on definitions of lymphatic regions, a comprehensive and detailed description of the three-dimensional (3D) nodal distribution is lacking. This article describes a new 3D atlas of lymph node topography based on the digital images of the Visible Human Male Anatomical (VHMA) data set. About 1,200 lymph nodes were localized in the data set and their distribution was compared with data from current cross-sectional lymphatic atlases. The identified nodes were delineated and then labeled with different colors that corresponded to their anatomical locations. A series of 2D illustrations, showing discrete locations, description, and distribution of major lymph nodes, was compiled to form a cross-sectional atlas. The resultant contours of all localized nodes in the VHMA data set were superimposed to develop a volumetric model. A 3D reconstruction was generated for the lymph nodes and surrounding structures. The volumetric lymph node topography was also integrated into the existing VOXEL-MAN digital atlas to obtain an interactive and photo-realistic visualization of the lymph nodes showing their proximity to blood vessels and surrounding organs. The lymph node topography forms part of our whole body atlas database, which includes organs, definitions, and parameters that are related to radiation therapy. The lymph node topography atlas could be utilized for visualization and exploration of the 3D lymphatic distribution to assist in defining the target volume for treatment based on the lymphatic spread surrounding the primary tumor.

The effective dose (Deff) for electron beams

BACKGROUND AND PURPOSE

Calculation of the effective dose and proposal of a dose specification method for the electron beams.

PATIENTS AND METHODS

In a homogenous water phantom the 3D dose distributions for electron beams of energy 6, 9, 12, 15, 18 and 21 MeV and beam size 10x10 cm were calculated. For a volume encompassed with 80, 85 and 90% isodose, the mean dose and the SD were calculated for each energy. Using the Brahme's formulae, the effective dose was calculated.

RESULTS

The larger the minimum dose (value of the encompassing isodose), the larger the mean dose and the smaller the SD. The mean doses and SD to the volume encompassed with 80, 85 and 90% are in the range of 91-94%, and 5.1-6.2%, 93-96% and 4.2-4.6%, 94-96% and 3.0-3.2%, respectively. Thus the effective dose for the volume encompassed with 80, 85 and 90% are about 90, 93 and 95%, respectively.

CONCLUSION

Taking into account the requirements regarding dose uniformity within the PTV and the sparing effect for normal tissue situated under the PTV, we propose to keep the 85% isodose as a minimum one and to prescribe the dose to the 90% isodose. The present method may be applied for single electron beams and typical cases.

A Monte Carlo multiple source model applied to radiosurgery narrow photon beams

Monte Carlo (MC) methods are nowadays often used in the field of radiotherapy. Through successive steps, radiation fields are simulated, producing source Phase Space Data (PSD) that enable a dose calculation with good accuracy. Narrow photon beams used in radiosurgery can also be simulated by MC codes. However, the poor efficiency in simulating these narrow photon beams produces PSD whose quality prevents calculating dose with the required accuracy. To overcome this difficulty, a multiple source model was developed that enhances the quality of the reconstructed PSD, reducing also the time and storage capacities. This multiple source model was based on the full MC simulation, performed with the MC code MCNP4C, of the Siemens Mevatron KD2 (6 MV mode) linear accelerator head and additional collimators. The full simulation allowed the characterization of the particles coming from the accelerator head and from the additional collimators that shape the narrow photon beams used in radiosurgery treatments. Eight relevant photon virtual sources were identified from the full characterization analysis. Spatial and energy distributions were stored in histograms for the virtual sources representing the accelerator head components and the additional collimators. The photon directions were calculated for virtual sources representing the accelerator head components whereas, for the virtual sources representing the additional collimators, they were recorded into histograms. All these histograms were included in the MC code, DPM code and using a sampling procedure that reconstructed the PSDs, dose distributions were calculated in a water phantom divided in 20000 voxels of 1 x 1 x 5 mm3. The model accurately calculates dose distributions in the water phantom for all the additional collimators; for depth dose curves, associated errors at 2sigma were lower than 2.5% until a depth of 202.5 mm for all the additional collimators and for profiles at various depths, deviations between measured and calculated values were less than 2.5% or 1 mm.

Conformal radiotherapy of urinary bladder cancer

Recent advances in radiotherapy (RT) are founded on the enhanced tumour visualisation capabilities of new imaging modalities and the precise deposition of individualised radiation dose distributions made possible with the new systems for RT planning and delivery. These techniques have a large potential to also improve the results of RT of urinary bladder cancer. Major challenges to take full advantage of these advances in the management of bladder cancer are to control, and, as far as possible, reduce bladder motion, and to reliably account for the related intestine and rectum motion. If these obstacles are overcome, it should be possible in the near future to offer selected patients with muscle invading bladder cancer an organ-sparing, yet effective combined-modality treatment as an alternative to radical surgery.

Adaptive radiation therapy for compensation of errors in patient setup and treatment delivery

In this paper, an adaptive radiation therapy algorithm is derived and evaluated using numerical simulations. Patient setup errors are considered and an off-line adaptive method to compensate for the effect of these is provided. The method consists of two parts, one for correction of patient position to account for the systematic error, and one for modulation of the fluence profiles to account for the random errors. The method is based on standard control theory for linear systems. It is investigated if this adaptive method can replace the use of a planning target volume (PTV) and therefore increase the possibilities to escalate the dose. Numerical simulations of treatments of a prostate patient indicate that this is the case. The simulations show that better organ-at-risk protection can be achieved when using the adaptation algorithm to correct for the geometrical uncertainties than when using a PTV.

Statistical methods for clinical verification of dose–response parameters related to esophageal stricture and AVM obliteration from radiotherapy

The purpose of this work is to provide some statistical methods for evaluating the predictive strength of radiobiological models and the validity of dose-response parameters for tumour control and normal tissue complications. This is accomplished by associating the expected complication rates, which are calculated using different models, with the clinical follow-up records. These methods are applied to 77 patients who received radiation treatment for head and neck cancer and 85 patients who were treated for arteriovenous malformation (AVM). The three-dimensional dose distribution delivered to esophagus and AVM nidus and the clinical follow-up results were available for each patient. Dose-response parameters derived by a maximum likelihood fitting were used as a reference to evaluate their compatibility with the examined treatment methodologies. The impact of the parameter uncertainties on the dose-response curves is demonstrated. The clinical utilization of the radiobiological parameters is illustrated. The radiobiological models (relative seriality and linear Poisson) and the reference parameters are validated to prove their suitability in reproducing the treatment outcome pattern of the patient material studied (through the probability of finding a worse fit, area under the ROC curve and chi2 test). The analysis was carried out for the upper 5 cm of the esophagus (proximal esophagus) where all the strictures are formed, and the total volume of AVM. The estimated confidence intervals of the dose-response curves appear to have a significant supporting role on their clinical implementation and use.

A model to predict bladder shapes from changes in bladder and rectal filling

The purpose of this study is to develop a model that quantifies in three dimensions changes in bladder shape due to changes in bladder and/or rectal volume. The new technique enables us to predict changes in bladder shape over a short period of time, based on known urinary inflow. Shortly prior to the treatment, the patient will be scanned using a cone beam CT scanner (x-ray volume imager) that is integrated with the linear accelerator. After (automated) delineation of the bladder, the model will be used to predict the short-term shape changes of the bladder for the time interval between image acquisition and dose delivery. The model was developed using multiple daily CT scans of the pelvic area of 19 patients. For each patient, the rigid bony structure in follow-up scans was matched to that of the planning CT scan, and the outer bladder and rectal wall were delineated. Each bladder wall was subdivided in 2500 domains. A fixed reference point inside the bladder was used to calculate for each bladder structure a "Mercator-like" 2D scalar map (similar to a height map of the globe), containing the distances from this reference point to each domain on the bladder wall. Subsequently, for all bladder shapes of a patient and for all domains on the wall individually, the distance to the reference point was fitted by a linear function of both bladder and rectal volume. The model uses an existing bladder structure to create a new structure via expansion (or contraction), until the expressed volume is reached. To evaluate the predictive power of the model, the jack-knife method was used. The errors in the fitting procedure depended on the part of the bladder and range from 0 to 0.5 cm (0.2 cm on average). It was found that a volume increase of 150 cc can lead to a displacement up to about 2.5 cm of the cranial part of the bladder. With the model, the uncertainty in the position of the bladder wall can be reduced down to a maximum value of about 0.5 cm in case the bladder volume increase is known. Furthermore, it was found that a change in rectal filling causes a shift of the bladder, while its shape is hardly influenced. In conclusion, we developed a model that describes the bladder shape and position as a function of the bladder volume and the rectal filling. The model accurately describes the complex shape of the bladder as it works on each domain of the bladder separately.

The clinical value of non-coplanar photon beams in biologically optimized intensity modulated dose delivery on deep-seated tumours

The aim of the present study is to compare the merits of different radiobiologically optimized treatment techniques using few-field planar and non-coplanar dose delivery on an advanced cancer of the cervix, with rectum and bladder as principal organs at risk. Classically, the rational for using non-coplanar beams is to minimize the overlap of beam entrance and exit regions and to find new beam directions avoiding organs at risk, in order to reduce damage to sensitive normal tissues. Two four-beam configurations have been extensively studied. The first consists of three evenly spaced coplanar beams and a fourth non-coplanar beam. A second tetrahedral-like configuration, with two symmetric non-coplanar beams at the same gantry angle and two coplanar beams, with optimized beam directions, was also tested. The present study shows that when radiobiologically optimized intensity modulated beams are applied to such a geometry, only a marginal increase in the treatment outcome can be achieved by non-coplanar beams compared to the optimal coplanar treatment. The main reason for this result is that the high dose in the beam-overlap regions is already optimally reduced by biologically optimized intensity modulation in the plane. The large number of degrees of freedom already incorporated in the treatment by the use of intensity modulation and radiobiological optimization, leads to the saturation of the benefit acquired by a further increase in the degrees of freedom with non-coplanar beams. In conclusion, the use of coplanar radiobiologically optimized intensity modulation simplifies the dose delivery, reducing the need for non-coplanar beam portals.

Organ motion, set-up variation and treatment margins in radical radiotherapy of urinary bladder cancer

BACKGROUND AND PURPOSE

A major challenge in conformal radiotherapy of bladder cancer is to determine adequate treatment margins. For this purpose, we therefore quantified the internal motion of the urinary bladder as well as the external patient set-up variation during a course of fractionated radiotherapy. In the light of the recently introduced ICRU-62 concept, the planning organ at risk volume, we also studied the internal motion of nearby organs at risk, the rectum and intestine.

MATERIAL AND METHODS

Weekly CT scans and electronic portal images (EPIs) were sampled from 20 patients during radical, conformal bladder irradiation (60-64 Gy/2 Gy in five fractions weekly). The planning scans were acquired with 70 ml of bladder contrast instilled, and patients were instructed to void before the treatment/repeat scanning sessions. Internal motion of the bladder, rectum and intestine was measured by 3-D image matching of the repeat scans to the patients' planning scans. Internal margins (CTV-to-ITV) were determined using both a direct empirical approach and an analytically derived margin recipe. The external patient set-up variability was determined by 2-D matching of front and lateral EPIs to corresponding digitally reconstructed radiographs.

RESULTS

A total of 149 CT scans (20 for planning, 129 during the treatment course) and 133 sets of EPIs were analysed. Bladder volumes were smaller during treatment than in the planning situation in 85% of the repeat scans. Nevertheless, we found the repeat scan bladder volumes to extend outside the planning scan bladder contours in 89% of the scans, on average with 9% of the volume (range: 0-47%). Eight patients (40%) had at least one repeat scan (25 scans in total) where displacements >15 mm were observed at one or more sides of the bladder. CTV-to-ITV margins of 10 mm inferior, 20 mm superior, 11 mm left, 8 mm right, 20 mm anterior and 14 mm posterior were required to simultaneously encompass all bladder deflections except for the largest outward deflection in all directions in 84% of the patients. Including patient set-up variation (CTV-to-PTV), we found that an additional safety margin of 2-6 mm had to be added in the various directions. The rectum expanded outside the planning contours in all repeat scans, on average with 24% of the volume (range: 2-69%). The volume of intestine found close to the bladder were significantly and negatively correlated to the bladder volume in almost half of the patients.

CONCLUSION

This study documented both a large internal motion of the bladder and a substantial patient set-up variation. Our current treatment margins have been adjusted according to the findings of this study. Considerable variation in position and volume of the rectum and intestine was also documented.

Limitations of a convolution method for modeling geometric uncertainties in radiation therapy. I. The effect of shift invariance

Convolution methods have been used to model the effect of geometric uncertainties on dose delivery in radiation therapy. Convolution assumes shift invariance of the dose distribution. Internal inhomogeneities and surface curvature lead to violations of this assumption. The magnitude of the error resulting from violation of shift invariance is not well documented. This issue is addressed by comparing dose distributions calculated using the Convolution method with dose distributions obtained by Direct Simulation. A comparison of conventional Static dose distributions was also made with Direct Simulation. This analysis was performed for phantom geometries and several clinical tumor sites. A modification to the Convolution method to correct for some of the inherent errors is proposed and tested using example phantoms and patients. We refer to this modified method as the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over different beam arrangements in the various phantom examples) was 21% with the Static dose calculation, 9% with Convolution, and reduced to 5% with the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over four clinical examples) was 9% for the Static method, 13% for Convolution, and 3% for Corrected Convolution. While Convolution can provide a superior estimate of the dose delivered when geometric uncertainties are present, the violation of shift invariance can result in substantial errors near the surface of the patient. The proposed Corrected Convolution modification reduces errors near the surface to 3% or less.

Limitations of a convolution method for modeling geometric uncertainties in radiation therapy. II. The effect of a finite number of fractions

Convolution methods can be used to model the effect of geometric uncertainties on the planned dose distribution in radiation therapy. This requires several assumptions, including that the patient is treated with an infinite number of fractions, each delivering an infinitesimally small dose. The error resulting from this assumption has not been thoroughly quantified. This is investigated by comparing dose distributions calculated using the Convolution method with the result of Stochastic simulations of the treatment. Additionally, the dose calculated using the conventional Static method, a Corrected Convolution method, and a Direct Simulation are compared to the Stochastic result. This analysis is performed for single beam, parallel opposed pair, and four-field box techniques in a cubic water phantom. Treatment plans for a simple and a complex idealized anatomy were similarly analyzed. The average maximum error using the Static method for a 30 fraction simulation for the three techniques in phantoms was 23%, 11% for Convolution, 10% for Corrected Convolution, and 10% for Direct Simulation. In the two anatomical examples, the mean error in tumor control probability for Static and Convolution methods was 7% and 2%, respectively, of the result with no uncertainty, and 35% and 9%, respectively, for normal tissue complication probabilities. Convolution provides superior estimates of the delivered dose when compared to the Static method. In the range of fractions used clinically, considerable dosimetric variations will exist solely because of the random nature of the geometric uncertainties. However, the effect of finite fractionation appears to have a greater impact on the dose distribution than plan evaluation parameters.

Margins for translational and rotational uncertainties: a probability-based approach

PURPOSE

To define margins for systematic rotations and translations, based on known statistical distributions of these deviations.

METHODS AND MATERIALS

The confidence interval-based expansion method for translations, known as the "rolling ball algorithm," was extended to include rotations. This new method, which we call the Rotational and Translational Confidence Limit (RTCL) method, is exact for a point with arbitrary rotations and translations or for a finite shape with rotations only. The method was compared with two existing expansion methods: a rolling ball algorithm without rotations, and a convolution (blurring) method which included rotations. On the basis of these methods, planning target volumes (PTVs, expanded clinical target volumes [CTVs]) were constructed for a number of shapes (a sphere, a sphere with an extension, and three prostate cases), and evaluated in several ways by means of a Monte Carlo method. The accuracy of each method was measured by determining the probability of finding the CTV completely inside the PTV (P(CTVinPTV)), using parameters that yield a 90% probability for a sphere-shaped CTV without rotations. Furthermore, with the expansion parameters adjusted to give an equal P(CTVinPTV) for all methods, PTV volumes were compared.

RESULTS

With the expansion algorithm parameters chosen to yield P(CTVinPTV) = 90% for a sphere, an average P(CTVinPTV) of 84%, 57%, and 46% was obtained for the other shapes, using the RTCL method, coverage probability, and rolling ball, respectively. With the parameters adjusted to yield an equal P(CTVinPTV) for all methods, the PTV volume was on average 8% larger for the coverage probability method and 15% larger for the rolling ball algorithm compared to the RTCL method.

CONCLUSION

The RTCL method provides an accurate way to include the effects of systematic rotations in the margin. Compared to other algorithms, the method is less sensitive to the shape of the CTV, and, given a fixed probability of finding the CTV inside the PTV, a smaller PTV volume can be obtained.

Intensity-modulated radiotherapy: current status and issues of interest

PURPOSE

To develop and disseminate a report aimed primarily at practicing radiation oncology physicians and medical physicists that describes the current state-of-the-art of intensity-modulated radiotherapy (IMRT). Those areas needing further research and development are identified by category and recommendations are given, which should also be of interest to IMRT equipment manufacturers and research funding agencies.

METHODS AND MATERIALS

The National Cancer Institute formed a Collaborative Working Group of experts in IMRT to develop consensus guidelines and recommendations for implementation of IMRT and for further research through a critical analysis of the published data supplemented by clinical experience. A glossary of the words and phrases currently used in IMRT is given in the. Recommendations for new terminology are given where clarification is needed.

RESULTS

IMRT, an advanced form of external beam irradiation, is a type of three-dimensional conformal radiotherapy (3D-CRT). It represents one of the most important technical advances in RT since the advent of the medical linear accelerator. 3D-CRT/IMRT is not just an add-on to the current radiation oncology process; it represents a radical change in practice, particularly for the radiation oncologist. For example, 3D-CRT/IMRT requires the use of 3D treatment planning capabilities, such as defining target volumes and organs at risk in three dimensions by drawing contours on cross-sectional images (i.e., CT, MRI) on a slice-by-slice basis as opposed to drawing beam portals on a simulator radiograph. In addition, IMRT requires that the physician clearly and quantitatively define the treatment objectives. Currently, most IMRT approaches will increase the time and effort required by physicians, medical physicists, dosimetrists, and radiation therapists, because IMRT planning and delivery systems are not yet robust enough to provide totally automated solutions for all disease sites. Considerable research is needed to model the clinical outcomes to allow truly automated solutions. Current IMRT delivery systems are essentially first-generation systems, and no single method stands out as the ultimate technique. The instrumentation and methods used for IMRT quality assurance procedures and testing are not yet well established. In addition, many fundamental questions regarding IMRT are still unanswered. For example, the radiobiologic consequences of altered time-dose fractionation are not completely understood. Also, because there may be a much greater ability to trade off dose heterogeneity in the target vs. avoidance of normal critical structures with IMRT compared with traditional RT techniques, conventional radiation oncology planning principles are challenged. All in all, this new process of planning and treatment delivery has significant potential for improving the therapeutic ratio and reducing toxicity. Also, although inefficient currently, it is expected that IMRT, when fully developed, will improve the overall efficiency with which external beam RT can be planned and delivered, and thus will potentially lower costs.

CONCLUSION

Recommendations in the areas pertinent to IMRT, including dose-calculation algorithms, acceptance testing, commissioning and quality assurance, facility planning and radiation safety, and target volume and dose specification, are presented. Several of the areas in which future research and development are needed are also indicated. These broad recommendations are intended to be both technical and advisory in nature, but the ultimate responsibility for clinical decisions pertaining to the implementation and use of IMRT rests with the radiation oncologist and radiation oncology physicist. This is an evolving field, and modifications of these recommendations are expected as new technology and data become available.