Monte Carlo based modulated electron beam treatment planning using a few-leaf electron collimator--feasibility study

Energy modulated electron beam therapy with conventional clinical accelerators has lagged behind photon IMRT despite its potential to achieve highly conformal dose distributions in superficial targets. One of the reasons for this is the absence of an automated collimating device that allows for the flexible delivery of a series of variable field openings. Electron-specific multileaf collimators attached to the bottom of the applicator require the use of a large number of motors and suffer from being relatively bulky and impractical for head and neck sites. In this work, we investigate the treatment planning aspects of a proposed 'few-leaf' electron collimator (FLEC) that consists of four motor-driven trimmer bars at the end of the applicator. The device is designed to serve as an accessory to standard equipment and allows for the shaping of any irregular field by combination of rectangular fieldlets. Using a Monte Carlo model of the FLEC, dose distributions are optimized using a simulated annealing (SA) inverse planning algorithm based on a limited number of Monte Carlo pre-generated, realistic phantom-specific dose kernels and user-specified dose-volume constraints. Using a phantom setup with an artificial target enclosed by organs at risk (OAR) as well as using a realistic patient case, we demonstrate that highly conformal distributions can be generated. Estimates of delivery times are made and show that a full treatment fraction can be kept to 15 min or less.

Adjoint Monte Carlo method for prostate external photon beam treatment planning: an application to 3D patient anatomy

Recently, the theoretical framework of the adjoint Monte Carlo (AMC) method has been developed using a simplified patient geometry. In this study, we extended our previous work by applying the AMC framework to a 3D anatomical model called VIP-Man constructed from the Visible Human images. First, the adjoint fluxes for the prostate (PTV) and rectum and bladder (organs at risk (OARs)) were calculated on a spherical surface of 1 m radius, centred at the centre of gravity of PTV. An importance ratio, defined as the PTV dose divided by the weighted OAR doses, was calculated for each of the available beamlets to select the beam angles. Finally, the detailed doses in PTV and OAR were calculated using a forward Monte Carlo simulation to include the electron transport. The dose information was then used to generate dose volume histograms (DVHs). The Pinnacle treatment planning system was also used to generate DVHs for the 3D plans with beam angles obtained from the AMC (3D-AMC) and a standard six-field conformal radiation therapy plan (3D-CRT). Results show that the DVHs for prostate from 3D-AMC and the standard 3D-CRT are very similar, showing that both methods can deliver prescribed dose to the PTV. A substantial improvement in the DVHs for bladder and rectum was found for the 3D-AMC method in comparison to those obtained from 3D-CRT. However, the 3D-AMC plan is less conformal than the 3D-CRT plan because only bladder, rectum and PTV are considered for calculating the importance ratios. Nevertheless, this study clearly demonstrated the feasibility of the AMC in selecting the beam directions as a part of a treatment planning based on the anatomical information in a 3D and realistic patient anatomy.

4D Monte Carlo simulation of proton beam scanning: modelling of variations in time and space to study the interplay between scanning pattern and time-dependent patient geometry

When dosimetric effects in time-dependent geometries are studied, usually either the results of individual three-dimensional (3D) calculations are combined or probability-based approaches are applied. These methods may become cumbersome and time-consuming if high time resolution is required or if the geometry is complex. Furthermore, it is difficult to study double-dynamic systems, e.g., to investigate the influence of time-dependent beam delivery (i.e., magnetically moving beam spots in proton beam scanning) on the dose deposition in a moving target. We recently introduced the technique of 4D Monte Carlo dose calculation to model continuously changing geometries. In intensity modulated proton therapy, dose is delivered by individual pristine Bragg curves. Dose spots are positioned in the patient by varying magnetic field and beam energy. If the movement of these dose spots occurs during significant respiratory motion, interplay effects can take place. Because of the inhomogeneity of individual subfields, the consequences of motion can be more severe than in conventional proton therapy. We demonstrate how the technique of 4D Monte Carlo can be used to study interplay effects in proton beam scanning. Time-dependent beam delivery to a changing patient geometry is simulated in a single 4D dose calculation. Interplay effects between respiratory motion and beam scanning speed are demonstrated.

[Optic nerve sheath meningeoma--from expectation to active treatment]

The management of optic nerve sheath meningeoma has been controversial and difficult. The conservative strategy was "to wait and see" until the patient became blind on the affected eye. Surgical excision had poor outcome and, with the exception of a few anecdotal case stories, led to blindness or postoperative eye motility disturbances. It has been long known that radiation therapy can prevent tumour progression, but the end result was very often visual deterioration and blindness. A few recent well documented studies describe how stereotactic fractionated conformal radiotherapy can provide stabilisation or improvement in the visual function in optic nerve sheath meningeoma. This article gives an overview, illustrated by a case story.

Monte Carlo dose calculations for spot scanned proton therapy

Density heterogeneities can have a profound effect on dose distributions for proton therapy. Although analytical calculations in homogeneous media are relatively straightforward, the modelling of the propagation of the beam through density heterogeneities can be more problematical. At the Paul Scherrer Institute, an in-house dedicated Monte Carlo (MC) code has been used for over a decade to assess the possible deficiencies of the analytical calculations in patient geometries. The MC code has been optimized for speed, and as such traces primary protons only through the treatment nozzle and patient's CT. Contributions from nuclear interactions are modelled analytically with no tracing of secondary particles. The MC code has been verified against measured data in water and experimental proton radiographs through a heterogeneous anthropomorphic phantom. In comparison to the analytical calculation, the MC code has been applied to both spot scanned and intensity modulated proton therapy plans, and to a number of cases containing titanium metal implants. In summary, MC-based dose calculations could provide an invaluable tool for independently verifying the calculated dose distribution within a patient geometry as part of a comprehensive quality assurance protocol for proton treatment plans.

Near surface photon energy spectra outside a 6 MV field edge

The purpose of this study was to investigate the difference between a 6 MV linear accelerator x-ray energy spectrum outside the field edge near a phantom surface, and the corresponding spectrum on the central axis. The Monte Carlo code MCNP-4A was used to calculate the spectra on the central axis and at 1, 2, 5 and 10 cm from the edge of a 4 x 4 cm2, 10 x 10 cm2 and 15 x 15 cm2 field. Compared to the spectrum on the central axis, the spectra outside the field edge showed two distinct regions: a broad peak below about 0.5 MeV, and a lower amplitude, less rapidly changing region at higher energies from 0.5 to 6 MeV. The lower energy peak was due to scattered photons, and the higher energy component was due mainly to primary photons transmitted through the jaws of the secondary collimator. The potential impact of these spectral differences on critical organ photon dosimetry was determined by calculating the ratio of the sensitivity of a Scanditronix EDD-5 diode and of a LiF:Mg:Ti thermoluminescent dosimeter (TLD) outside the field edge to their respective sensitivity at the calibration position on the central axis. The lower energy peak combined with the non-uniform energy sensitivity of each detector produced up to a two-thirds overestimate of x-ray dose outside the field by the diode, whereas the response ratio of the TLD was about unity. These results indicated that a similar evaluation was required for profile measurements of a dynamic wedged field and measurements in an intensity modulated beam with either type of detector.

Influence of calculation model on dose distribution in stereotactic radiotherapy for pulmonary targets

PURPOSE

To compare the pencil beam (PB) and collapsed cone (CC)-based three-dimensional dose calculation used for stereotactic irradiation of pulmonary targets.

METHODS AND MATERIALS

Three-dimensional conformal dose distributions (using 6-MV and 18-MV photon beams) were generated for 33 pulmonary targets using the PB algorithm implemented in the Helax-TMS treatment planning system and then recalculated with the CC algorithm of TMS using an identical beam setup and parameters. The differences were analyzed by evaluating the dose-volume histograms for the planning target volume (PTV) and clinical target volume (CTV) and evaluating the computed absolute monitor units (MUs). The influence of the photon energy was also studied. For three cases, the results were compared with Monte-Carlo calculations.

RESULTS

Use of the CC model typically showed increased dose inhomogeneity. Owing to a more accurate modeling of secondary charged particle disequilibrium at the tumor-lung interface, the beam penumbra is broadened. The median and mean target dose decreased by 13.9% and 11.2% for the PTV and 9.2% and 9.4% for the CTV, respectively, using the CC algorithm. Consequently, the average PTV dose coverage decreased by 7.1% (SD, 6.5%). On average, the MUs calculated to achieve the prescribed dose were 5.4% (SD, 5.8%) greater for the CC algorithm. The difference in MUs between the PB and CC increased with decreasing PTV size and high photon energy (18 MV; r = -0.68), reaching 26% at the maximum.

CONCLUSION

The absorbed dose at the lung-tumor interface calculated by the PB algorithm was considerably greater than the dose calculated using the CC algorithm. In small targets (PTV < or = 100 cm(3)) and for 18-MV photons, the MUs calculated with PB may lead to an insufficient dose to the target volume.

Impact of tumor repopulation on radiotherapy planning

PURPOSE

Biologic/functional imaging (e.g., fluorodeoxyglucose/3'-deoxy-3'-fluorothymidine-positron emission tomography) is promising to provide information on tumor cell repopulation. Such information is important in the design of biologically conformal radiotherapy for cancer. The questions remaining unclear are whether it is necessary to escalate the dose to the regions with rapid cell repopulation in the tumor target and, if so, by how much. The purpose of this work was to address these questions using radiobiologic modeling.

METHODS AND MATERIALS

The generalized linear-quadratic model, extended to account for the effect of clonogenic cell repopulation, was used to calculate the cell-killing efficiency of radiotherapy. The standard Poisson tumor control probability (TCP) model was used to bridge cell killing to treatment outcome. Prostate cancer was chosen as the example for this study. In situ measurements of prostate cancer patients have shown that the potential doubling time of tumor cells has a large variation, ranging from 15 to 170 days. On the basis of the linear-quadratic and TCP parameters (alpha = 0.14 Gy(-1), alpha/beta = 3.1 Gy, and the number of clonogens K = 10(6)-10(7) cells) determined in earlier studies, we evaluated the influence of tumor cell repopulation during protracted treatment courses on treatment outcome. The dose escalations, which can be used to combat aggressive cell repopulation in regions with different doubling times (15-170 days) and sizes (5, 10, 15, and 40 cm(3) of a 40-cm(3) tumor), were calculated for commonly practiced radiotherapy modalities. The influence of linear-quadratic parameters on this calculation was also considered.

RESULTS

The impact of tumor cell repopulation on TCP and the corresponding dose escalation required to account for this impact were investigated for both external beam radiotherapy and permanent implantation. The results indicated that for regions with aggressive tumor cell growth, dose escalation is necessary to compensate for the repopulation effect. For example, for tumors with an effective doubling time changing from 42 days to 15 days, the prescription dose of external beam radiotherapy needs to be increased from 75.6 to 81 Gy to maintain a target TCP of 80% for intermediate-risk prostate cancer. For (125)I implants, dose escalation from 152 to 160 Gy is required for the same target TCP. These data were calculated on the basis of an alpha/beta ratio of 3.1 Gy. Greater dose escalations are required if the alpha/beta ratio is 1.5 Gy (e.g., 88 Gy for external beam radiotherapy or 180 Gy for (125)I implantation for the same treatment outcome). Our study results showed that it is important to cover the entire tumor volume, including all aggressive spots, with the desired prescription dose, especially for low-dose-rate brachytherapy.

CONCLUSION

Dose escalation is necessary to offset the accelerated tumor cell repopulation during prolonged treatment courses. This study provides a preliminary estimate of the dose escalation for prostate cancer based on the in situ measurements of potential doubling time and radiobiologic models. The proposed dose prescriptions are technically feasible for clinical trials.

Interpolation and extrapolation of dose measurements with different detector sizes to improve the spatial resolution of radiotherapy dosimetry as demonstrated for helical tomotherapy

A new technique for intensity modulated radiation therapy (IMRT) delivery is helical tomotherapy (HT). Like most IMRT delivery methods, HT utilizes many small fields as part of the treatment plan, which can be difficult to characterize. A novel technique for small field characterization, based on inter- and extrapolation of ion chamber readings, is presented in the context of HT. As a fan beam is characterized by its thickness and output factor, plane parallel chambers with different active volumes were used to scan the fan beam profiles. The fan beam thickness (FBT) can be determined from the thickness measured with the chamber by extrapolating to an infinitesimally small chamber size. The effective output was derived from the integral under the dose profile divided by the FBT. This was done for five FBTs and demonstrated a sharp fall off in dose when the FBT decreased below 8 mm. Similar techniques can be applied to other IMRT techniques to improve the characterization of various beam parameters.

Planning evaluation of radiotherapy for complex lung cancer cases using helical tomotherapy

Lung cancer treatment is one of the most challenging fields in radiotherapy. The aim of the present study was to investigate what role helical tomotherapy (HT), a novel approach to the delivery of highly conformal dose distributions using intensity-modulated radiation fan beams, can play in difficult cases with large target volumes typical for many of these patients. Tomotherapy plans were developed for 15 patients with stage III inoperable non-small-cell lung cancer. While not necessarily clinically indicated, elective nodal irradiation was included for all cases to create the most challenging scenarios with large target volumes. A 2 cm margin was used around the gross tumour volume (GTV) to generate primary planning target volume (PTV2) and 1 cm margin around elective nodes for secondary planning target volume (PTV1) resulting in PTV1 volumes larger than 1000 cm3 in 13 of the 15 patients. Tomotherapy plans were created using an inverse treatment planning system (TomoTherapy Inc.) based on superposition/convolution dose calculation for a fan beam thickness of 25 mm and a pitch factor between 0.3 and 0.8. For comparison, plans were created using an intensity-modulated radiation therapy (IMRT) approach planned on a commercial treatment planning system (TheraplanPlus, Nucletron). Tomotherapy delivery times for the large target volumes were estimated to be between 4 and 19 min. Using a prescribed dose of 60 Gy to PTV2 and 46 Gy to PTV1, the mean lung dose was 23.8+/-4.6 Gy. A 'dose quality factor' was introduced to correlate the plan outcome with patient specific parameters. A good correlation was found between the quality of the HT plans and the IMRT plans with HT being slightly better in most cases. The overlap between lung and PTV was found to be a good indicator of plan quality for HT. The mean lung dose was found to increase by approximately 0.9 Gy per percent overlap volume. Helical tomotherapy planning resulted in highly conformal dose distributions. It allowed easy achievement of two different dose levels in the target simultaneously. As the overlap between PTV and lung volume is a major predictor of mean lung dose, future work will be directed to control of margins. Work is underway to investigate the possibility of breath-hold techniques for tomotherapy delivery to facilitate this aim.

Dose calibration of nonconventional treatment systems applied to helical tomotherapy

Current dosimetric protocols based on the absorbed dose (AAPM TG-51 and IAEA TRS-398 protocols) require calibration measurements under reference conditions. For some radiotherapy systems, this requirement cannot be met, and calibration has to be performed under nonreference experimental conditions. In order to solve this problem, both protocols can be extended by inclusion of the measured-to-reference conversion factor, k(mr). In order to determine this factor, basic dosimetric quantities, like stopping power ratios, mass attenuation coefficients and chamber correction factors have to be calculated. If measurements are not feasible, accurate Monte Carlo modeling is required. The extension of the protocols is illustrated using the case of the helical tomotherapy radiation unit, where the typical calibration measurement conditions are the 10 x 5 cm2 field size and the 85 cm surface source distance, limited by the system design. It was calculated that the k(mr) factor for this conditions is close to unity (0.997+/-0.001). In addition, the deviation of the measurement conditions from the reference conditions results in the change of the quality conversion factor (approximately 0.995-0.998, depending on the ionization chamber used). This change is the same regardless of the used calibration protocol. For smaller field sizes the corrections become more significant, resulting in the total correction factor compared to the reference conditions of up to 1.5% for the smallest considered field size of 2 x 2 cm2.

Dosimetric accuracy of Kodak EDR2 film for IMRT verifications

Patient-specific intensity-modulated radiotherapy (IMRT) verifications require an accurate two-dimensional dosimeter that is not labor-intensive. We assessed the precision and reproducibility of film calibrations over time, measured the elemental composition of the film, measured the intermittency effect, and measured the dosimetric accuracy and reproducibility of calibrated Kodak EDR2 film for single-beam verifications in a solid water phantom and for full-plan verifications in a Rexolite phantom. Repeated measurements of the film sensitometric curve in a single experiment yielded overall uncertainties in dose of 2.1% local and 0.8% relative to 300 cGy. 547 film calibrations over an 18-month period, exposed to a range of doses from 0 to a maximum of 240 MU or 360 MU and using 6 MV or 18 MV energies, had optical density (OD) standard deviations that were 7%-15% of their average values. This indicates that daily film calibrations are essential when EDR2 film is used to obtain absolute dose results. An elemental analysis of EDR2 film revealed that it contains 60% as much silver and 20% as much bromine as Kodak XV2 film. EDR2 film also has an unusual 1.69:1 silver:halide molar ratio, compared with the XV2 film's 1.02:1 ratio, which may affect its chemical reactions. To test EDR2's intermittency effect, the OD generated by a single 300 MU exposure was compared to the ODs generated by exposing the film 1 MU, 2 MU, and 4 MU at a time to a total of 300 MU. An ion chamber recorded the relative dose of all intermittency measurements to account for machine output variations. Using small MU bursts to expose the film resulted in delivery times of 4 to 14 minutes and lowered the film's OD by approximately 2% for both 6 and 18 MV beams. This effect may result in EDR2 film underestimating absolute doses for patient verifications that require long delivery times. After using a calibration to convert EDR2 film's OD to dose values, film measurements agreed within 2% relative difference and 2 mm criteria to ion chamber measurements for both sliding window and step-and-shoot fluence map verifications. Calibrated film results agreed with ion chamber measurements to within 5 % /2 mm criteria for transverse-plane full-plan verifications, but were consistently low. When properly calibrated, EDR2 film can be an adequate two-dimensional dosimeter for IMRT verifications, although it may underestimate doses in regions with long exposure times.

Accurate convolution/superposition for multi-resolution dose calculation using cumulative tabulated kernels

Convolution/superposition (C/S) is regarded as the standard dose calculation method in most modern radiotherapy treatment planning systems. Different implementations of C/S could result in significantly different dose distributions. This paper addresses two major implementation issues associated with collapsed cone C/S: one is how to utilize the tabulated kernels instead of analytical parametrizations and the other is how to deal with voxel size effects. Three methods that utilize the tabulated kernels are presented in this paper. These methods differ in the effective kernels used: the differential kernel (DK), the cumulative kernel (CK) or the cumulative-cumulative kernel (CCK). They result in slightly different computation times but significantly different voxel size effects. Both simulated and real multi-resolution dose calculations are presented. For simulation tests, we use arbitrary kernels and various voxel sizes with a homogeneous phantom, and assume forward energy transportation only. Simulations with voxel size up to 1 cm show that the CCK algorithm has errors within 0.1% of the maximum gold standard dose. Real dose calculations use a heterogeneous slab phantom, both the 'broad' (5 x 5 cm2) and the 'narrow' (1.2 x 1.2 cm2) tomotherapy beams. Various voxel sizes (0.5 mm, 1 mm, 2 mm, 4 mm and 8 mm) are used for dose calculations. The results show that all three algorithms have negligible difference (0.1%) for the dose calculation in the fine resolution (0.5 mm voxels). But differences become significant when the voxel size increases. As for the DK or CK algorithm in the broad (narrow) beam dose calculation, the dose differences between the 0.5 mm voxels and the voxels up to 8 mm (4 mm) are around 10% (7%) of the maximum dose. As for the broad (narrow) beam dose calculation using the CCK algorithm, the dose differences between the 0.5 mm voxels and the voxels up to 8 mm (4 mm) are around 1% of the maximum dose. Among all three methods, the CCK algorithm is demonstrated to be the most accurate one for multi-resolution dose calculations.

Consistency test of the electron transport algorithm in the GEANT4 Monte Carlo code

In this work, the condensed history algorithm in GEANT4 (version 4.6.2.p01) is examined. We performed simulations of an ionization chamber composed of water for 1.25 MeV incident photon beams under Fano conditions, and evaluated the consistency of the cavity response for several combinations of electron transport parameters. GEANT4 permits electrons to reach geometric boundaries in large steps, and underestimates lateral displacement near interfaces. Step size artefacts due to distortions in electron fluence and angular distributions reduce the cavity dose by up to 39%. Accurate cavity response can be achieved using severe user-imposed step size restrictions. We suggest that improvements in the electron transport algorithm in GEANT4 should address the handling of boundary crossing.

[Radiation therapy for prostate cancer]

The advent of image-guided or CT-based treatment planning, such as 3D-conformal or intensity modulated radiation therapy has allowed safe delivery of high- dose radiation therapy in men with prostate cancer with an acceptable rate of side effects and complications. External-beam radiation therapy designed to decrease exposure of normal tissues using methods such as 3D-conformal treatment planning is under clinical evaluation. Randomized trial shows escalation of radiation dose leads to improved biochemical disease-free survival for a select group of patients. Interstitial brachytherapy has been also employed for patients with selected for favorable characteristics, including low Gleason score, low PSA level, and stage T1 to T2 tumors. Long-term follow-up of patients is necessary to assess treatment efficacy and side effects.

Surgery or radiation: what is the optimal management for locally advanced prostate cancer?

INTRODUCTION

To date, randomized trials comparing radiotherapy to surgery for adenocarcinoma of the prostate are few. Lacking, are randomized comparisons between treatment modalities for the patient with high-risk locally advanced disease. Hence, there is a need to determine which approach offers superior results in these patients who comprise a significant proportion of those dying of prostate cancer. In this short review we highlight key studies that may provide interim answers while awaiting definitive results from randomized studies.

MATERIAL AND METHODS

A MEDLINE literature review was performed of studies evaluating current treatment modalities for high-risk (TNM stage >T2b, PSA>10, Gleason > or =8) prostate cancer. Publications from 1975 to present were searched using the keywords: prostate cancer, locally advanced prostate cancer, high-risk prostate cancer, prostatectomy, external beam radiation, brachytherapy, and PSA-doubling time.

RESULTS

Comparisons of different treatment modalities are difficult due to many factors, from uncertainties in clinical staging to the questionable equivalence of PSA failure. However, the general consensus is that low dose rate brachytherapy monotherapy is not ideal for high-risk patients. There are several options for combination therapy which show moderately good survival results. Because of the lack of prospective randomized trials comparing these approaches, matched analyses with uniform patient treatment and pathological review may provide an interim answer.

CONCLUSION

The optimal management for patients with locally advanced prostate cancer is unclear. While randomized clinical trials will eventually shed light on this question, interim solutions may provide some answers in the short term.

Image-guided radiotherapy for prostate cancer by CT–linear accelerator combination: Prostate movements and dosimetric considerations

PURPOSE

Multiple studies have indicated that the prostate is not stationary and can move as much as 2 cm. Such prostate movements are problematic for intensity-modulated radiotherapy, with its associated tight margins and dose escalation. Because of these intrinsic daily uncertainties, a relative generous "margin" is necessary to avoid marginal misses. Using the CT-linear accelerator combination in the treatment suite (Primatom, Siemens), we found that the daily intrinsic prostate movements can be easily corrected before each radiotherapy session. Dosimetric calculations were performed to evaluate the amount of discrepancy of dose to the target if no correction was done for prostate movement.

METHODS AND MATERIALS

The Primatom consists of a Siemens Somatom CT scanner and a Siemens Primus linear accelerator installed in the same treatment suite and sharing a common table/couch. The patient is scanned by the CT scanner, which is movable on a pair of horizontal rails. During scanning, the couch does not move. The exact location of the prostate, seminal vesicles, and rectum are identified and localized. These positions are then compared with the planned positions. The daily movement of the prostate and rectum were corrected for and a new isocenter derived. The patient was treated immediately using the new isocenter.

RESULTS

Of the 108 patients with primary prostate cancer studied, 540 consecutive daily CT scans were performed during the last part of the cone down treatment. Of the 540 scans, 46% required no isocenter adjustments for the AP-PA direction, 54% required a shift of > or =3 mm, 44% required a shift of >5 mm, and 15% required a shift of >10 mm. In the superoinferior direction, 27% required a shift of >3 mm, 25% required a shift of >5 mm, and 4% required a shift of >10 mm. In the right-left direction, 34% required a shift of >3 mm, 24% required a shift of >5 mm, and 5% required a shift of >10 mm. Dosimetric calculations for a typical case of prostate cancer using intensity-modulated radiotherapy with 5-mm margin coverage from the clinical target volume (prostate gland) was performed. With a posterior shift of 10 mm for the prostate, the dose coverage dropped from 95-107% to 71-100% coverage.

CONCLUSION

We have described a technique that corrects for the daily prostate motion, allowing for extremely precise prostate cancer treatment. This technique has significant implications for dose escalation and for decreasing rectal complications in the treatment of prostate cancer.

Evaluation of inter- and intrafraction organ motion during intensity modulated radiation therapy (IMRT) for localized prostate cancer measured by a newly developed on-board image-guided system

PURPOSE

To investigate prostatic organ motion at both setup and intrafraction using an onboard image-guided system. An intrafraction field-based repositioning method also was evaluated.

MATERIALS AND METHODS

A dual fluoroscopy with amorphous-silicon flat panel (DFFP) system was used for the three-dimensional registration of implanted markers in the prostate of eight organ-confined cancer patients planned for treatment with intensity modulated radiation therapy (IMRT). Day-to-day motion errors were quantified and intrafraction displacements of more than +/-1 mm were corrected.

RESULTS

Among 214 fractions and 565 system views, day-to-day mean magnitude of marker discrepancy +/- standard deviation (SD) was 1.76 +/- 1.4 mm, 3.14 +/- 1.6 mm, and 3.78 +/- 2.4 mm in the right-left, cranial-caudal, and anterior-posterior directions, respectively. The intrafractional mean magnitude +/- SD of marker displacement was 0.45 +/- 0.7 mm, 1.08 +/- 1.38 mm and 1.45 +/- 1.70 mm in the right-left, cranial-caudal, and anterior-posterior directions, respectively. Intrafraction corrected sessions (84/214) showed a median (range) of motion of 0.1 mm (-1.2 to 0.7 mm), -0.2 mm (-2.1 to 1.1 mm), and -0.2 mm (-1.7 to 2.0 mm) in the right-left, cranial-caudal, and anterior-posterior directions, respectively.

CONCLUSION

Motion uncertainty can be considerably decreased with daily use of the DFFP system. Reduced intrafraction organ motion clearly endorsed the value of the repositioning approach, allowing a safer dose escalation protocol.

[A new area for radiotherapy with favourable features]

Radiotherapy is a complex medical speciality involving technology research, biology research and clinical research. All these basic researches are performed in order to optimise the management of cancer treatment patients. The aim of the present review is to present radiotherapy as a moving speciality whatever the concerned section. It will be particularly described the new approaches in terms of technology but also clinical developments.

Low-dose megavoltage cone-beam CT for radiation therapy

PURPOSE

The objective of this work was to demonstrate the feasibility of acquiring low-exposure megavoltage cone-beam CT (MV CBCT) three-dimensional (3D) image data of sufficient quality to register the CBCT images to kilovoltage planning CT images for patient alignment and dose verification purposes.

METHODS AND MATERIALS

A standard clinical 6-MV Primus linear accelerator, operating in arc therapy mode, and an amorphous-silicon (a-Si) flat-panel electronic portal-imaging device (EPID) were employed. The dose-pulse rate of 6-MV Primus accelerator beam was windowed to expose an a-Si flat panel by using only 0.02 to 0.08 monitor unit (MUs) per image. A triggered image-acquisition mode was designed to produce a high signal-to-noise ratio without pulsing artifacts. Several data sets were acquired for an anthropomorphic head phantom and frozen sheep and pig cadaver head, as well as for a head-and-neck cancer patient on intensity-modulated radiotherapy (IMRT). For each CBCT image, a set of 90 to 180 projection images incremented by 1 degree to 2 degrees was acquired. The two-dimensional (2D) projection images were then synthesized into a 3D image by use of cone-beam CT reconstruction. The resulting MV CBCT image set was used to visualize the 3D bony anatomy and some soft-tissue details. The 3D image registration with the kV planning CT was performed either automatically by application of a maximization of mutual information (MMI) algorithm or manually by aligning multiple 1D slices.

RESULTS

Low-noise 3D MV CBCT images without pulsing artifacts were acquired with a total delivered dose that ranged from 5 to 15 cGy. Acquisition times, including image readout, were on the order of 90 seconds for 180 projection images taken through a continuous gantry rotation of 180 degrees. The processing time of the data required an additional 90 seconds for the reconstruction of a 256(3) cube with 1.0-mm voxel size. Implanted gold markers (1 mm x 3 mm) were easily visible or all exposure levels without artifacts. In general, the presence of high Z materials such as tooth fillings or implanted markers did not result in visible streak artifacts. The registration of structures such as the spinal canal and the nasopharynx in the MV CBCT and kV CT data sets was possible with millimeter and degree accuracy as assessed by displacement simulations and subsequent visual evaluation.

CONCLUSIONS

We believe that the quality of these images, along with the rapid acquisition and reconstruction times, demonstrates that MV CBCT performed by use of a standard linear accelerator equipped with a flat-panel imager can be applied clinically for patient alignment.

Measurements and treatment planning calculations of electron dose distributions below bolus edges

Electron dose distributions below bolus edges of various thicknesses and angles were measured using a diode in a water phantom. The measurements were compared with treatment planning calculations using the gamma method. Below 5 mm boluses, the dose variations were acceptable for all angles, while for boluses thicker than 5mm, the lowest edge gave the smallest dose variations. The calculated dose distributions agreed well with the measured dose distributions.

Online compensation for target motion with scanned particle beams: simulation environment

Target motion is one of the major limitations of each high precision radiation therapy. Using advanced active beam delivery techniques, such as the magnetic raster scanning system for particle irradiation, the interplay between time-dependent beam and target position heavily distorts the applied dose distribution. This paper presents a simulation environment in which the time-dependent effect of target motion on heavy-ion irradiation can be calculated with dynamically scanned ion beams. In an extension of the existing treatment planning software for ion irradiation of static targets (TRiP) at GSI, the expected dose distribution is calculated as the sum of several sub-distributions for single target motion states. To investigate active compensation for target motion by adapting the position of the therapeutic beam during irradiation, the planned beam positions can be altered during the calculation. Applying realistic parameters to the planned motion-compensation methods at GSI, the effect of target motion on the expected dose uniformity can be simulated for different target configurations and motion conditions. For the dynamic dose calculation, experimentally measured profiles of the beam extraction in time were used. Initial simulations show the feasibility and consistency of an active motion compensation with the magnetic scanning system and reveal some strategies to improve the dose homogeneity inside the moving target. The simulation environment presented here provides an effective means for evaluating the dose distribution for a moving target volume with and without motion compensation. It contributes a substantial basis for the experimental research on the irradiation of moving target volumes with scanned ion beams at GSI which will be presented in upcoming papers.

MCDE: a new Monte Carlo dose engine for IMRT

A new accurate Monte Carlo code for IMRT dose computations, MCDE (Monte Carlo dose engine), is introduced. MCDE is based on BEAMnrc/DOSXYZnrc and consequently the accurate EGSnrc electron transport. DOSXYZnrc is reprogrammed as a component module for BEAMnrc. In this way both codes are interconnected elegantly, while maintaining the BEAM structure and only minimal changes to BEAMnrc.mortran are necessary. The treatment head of the Elekta SLiplus linear accelerator is modelled in detail. CT grids consisting of up to 200 slices of 512 x 512 voxels can be introduced and up to 100 beams can be handled simultaneously. The beams and CT data are imported from the treatment planning system GRATIS via a DICOM interface. To enable the handling of up to 50 x 10(6) voxels the system was programmed in Fortran95 to enable dynamic memory management. All region-dependent arrays (dose, statistics, transport arrays) were redefined. A scoring grid was introduced and superimposed on the geometry grid, to be able to limit the number of scoring voxels. The whole system uses approximately 200 MB of RAM and runs on a PC cluster consisting of 38 1.0 GHz processors. A set of in-house made scripts handle the parallellization and the centralization of the Monte Carlo calculations on a server. As an illustration of MCDE, a clinical example is discussed and compared with collapsed cone convolution calculations. At present, the system is still rather slow and is intended to be a tool for reliable verification of IMRT treatment planning in the case of the presence of tissue inhomogeneities such as air cavities.

Regularization of inverse planning for intensity-modulated radiotherapy

The performance of a variational regularization technique to improve robustness of inverse treatment planning for intensity modulated radiotherapy is analyzed and tested. Inverse treatment planning is based on the numerical solutions to the Fredholm integral equation of the first kind which is ill-posed. Therefore, a fundamental problem with inverse treatment planning is that it may exhibit instabilities manifested in nonphysical oscillations in the beam intensity functions. To control the instabilities, we consider a variational regularization technique which can be applied for the methods which minimize a quadratic objective function. In this technique, the quadratic objective function is modified by adding of a stabilizing functional that allows for arbitrary order regularization. An optimal form of stabilizing functional is selected which allows for both regularization and good approximation of beam intensity functions. The regularized optimization algorithm is shown, by comparison for a typical case of a head-and-neck cancer treatment, to be significantly more accurate and robust than the standard approach, particularly for the smaller beamlet sizes.

Performance analysis of a film dosimetric quality assurance procedure for IMRT with regard to the employment of quantitative evaluation methods

A system for dosimetric verification of intensity-modulated radiotherapy (IMRT) treatment plans using absolute calibrated radiographic films is presented. At our institution this verification procedure is performed for all IMRT treatment plans prior to patient irradiation. Therefore clinical treatment plans are transferred to a phantom and recalculated. Composite treatment plans are irradiated to a single film. Film density to absolute dose conversion is performed automatically based on a single calibration film. A software application encompassing film calibration, 2D registration of measurement and calculated distributions, image fusion, and a number of visual and quantitative evaluation utilities was developed. The main topic of this paper is a performance analysis for this quality assurance procedure, with regard to the specification of tolerance levels for quantitative evaluations. Spatial and dosimetric precision and accuracy were determined for the entire procedure, comprising all possible sources of error. The overall dosimetric and spatial measurement uncertainties obtained thereby were 1.9% and 0.8 mm respectively. Based on these results, we specified 5% dose difference and 3 mm distance-to-agreement as our tolerance levels for patient-specific quality assurance for IMRT treatments.