P Haering

Intensity modulated radiotherapy (IMRT) for recurrent, residual, or untreated skull-base meningiomas: preliminary clinical experience

OBJECTIVE To investigate the feasibility of using intensity modulated radiotherapy (IMRT) for complex-shaped benign meningiomas of the skull base and report clinical experience. METHODS Twenty patients with benign skull-base meningiomas WHO degrees I (histopathologically proven in 16/20) were treated with IMRT between June 1998 and August 1999. Each tumor was complex in shape and adherent to, or encompassed, organs at risk (cranial nerves, optic apparatus, and brainstem). All patients, immobilized in a customized head mask integrated into a stereotactic system, were planned on an inverse treatment planning system using 5 or 7 coplanar, equidistant beams and 5 intensity steps. Each treatment plan was verified extensively before treatment. Follow-up with MRI and clinical examination was performed at 6 and 18 weeks and every 6 months thereafter. RESULTS Target volumes ranged from 27 to 278 cc (median: 108 cc). Mean dose in 32 fractions ranged between 55.8 and 58.2 Gy. At median follow-up of 36 months (range: 31-43 months), pre-existing neurologic symptoms improved in 12/20 (60%), remained stable in 7/20 (35%), and worsened in 1 (5%) patient. Radiographic follow-up revealed significant tumor shrinkage 6 weeks post-IMRT in 2 patients and partial remission in 3 more patients at 9-17 months; other tumor volumes remained stable. There was no radiation-induced peritumoral edema, increase in tumor size, or new onset of neurologic deficits. Transient acute treatment side effects included nausea and vomiting and single occurrences of conjunctivitis/increased tearing and serous tympanitis. CONCLUSION IMRT in the treatment of central nervous system meningiomas is feasible and safe, offering highly conformal irradiation for complex-shaped skull-base tumors while sparing adjacent critical structures. If the tumor remissions seen here are found in the ongoing treatments, IMRT may be considered the treatment of choice for inoperable or subtotally resected meningiomas and for otherwise difficult-to-treat, complex-shaped tumors of the central nervous system adjacent to critical structures, with the potential of dose escalation for malignant tumors.

4D-CT-based target volume definition in stereotactic radiotherapy of lung tumours: Comparison with a conventional technique using individual margins

PURPOSE To investigate the dosimetric benefit of integration of 4D-CT in the planning target volume (PTV) definition process compared to conventional PTV definition using individual margins in stereotactic body radiotherapy (SBRT) of lung tumours. MATERIAL AND METHODS Two different PTVs were defined: PTV(conv) consisting of the helical-CT-based clinical target volume (CTV) enlarged isotropically for each spatial direction by the individually measured amount of motion in the 4D-CT, and PTV(4D) encompassing the CTVs defined in the 4D-CT phases displaying the extremes of the tumour position. Tumour motion as well as volumetric and dosimetric differences and relations of both PTVs were evaluated. RESULTS Volumetric examinations revealed a significant reduction of the mean PTV by 4D-CT from 57.7 to 40.7 cm(3) (31%) (p<0.001). A significant inverse correlation was found for the motion vector and the amount of inclusion of PTV(4D) in PTV(conv) (r=-0.69, 90% confidence limits: -0.87 and -0.34, p=0.007). Mean lung dose (MLD) was decreased significantly by 17% (p<0.001). CONCLUSIONS In SBRT of lung tumours the mere use of individual margins for target volume definition cannot compensate for the additional effects that the implementation of 4D-CT phases can offer.

Radiobiological investigation of dose-rate effects in intensity-modulated radiation therapy.

Background and Purpose:Intensity-modulated radiation therapy (IMRT) has proven extraordinary capability in physical terms such as target conformity, dose escalation in the target volume, and sparing of neighboring organs at risk. The radiobiological consequences of the protracted dose delivery for cell survival and cell cycle progression are still unclear and shall be examined in this study.Material and Methods:Human lymphoblasts (TK6) and human melanoma cells (MeWo) were irradiated with protocols of increasing dose protraction. In addition, a new biophysical phantom was developed and used to transfer clinical IMRT plans to experimental cell irradiation. Clonogenic cell survival and cell cycle analysis were performed after various irradiation experiments.Results:In a first series of experiments, melanoma cells showed a highly significant increase of survival of 6.0% after protracted dose delivery of 2 Gy compared to conventional fast application with the same dose. Lymphoblastoid cells also showed a significant increase of survival of 2.2%. Experiments with patient plans in the phantom confirmed the trend of increased cell survival after protracted dose delivery. Cells were irradiated at 13 points in four different IMRT plans. In comparison to irradiation with application of the same dose in a classic four-field box, a significantly increased survival of 5.1% (mean value) was determined.Conclusion:Even at fraction times of 15–30 min the protracted dose delivery increases the survival rates in cell culture. The altered survival rates indicate the importance of the dose rate in the effectivity of IMRT. Besides physical parameters the consideration of biological factors might contribute to the optimization of IMRT in the future.Hintergrund und Ziel:Die intensitätsmodulierte Strahlentherapie (IMRT) ist ein modernes Radiotherapieverfahren, welches unter physikalischen Gesichtspunkten wie der Zielkonformität, Dosiseskalation und Schonung von Risikostrukturen hervorragende Ergebnisse erzielen kann. Doch die strahlenbiologischen Konsequenzen für Zellüberleben und Zellzyklusprogression, die sich aus der protrahierten Dosisapplikation ergeben könnten, sind noch unklar und sollen in dieser Arbeit untersucht werden.Material und Methodik:Humane Lymphoblasten (TK6) und humane Melanomzellen (MeWo) wurden mit Protokollen ansteigender Dosisprotrahierung bestrahlt. Zudem wurde ein neuartiges biophysikalisches Phantom entwickelt, welches die Übertragung klinischer IMRT-Pläne in ein vielseitiges experimentelles Setup ermöglicht. Klonogenes Zellüberleben sowie Zellzyklusprogression nach verschiedenen Bestrahlungsexperimenten wurden untersucht.Ergebnisse:In einer ersten Versuchsreihe zeigten die Melanomzellen ein signifikant um 6,0% erhöhtes Zellüberleben, wenn 2 Gy stark protrahiert appliziert wurden, verglichen mit schneller herkömmlicher Bestrahlung. Auch die Lymphoblasten zeigten ein um 2,2% signifikant erhöhtes Überleben. Die Experimente im Phantom mit Patientenplänen bestätigten den Trend des erhöhten Überlebens nach Dosisprotrahierung. Die Zellen wurden an 13 verschiedenen Punkten in vier IMRT-Plänen bestrahlt. Im Vergleich zur Bestrahlung mit der gleichen Dosis in einer konventionellen Vierfelderbox war das Überleben nach IMRT durchschnittlich um 5,1% erhöht.Schlussfolgerung:Selbst bei Fraktionszeiten von 15–30 min führt die protrahierte Dosisapplikation zu einem erhöhten Zellüberleben in Zellkultur. Die veränderten Überlebensraten zeigen die Bedeutung der Dosisrate für die Effektivität der IMRT. Neben physikalischen Parametern der Planbeurteilung müssen auch biologische Parameter zur weiteren Optimierung der IMRT herangezogen werden.

Influence of intra-fractional breathing movement in step-and-shoot IMRT

Efforts have been made to extend the application of intensity-modulated radiotherapy to a variety of organs. One of the unanswered questions is whether breathing-induced organ motion may lead to a relevant over- or underdosage, e.g., in treatment plans for the irradiation of lung cancer. Theoretical considerations have been made concerning the different kinds of IMRT but there is still a lack of experimental data. We examined 18 points in a fraction of a clinical treatment plan of a NSCLC delivered in static IMRT with a new phantom and nine ionization chambers. Measurements were performed at a speed of 12 and 16 breathing cycles per minute. The dose differences between static points and moving target points ranged between -2.4% and +5.5% (mean: +0.2%, median: -0.1%) when moving with 12 cycles min(-1) and between -3.6% and +5.0% (mean: -0.4%, median: -0.6%) when moving with 16 cycles min(-1). All differences of measurements with and without movements were below 5%, with one exception. In conclusion, our results underline that at least in static IMRT breathing effects (concerning target dose coverage) due to interplay effects between collimator leaf movement and target movement are of secondary importance and will not reduce the clinical value of IMRT in the step-and-shoot technique for irradiation of thoracic targets.

INTENSITY-MODULATED RADIOTHERAPY OF THE FEMALE BREAST

Current methods for intensity-modulated radiotherapy (IMRT) in breast cancer use forward planning based on equivalent radiological path length to design intensity modulated tangential beams. Compared to conventional tangential techniques, dose reduction of organs at risk is limited using these techniques. We developed a method for intensity modulation of multiple beams for adjuvant radiotherapy of breast cancer by application of a virtual bolus defined on CT for inverse optimization. This method enables multibeam IMRT, which provides improved sparing of lung and heart tissue. In this paper, we present the general aspects of this approach and an evaluation of the optimum beam configuration for IMRT based on inverse treatment planning. We compared this method to conventional techniques. Different clinical examples illustrate the possible indications and feasibility of this new approach. This method is superior to conventional techniques because of the reduction of high-dose area of a substantial cardiac volume in those cases where the parasternal lymph nodes are part of the target volume.

Dosimetric integration of daily mega-voltage cone-beam CT for image-guided intensity-modulated radiotherapy

PurposeThe goal of this work was to compare different methods of incorporating the additional dose of mega-voltage cone-beam CT (MV-CBCT) for image-guided intensity modulated radiotherapy (IMRT) of different tumor entities.Material and methodsThe absolute dose delivered by the MV-CBCT was calculated and considered by creating a scaled IMRT plan (scIMRT) by renormalizing the clinically approved plan (orgIMRT) so that the sum with the MV-CBCT dose yields the same prescribed dose. In the other case, a newly optimized plan (optIMRT) was generated by including the dose distribution of the MV-CBCT as pre-irradiation. Both plans were compared with the orgIMRT plan and a plan where the last fraction was skipped.ResultsNo significant changes were observed regarding the 95% conformity index of the target volume. The mean dose of the organs at risk (OAR) increased by approx. 7% for the scIMRT plan and 5% for the optIMRT plan. A significant increase of the mean dose to the outline contour was observed, ranging from 3.1 ± 1.3% (optIMRT) to 13.0 ± 6.1% (scIMRT) for both methods over all entities. If the dose of daily MV-CBCT would have been ignored, the additional dose accumulated to nearly a whole treatment fraction with a general increase of approx. 10% to the OARs and approx. 4% to the target volume.ConclusionBoth methods of incorporating the additional MV-CBCT dose into the treatment plan are suitable for clinical practice. The dose distribution of the target volume could be achieved as conformal as with the orgIMRT plan, while only a moderate increase of mean dose to OAR was observed.ZusammenfassungZielsetzungVergleich verschiedener Methoden zur Integration der zusätzlichen Dosis von täglichen Megavolt-Cone-Beam-CTs (MV-CBCT) für die bildgestützte intensitätsmodulierte Strahlentherapie (IMRT) verschiedener Tumorentitäten.Material und MethodenDer absolute zusätzliche Dosisbeitrag durch das MV-CBCT wurde jeweils berechnet. In einem Fall wurde durch Re-Normalisierung ein skalierter IMRT-Plan (scIMRT) erzeugt, bei dem die Summe aus IMRT-Plan und MV-CBCT die verschriebene Dosis des klinisch akzeptierten IMRT-Plans (orgIMRT) ergab. Im anderen Fall wurde ein neu optimierter IMRT-Plan (optIMRT) erstellt, der die zusätzliche Dosis des MV-CBCT als Vorbelastung berücksichtigt. Beide Pläne wurden mit dem orgIMRT-Plan und einem Plan, bei dem die letzte Fraktion weggelassen wurde, verglichen.ErgebnisseEs zeigten sich keine signifikanten Veränderungen des 95%-Konformität-Index des Zielvolumens. Die mittlere Dosisbelastung der Risikoorgane (OAR) erhöhte sich beim scIMRT-Plan um etwa 7% und beim optIMRT-Plan um etwa 5%. Es zeigte sich ein signifikanter Anstieg der mittleren Dosis der Außenkontur des Patienten im Bereich von 3,1 ± 1,3% (optIMRT) bis 13,0 ± 6,1% (scIMRT) bei beiden Methoden über alle Lokalisationen. Der zusätzliche Dosisbeitrag eines täglichen MV-CBCT lag in der Größenordnung einer Fraktion und führte insgesamt zu einer Zunahme der Belastung der Risikoorgane um etwa 10% und der Dosis im Zielvolumen um etwa 4%.SchlussfolgerungenBeide Methoden zur Integration der zusätzlichen Dosis eines MV-CBCT in den Bestrahlungsplan eigenen sich für die klinische Routine. Die Dosisverteilung des Zielvolumens war im Vergleich zur IMRT ähnlich konformal, bei moderatem Anstieg der mittleren Dosisbelastung der Risikoorgane.

Feasibility of polymer gel-based measurements of radiation isocenter accuracy in magnetic fields

For conventional irradiation devices, the radiation isocenter accuracy is determined by star shot measurements on films. In magnetic resonance (MR)-guided radiotherapy devices, the results of this test may be altered by the magnetic field and the need to align the radiation and imaging isocenter may require a modification of measurement procedures. Polymer dosimetry gels (PG) may offer a way to perform both, the radiation and imaging isocenter test, however, first it has to be shown that PG reveal results comparable to the conventionally applied films. Therefore, star shot measurements were performed at a linear accelerator using PG as well as radiochromic films. PG were evaluated using MR imaging and the isocircle radius and the distance between the isocircle center and the room isocenter were determined. Two different types of experiments were performed: i) a standard star-shot isocenter test and (ii) a star shot, where the detectors were placed between the pole shoes of an experimental electro magnet operated either at 0 T or 1 T. For the standard star shot, PG evaluation was independent of the time delay after irradiation (1 h, 24 h, 48 h and 216 h) and the results were comparable to those of film measurements. Within the electro magnet, the isocircle radius increased from 0.39  ±  0.01 mm to 1.37  ±  0.01 mm for the film and from 0.44  ±  0.02 mm to 0.97  ±  0.02 mm for the PG-measurements, respectively. The isocenter distance was essentially dependent on the alignment of the magnet to the isocenter and was between 0.12  ±  0.02 mm and 0.82  ±  0.02 mm. The study demonstrates that evaluation of the PG directly after irradiation is feasible, if only geometrical parameters are of interest. This allows using PG for star shot measurements to evaluate the radiation isocenter accuracy with comparable accuracy as with radiochromic films.

SU‐F‐T‐316: A Model to Deal with Dosimetric and Delivery Uncertainties in Radiotherapy Treatment Planning

PURPOSE The conventional way of dealing with uncertainties resulting from dose calculation or beam delivery in IMRT, is to do verification measurements for the plan in question. Here we present an alternative based on recommendations given in the AAPM 142 report and treatment specific parameters that model the uncertainties for the plan delivery. METHODS Basis of the model is the assignment of uncertainty parameters to all segment fields or control point sequences of a plan. The given field shape is analyzed for complexity, dose rate, number of MU, field size related output as well as factors for in/out field position and penumbra regions. Together with depth related uncertainties, a 3D matrix is generated by a projection algorithm. Patient anatomy is included as uncertainty CT data set as well. Therefore, object density is classified in 4 categories close to water, lung, bone and gradient regions with additional uncertainties. The result is then exported as a DICOM dose file by the software tool (written in IDL, Exelis), having the given resolution and target point. RESULTS Uncertainty matrixes for several patient cases have been calculated and compared side by side in the planning system. The result is not quite always intuitive but it clearly indicates high and low uncertainties related to OARs and target volumes as well as to measured gamma distributions.ConclusionThe imported uncertainty datasets may help the treatment planner to understand the complexity of the treatment plan. He then might decide to change the plan to produce a more suited uncertainty distribution, e.g. by changing the beam angles the high uncertainty spots can be influenced or try to use another treatment setup, resulting in a plan with lower uncertainties. A next step could be to include such a model into the optimization algorithm to add a new dose uncertainty constraint.

SU-E-T-773: Use of a Commercial TPS for Deriving Exit Dose Distribution for Patient Specific QA with An EPID

Purpose: A method to derive exit dose distributions with a commercial treatment planning system for comparison to Epid measured doses. Methods: In contrast to published methods, we present an approach that uses a treatment planning system (Raystation, Raysearch) to calculate exit dose patterns based on a modified patient CT dataset. The EPID is assumed as water equivalent and therefore is represented as water filled ring in the images. This is done by: -Export a renamed copy of the patient CT and plan data set. —Manipulate the CT-data to a field of view of 1000 mm. — Insert the water filled cylinder ring, centered to the target point. — Manipulate plan file with the new target and dose prescription point. — Import and recalculate dose on the manipulated data set. — Export and extract exit field dose matrix from the cylinder (optional entrance dose). — Calibrate matrix to size and rebin data from the cylinder to EPID equivalent data. — Correct Epid measurement for scatter, position and field size. -Evaluate and compare data in Verisoft (PTW). Data manipulation and extraction is done by a simple tool (IDL, Exelis). Results: This method was tested on a Siemens Artiste 6MV for field sizes up to 27×27cm2 limited by used geometry and EPID size. First phantom measurements show good results for fields up to 20×20cm (pass rate > 95% for 3%, 3mm Gamma index) while larger fields have higher discrepancies towards the field edges. This might Result from off axis softening of the beam and the higher sensitivity of the detector to beam scatter. Conclusion: This method might simplify the use of exit dosimetry with the EPID for patient specific QA as it uses the dose calculation of a commercial treatment planning system. The concept was proven by phantom data sets, giving acceptable results.

SU‐E‐T‐124: A Modified Winston Lutz Test Enabling Beam to Laser Angle Measurements

PURPOSE To present a modified Winston-Lutz-Test procedure able to measure beam and laser angles. METHODS Room lasers have not only to indicate the isocenter spot but should also be aligned to the central beam axis. Therefore a modified WL test, based on a cube phantom made of low density foam material was developed. The classical steel sphere in the center is surrounded by 8 additional smaller spheres located near the cube corners. Surface markers on the cube indicate the position of the spheres and are used for easy setup to the lasers. Measurements are made with a field size covering all spheres in the well known way, ideally with a gantry mounted EPID. Result is an image of in total 9 spheres that is influenced by the distances and incoming beam directions. An automated template based detection algorithm then searches the image for the spheres as well as for the outside field boundaries. Knowing the phantom geometry, it is now easy to calculate the following parameters: Position of center sphere and laser to central axis of the beam, beam angle to the orientation of the phantom and the distance of the cube to the radiation source. Calculation result s then can be used to correct the phantom position and orientation. A transfer device equipped with a finder sight then allows to set the lasers. RESULTS Test measurements were taken at a Siemens Artiste. Here the detection accuracy for angles and positions was tested. For smaller angles the automated detection works quite well within an accuracy of around 0.1° (max error 0.2°). Position detection was below 1/10mm and showed clearly the effects of Gantry and collimator sag. CONCLUSIONS This method detects both, positions and angles of laser and beam, enabling a higher precision laser setup.