Prostate Radiotherapy Treatment Research Articles

Influence of the electron energy and number of beams on the absorbed dose distributions in radiotherapy of deep seated targets

With the advent of compact laser-based electron accelerators, there has been some renewed interest on the use of such charged particles for radiotherapy purposes. Traditionally, electrons have been used for the treatment of fairly superficial lesions located at depths of no more than 4cm inside the patient, but lately it has been proposed that by using very high energy electrons, i.e. those with an energy in the order of 200–250MeV it should be possible to safely reach deeper targets. In this paper, we used a realistic patient model coupled with detailed Monte Carlo simulations of the electron transport in such a patient model to examine the characteristics of the resultant absorbed dose distributions as a function of both the electron beam energy as well as the number of beams for a particular type of treatment, namely, a prostate radiotherapy treatment. Each treatment is modeled as consisting of nine, five or three beam ports isocentrically distributed around the patient. An optimization algorithm is then applied to obtain the beam weights in each treatment plan. It is shown that for this particularly challenging case, both excellent target coverage and critical structure sparing can be obtained for energies in the order of 150MeV and for as few as three treatment ports, while significantly reducing the total energy absorbed by the patient with respect to a conventional megavoltage x-ray treatment.

Calypso® 4D Localization System: a review

AbstractPurposeCalypso® 4D Localization System is a system based on electromagnetic transponders detection enabling precise 3D localisation and continuous tracking of tumour target. This review intended to provide information in order to (1) show how Calypso® 4D Localization System works, (2) to present advantages and disadvantages of this system, (3) to gather information from several clinical studies and, finally, (4) to refer Calypso® System as a tool in dynamic multileaf collimator studies for target motion compensation.MethodsA structured search was carried out on B-On platform. The key words used in this research were ‘Calypso’, ‘Transponder’, ‘Electromagnetic Localization’, ‘Electromagnetic Tracking’, ‘Target Localization’, ‘Intrafraction Motion’ and ‘DMLC’.ReviewTreatment the implanted transponders are excited by an electromagnetic field and resonate back. These frequencies are detected and Calypso® software calculates the position of the transponders. If the movement detected is larger than the limits previously defined, irradiation can be stopped. The system has been proven to be submillimetre accurate.DiscussionCalypso® System has been presented as an accurate tool in prostate radiotherapy treatments. The application of this system to other clinical sites is being developed.ConclusionThe Calypso® System allows real-time localisation and monitoring of the target, without additional ionising radiation administration. It has been a very useful tool in prostate cancer treatment.

Superheated emulsions and track etch detectors for photoneutron measurements

This paper describes the criteria behind the selection of neutron detection techniques for photoneutron dosimetry as well as the methods adopted to obtain dosimetric readouts. The work was conducted within the framework of Working Group 9 (WG9 Radiation Protection Dosimetry in Medicine), coordinated by the European Radiation Dosimetry Group (EURADOS). WG9 research aims at estimating the risk of second cancer induction due to radiation therapy. Therefore, a comprehensive experimental programme was devised to measure doses received by non-target organs-at-risk (OAR) during radiation therapy. The techniques described in this work were selected and used for the neutron dosimetric assessment during in-phantom simulations of clinical prostate radiotherapy treatments, carried out in three European facilities. Non-conformal standard fields were used as a common reference between different facilities.Performing neutron measurements near linacs is a complex task, because of the intense pulsed photon primary field. Therefore, photon insensitive dosimeters such as superheated emulsions (SE) and solid state nuclear track detectors (SSNTD) were chosen. Their readout procedures were carefully assessed. Methods were developed to count the large number of tracks and bubbles in SE. These are described in detail in the present work, along with a brief introduction to the detector physics.

Dose deformation-invariance in adaptive prostate radiation therapy: Implication for treatment simulations

PurposeTo investigate dose deformation-invariance in adaptive prostate radiation treatment. MethodsA 19 patient prostate cancer-cohort with 8–13 CTs/patient is used. The 79.2Gy plans are developed on the reference image using seven 6 and 18MV intensity-modulated beams with identical RTOG 0126 objectives. Dose on the subsequent images is evaluated in two ways: (A1) Dose is recalculated on each image. (A2) The initially planned dose distribution is copied to each image. A2 assumes dose-invariance in the accelerator-coordinate-system. Effects of patient miss-alignment are simulated by 27 per-patient image shifts; 0 and ±10mm in left–right, anterior–posterior and superior–inferior directions. The per-voxel dose differences for each patient image, total accumulated patient dose, and dose–volume metrics (CTV-D98 and -D90, bladder- and rectum-D50, -D35, -D25 and -D15) are used to compare A1 and A2. ResultsThe per-voxel mean percent difference in A1 and A2 dose over all patient images at 6MV is (0.01±1.56)% and at 18MV is (0.00±0.96)%. For 6MV and 18MV plans, the root-mean-square-percentage-error (RMSPE) in A2 over all patient image shifts are CTV-D98=0.94 and 0.55, CTV-D90=0.92 and 0.55, rectum-D50, -D35, -D25 and -D15=1.00, 0.96, 0.86, 0.80 and 0.84, 0.96, 0.92, 1.05; and bladder-D50, -D35, -D25, -D15=1.07, 0.88, 0.78, 0.72 and 1.61, 0.93, 0.67, 0.51. The dose differences are not correlated to the dice-similarity coefficients; with respective correlation-coefficients for CTV, rectum and bladder being −0.17, −0.17 and 0.081. ConclusionsAssumption of shift- and deformation-invariant dose distributions on an average introduces <2% error in evaluated dose–volume metrics for 6 and 18MV IMRT prostate plans. Use of invariant dose distributions has a potential to reduce online re-planning time and permit pre-planning based on tissue deformation models.

Daily Online Bony Correction is Required for Prostate Patients Without Fiducial Markers or Soft-tissue Imaging

Aim To compare online position verification strategies with offline correction protocols for patients undergoing definitive prostate radiotherapy. Materials and methods We analysed 50 patients with implanted fiducial markers undergoing curative prostate radiation treatment, all of whom underwent daily kilovoltage imaging using an on-board imager. For each treatment, patients were set-up initially with skin tattoos and in-room lasers. Orthogonal on-board imager images were acquired and the couch shift to match both bony anatomy and the fiducial markers recorded. The set-up error using skin tattoos and offline bone correction was compared with online bone correction. The fiducial markers were used as the reference. Results Data from 1923 fractions were analysed. The systematic error was ≤1 mm for all protocols. The average random error was 2–3 mm for online bony correction and 3–5 mm for skin tattoos or offline-bone. Online-bone showed a significant improvement compared with offline-bone in the number of patients with >5 mm set-up errors for >10% ( P < 0.001) and >20% ( P < 0.003) of their fractions. Conclusions Online correction to bony anatomy reduces both systematic and random set-up error in patients undergoing prostate radiotherapy, and is superior to offline correction methods for those patients not suitable for fiducial markers or daily soft-tissue imaging.

Treatment planning considerations in contrast-enhanced radiotherapy: energy and beam aperture optimization

It has been shown that the use of kilovoltage x-rays in conjunction with a contrast agent incorporated into the tumor can lead to acceptable treatment plans with regard to the absorbed dose distribution produced in the target as well as in the tissue and organs at risk surrounding it. In this work, several key aspects related to the technology and irradiation techniques necessary to clinically implement this treatment modality are addressed by means of Monte Carlo simulation. The Zubal phantom was used to model a prostate radiotherapy treatment, a challenging site due to the depth of the prostate and the presence of bony structures that must be traversed by the x-ray beam on its way to the target. It is assumed that the concentration levels of the enhancing agent present in the tumor are at or below 10 mg per 1 g of tissue. The Monte Carlo code PENELOPE was used to model a commercial x-ray tube having a tungsten target. X-ray energy spectra for several combinations of peak electron energy and added filtration were obtained. For each energy spectrum, a treatment plan was calculated, with the PENELOPE Monte Carlo code, by modeling the irradiation of the patient as 72 independent conformal beams distributed at intervals of 5° around the phantom in order to model a full x-ray source rotation. The Cimmino optimization algorithm was then used to find the optimum beam weight and energy for different treatment strategies. It is shown that for a target dose prescription of 72 Gy covering the whole tumor, the maximum rectal wall and bladder doses are kept below 52 Gy for the largest concentration of contrast agent of 10 mg per 1 g of tissue. It is also shown that concentrations of as little as 5 mg per 1 g of tissue also render dose distributions with excellent sparing of the organs at risk. A treatment strategy to address the presence of non-uniform distributions of the contrast agent in the target is also modeled and discussed.

Real-time correction of magnetic field inhomogeneity-induced image distortions for MRI-guided conventional and proton radiotherapy

Image-guided radiotherapy has the potential to increase the success of treatment by decreasing uncertainties concerning tumour position and shape. We pursue integrated diagnostic quality MRI functionality with radiotherapy systems to boost the possibilities of image guidance by providing images with superior soft-tissue contrast during treatment. However, the use of MR images in radiotherapy can be hindered by geometrical distortions due to magnetic field inhomogeneity problems. A method for fast correction of these distortions is presented and implemented. Using a 20 cm square phantom containing a regular grid, a measure of residual deformation after correction is established. At very low gradient strength (which leads to large deformations) a maximum displacement of 2.9 mm is shown to be reduced to 0.63 mm. Next, the method is applied in vivo to the case of pelvic body contour extraction for prostate radiotherapy treatment planning. Here, again with low gradient strengths, distortions of up to 6 mm can be reduced to 2 mm. All results are provided within a lag time of 8 ms. We discuss implications of image distortions for MRI-guided photon and proton radiotherapy separately, since the dose-depth curves in these treatments are very different. We argue that, although field inhomogeneities cannot be prevented from occurring, distortion correction is not always necessary in practice. This work opens new possibilities for investigating on-line MRI-based plan adaptations and ultimately MRI-based treatment planning.

Acurácia na reprodutibilidade do posicionamento diário de pacientes submetidos a radioterapia conformada (RT3D) para câncer de próstata

OBJETIVO: Avaliar a reprodutibilidade do posicionamento de pacientes com diagnóstico de câncer de próstata submetidos a radioterapia conformada. MATERIAIS E MÉTODOS: Foram avaliados 960 (posições anterior e lateral) filmes radiológicos, de um total de 120 pacientes que receberam radioterapia conformada na próstata com técnica isocêntrica. As imagens foram obtidas em acelerador linear de partículas 6 MV. Aplicou-se protocolo específico para planejamento e tratamento da próstata, com o paciente em posição supina, mãos colocadas sobre o tórax, pés apoiados em suporte apropriado. Diariamente, os pacientes foram posicionados conforme demarcações na pele, coincidentes com os lasers da sala. Os filmes radiológicos foram comparados com as radiografias reconstruídas digitalmente (digitally reconstructed radiography - DRR) em sistema de planejamento computadorizado Eclipse, a partir das tomografias. As radiografias de posicionamento foram realizadas no primeiro dia e após, semanalmente, até o término do tratamento. RESULTADOS: As médias dos deslocamentos observados foram de 1,99 ± 1,25 mm no sentido crânio-caudal, 1,37 ± 0,84 mm no látero-lateral e 1,94 ± 1,10 mm no ântero-posterior. CONCLUSÃO: O uso de protocolos específicos para posicionamento dos pacientes é possível na prática clínica, possibilita reprodutibilidade adequada e rápida correção dos possíveis erros.

A Monte Carlo Comparison of Three Different Media for Contrast Enhanced Radiotherapy of the Prostate

Contrast-enhanced radiotherapy makes use of a kilovoltage X-ray beam, either from a diagnostic X-ray tube or modified megavoltage linear accelerator, in conjunction with a high-Z contrast medium deposited into the target volume to enhance the absorption of radiation. In this work, using the Monte Carlo code PENELOPE and the voxelized Zubal phantom to model a prostate radiotherapy treatment, a comparison between the physical absorbed dose distributions rendered by three different enhancing agents namely bismuth, gadolinium, and iodine is performed. It is assumed that there exists a concentration of 10 mg of enhancing agent per 1 g of tissue in the target volume while in the background a concentration of 1.5 mg per 1 g of tissue is present. The X-ray beam energy spectrum was obtained by means of Monte Carlo simulation of a tungsten target upon which a 220 keV mono-energetic electron pencil beam is made to impinge, and the resultant photon beam is heavily filtrated by 0.2 cm of copper. The treatment delivery is simulated as a 3608 arc collimated to conform to the target from every direction. Cumulative dose-volume histograms and isodose curves are presented for the target as well as five organs-at-risk, namely rectal wall, bladder, femoral heads, skin, and bone marrow. It is shown that under these conditions clinically acceptable treatment plans are obtained for all three contrast agents. A 72 Gy dose to 100% of the target volume results in maximum absorbed doses to the above mentioned organs-at-risk of 65, 56, 44, 32 and 65 Gy respectively when bismuth is used as the contrast agent, but the results obtained with gadolinium follow closely.

Using Bayesian logistic regression to evaluate a new type of dosimetric constraint for prostate radiotherapy treatment planning

Modern radiotherapy treatments can be optimized using dose-volume constraints which specify the volume of tumor and organs-at-risk receiving a given threshold dose. Careful derivation and evaluation of rectal constraints is essential to allow safe dose escalation in radiotherapy of prostate cancer. The authors present a new type of hybrid dosimetric constraint which comprises both volumetric and spatial factors of the dose-distribution. The authors also propose a framework to evaluate these constraints. The authors used data from the RT01 prostate radiotherapy trial (ISRCTN 47772397) to derive this set of hybrid constraints for the rectum based on measures extracted from dose-surface maps. For comparison, the authors also derive a set of dose-volume constraints. In order to evaluate these dosimetric constraints, the authors propose a new framework for predicting radiation-induced toxicities using Bayesian logistic regression with high-order interactions. The predictive power of the new RT01-based constraints, as well as of two sets of rectal dose-volume constraints proposed in the recent literature-The constraints proposed by other researchers [C. Fiorino, G. Fellin, T. Rancati, V. Vavassori, C. Bianchi, V. C. Borca, G. Girelli, M. Mapelli, L. Menegotti, S. Nava, and R. Valdagni, "Clinical and dosimetric predictors of late rectal syndrome after 3D-CRT for localized prostate cancer: Preliminary results of a multicenter prospective study," Int. J. Radiat. Oncol., Biol., Phys. 70, 1130-1137 (2008)] and the constraints used in the conventional or hypofractionated high dose intensity modulated radiotherapy for prostate cancer (CHHiP) trial [C. P. South, V. S. Khoo, O. Naismith, A. Norman, and D. P. Dearnaley, "A comparison of treatment planning techniques used in two randomised UK external beam radiotherapy trials for localised prostate cancer," Clin. Oncol. (R Coll. Radiol) 20, 15-21 (2008)]--were evaluated using a tenfold cross-validation with follow-up data from the RT01 trial. The predictive power was quantified using receiver-operator characteristic (ROC) curves. Toxicities considered were rectal bleeding, loose stools, and a global toxicity score. Dose-volume constraints had less predictive power than the new type of hybrid constraints. A probabilistic model for predicting rectal bleeding based on the dose-volume constraints proposed by other researchers [C. Fiorino, G. Fellin, T. Rancati, V. Vavassori, C. Bianchi, V. C. Borca, G. Girelli, M. Mapelli, L. Menegotti, S. Nava, and R. Valdagni, "Clinical and dosimetric predictors of late rectal syndrome after 3D-CRT for localized prostate cancer: Preliminary results of a multicenter prospective study," Int. J. Radiat. Oncol., Biol., Phys. 70, 1130-1137 (2008)], the CHHiP dose-volume constraints, the RT01-based dose-volume constraints, and the hybrid constraints resulted in average areas under the ROC curves of 0.56, 0.58, 0.62, and 0.67, respectively. For predicting loose stools, the corresponding values were 0.57, 0.53, 0.66, and 0.71, respectively. The areas under the respective ROC curves for predicting the global toxicity score were 0.58, 0.55, 0.61, and 0.63. Thus, imposing the new type of hybrid constraints when generating a treatment plan should result in a reduction in the incidence of radiation-induced late rectal toxicity.

SU‐FF‐T‐314: Patient‐Specific Quality Assurance Techniques for RapidArc Radiotherapy

Purpose: To develop a patient‐specific quality assurance protocol for RapidArc™ radiotherapy. Method and Materials: Following the commissioning of RapidArc™ treatment delivery, we tested several methods of patient‐specific quality assurance (QA). Ion chamber and film measurements were used as the “gold standard”. One ion chamber measurement was taken in a region of low‐dose gradient. Four film measurements were taken in three planes: axial, sagittal, coronal, and a coronal180° (testing setup uncertainty). Measurements were also taken using the Matrixx® device (IBA Dosimetry, Bartlett, TN) in two orientations, sagittal and coronal. The Matrixx® consists of a 24×24cm2 grid of ion chambers, providing a 2D absolute dose comparison. All plans were designed and calculated using Eclipse (Varian Medical Systems, Palo Alto, CA). Eleven patient plans were evaluated, consisting of 4 brain, 1 head&amp;neck boost, and 6 prostate radiotherapy treatments. Results: Ion chamber measurements were typically ∼2% greater than those predicted by Eclipse. Using a gamma index with 3%, 3mm, 5% threshold criteria, 88% of films had gamma pass rates of &gt;90%. Three of 7 axial films failed, but passed with thresholds of 30–40%. Similarly, Matrixx® measurements had gamma pass rates of &gt;90% for 100% of measurements. The average reading of 4 Matrixx ion chambers was significantly correlated with the single ion chamber results (Pearson correlation coefficient = 0.839, p = 0.002), and the 2D Matrixx results agreed well with film measurements (gamma pass rate &gt;90% for 10 of 11 comparisons). Conclusion: These QA measurements produced good agreement between planned and measured doses. Through utilizing ion chamber and film measurements, we have gained confidence in the Matrixx® for RapidArc QA. The chosen passing criteria were +/−3% for ion chamber readings, and gamma passing rates of &gt;90% for film and Matrixx® with 3%, 3mm, and 5% threshold.

An analysis of geometric uncertainty calculations for prostate radiotherapy in clinical practice

Margins are used in radiotherapy to assist in the calculation of planning target volumes. These margins can be determined by analysing the geometric uncertainties inherent to the radiotherapy planning and delivery process. An important part of this process is the study of electronic portal images collected throughout the course of treatment. Set-up uncertainties were determined for prostate radiotherapy treatments at our previous site and the new purpose-built centre, with margins determined using a number of different methods. In addition, the potential effect of reducing the action level from 5 mm to 3 mm for changing a patient set-up, based on off-line bony anatomy-based portal image analysis, was studied. Margins generated using different methodologies were comparable. It was found that set-up errors were reduced following relocation to the new centre. Although a significant increase in the number of corrections to a patient's set-up was predicted if the action level was reduced from 5 mm to 3 mm, minimal reduction in patient set-up uncertainties would be seen as a consequence. Prescriptive geometric uncertainty analysis not only supports calculation and justification of the margins used clinically to generate planning target volumes, but may also best be used to monitor trends in clinical practice or audit changes introduced by new equipment, technology or practice. Simulations on existing data showed that a 3 mm rather than a 5 mm action level during off-line, bony anatomy-based portal imaging would have had a minimal benefit for the patients studied in this work.

SU-FF-J-109: Conformal Prostate Treatment - a Closer Look at Dose Delivery Uncertainty and Adaptive Options

Purpose: To evaluate uncertainties in Tomotherapy conformal prostate treatment due to inter‐fraction organ variation. On board MVCT images were evaluated for reconstruction, comparing planned dose and delivered dose. Clinical feasibility of a closed feed back process for image guided adaptive process was studied. Method and Materials: Five prostate patients were randomly chosen for this study. The positional variations of inter‐fraction organ motion for each treatment fraction were systematically monitored and characterized using onboard MVCT images. Rectum and bladder were re‐contoured on MVCT images after each treatment and delivered dose was reconstructed using a sinogram that was generated during kVCT based treatment planning optimization. Reconstructeddoses were compared with treatment planningdoses for individual organs through dose volume histograms. Results: MVCT images were shown to be useful for dosereconstruction purposes. Three patients' treatment delivery showed large dose delivery variations for individual treatment fractions. Rectal doses were significantly higher (10–50%) due to large changes in rectal volume. Bladder dose varied less compared to rectal dose. However, two patients' delivered dose was close to planned dose.Conclusions: The current report describes an adaptive process for Tomotherapy prostate radiation treatment. Re‐planning with dose differences will ultimately allow one to adjust for any dose variation (in order to correct for under dosing or overdosing). However, at this point, re‐contouring, re‐planning, and verification of quality assurance are very labor intensive and require strategies for offline patient specific adaptive treatment. One may, however, sum dose differences for all fractions to evaluate actual total dose delivery. In the future, the development of automatic adaptive processes may pave the way for true closed loop intervention.

SU-FF-J-81: Estimation of Dose Variations During Prostate Radiation Treatment Due to Elastic Deformations of Soft Tissues

Objective: The true dose distribution is subject to change due to organ shifts and also deformations. The goal of this work is to estimate the effect of the elastic organ deformations on the dose distribution for the average prostate IMRT treatment. Methods: A prostate patient was surveyed during the radiation treatment (IRB approved). The original treatment plan was created based on the planning CT. On-board cone beam computed tomography (CBCT) images were taken before treatments for the first 5 days and once a week therafter. The prostate, seminal vesicles (SV), bladder and rectum segmented on each CBCT were deformably registered against the planning CT. The analysis was conducted with the help of C++ ITK image registration and segmentation package. Initially, a volume (e.g., prostate and bladder) segmented on the planning CT scan was aligned against corresponding volume on the daily CBCT scan using a translational filter. Then, the aligned volume on CBCT was elastically deformed (Finite Element elastic deformation method), and the computed deformation vector field was used to propagate the true delivered radiation dose distribution to the original CT-based volume of the patient. Results: For the plan with a planning target volume (PTV) defined as a combination of both prostate and SV with a 5-mm margin, the DVH parameters fluctuated as follows (Vx denotes percent volume above x% of the prescription dose): prostate mean V100 76%, mean V95 97%, compared to the original prostate 98.15% and 100%, rectum and bladder mean V50 49% and 75% correspondingly, compared to the original rectum and bladder 50% and 29%. Conclusions: This study shows that some deviations from the planned dose may occur due to elastic distortion of soft tissue geometry. The developed suite of applications can be used to monitor actual delivered dose to the targets and critical structures.

PTV margin for dose‐escalated radiation therapy of prostate cancer with daily online realignment using internal fiducial markers: Monte Carlo approach and dose population histogram (DPH) analysis

Using internal fiducial markers and electronic portal imaging (EPI) to realign patients has been shown to significantly reduce positioning uncertainties in prostate radiation treatment. This creates the possibility of decreasing the planning target volume (PTV) margin added on the clinical target volume (CTV), which in turn may allow for dose escalation. We compared the outcome of two plans: 70 Gy/35 fx, 10‐mm PTV margin without patient realignment (Reference Plan) and 78 Gy/39 fx, 5‐mm PTV margin with patient realignment (Escalated Plan). Four‐field‐oblique (gantry angles 35°, 90°, 270°, 325°) beam arrangement was used. Monte Carlo code was used to simulate the daily organ motion. Dose to each organ was calculated. Tumor control probability (TCP) and the effective dose to critical organs (Deff) were calculated using the biologically normalized dose‐volume histograms. By comparing the biological factors, we found that the prescription dose can be escalated to 78 Gy/39 fx with a 5‐mm PTV margin when using internal fiducial markers and EPI. Based on the available dose‐response data for intermediate risk prostate patients, this will result in a 20% increase of local control and significantly reduced rectal complications provided that less serial dose‐volume behavior of rectum is proven.PACS number: 87.50.‐a

Internal fiducial markers can assist dose escalation in treatment of prostate cancer: result of organ motion simulations

Use of internal fiducial markers and electronic portal imaging (EPI) to realign patients has been shown to significantly reduce positioning uncertainties in prostate radiation treatment. This creates the possibility of improving the treatment by decreasing the planning target volume (PTV) margin added to the clinical target volume (CTV), which in turn may allow dose escalation. Conformal treatment plans for three prostate cancer patients were evaluated by using different PTV margins with dose prescription of 70 Gy/35 fr initially. Two beam arrangements, 4-field-box (4FB) and 4-field-oblique (4FO), were used. Then, two dose escalation schemes, 74 Gy and 78 Gy, with tighter PTV margins, were chosen from the first simulation and were tested. A Monte Carlo model was developed to simulate the daily geometric uncertainty and calculate the dose to each organ. After the whole treatment, dose–volume histograms were produced and tumour control probability, prostate equivalent uniform dose and the effective dose to critical organs were calculated. By comparing these radiobiological metrics, optimized dose escalation schemes were found. The results show that using internal fiducial markers and EPI, the prescription dose can be escalated to 78 Gy/39 fr with a 4 mm PTV margin. Based on the available dose–response data for intermediate risk prostate patients, this is estimated to result in a 20% increase of local control and significantly reduced rectal complications.

Development of an adjoint Monte Carlo method for clinical prostate radiation treatment

Development of an adjoint Monte Carlo method for clinical prostate radiation treatment

TU‐EE‐A2‐06: Patient Setup Accuracy and Precision of Prostate Radiation Treatment with BrainLab ExacTrac X‐Ray System with Implanted Fiducials

Purpose: To confirm the intra‐radiation treatment motion of the prostate is small enough so that daily positioning of the prostate is a valid effort. To test the automatic target alignment of the ExacTrac X‐ray® system. Method and Materials: The target positioning data of 51 radiation treatment sessions were collected retrospectively from 5 prostate cancer patients treated with IMRT between June and August in 2004. All patients were in supine treatment position and immobilized with HipFix (Med‐Tec, Inc.) without any padding or mold on the couch. Four gold fiducial markers (φ 1.2 mm, 3.0 mm long, Med‐Tec, Inc.) were implanted in the patient's prostate under ultrasonic guidance prior to the planning CT scan. Prior to the treatment, fiducials were localized using stereoscopic kV x‐ray imaging system (ExacTrac X‐ray®, BrainLab) and the couch was automatically shifted to a correct the target position. There are two sets of data collected. 1) pre‐treatment target deviation after the automatic positioning with kV x‐ray images. 2) post‐treatment target deviation. Results: 1) The average pre‐treatment deviations of the target after the automatic positioning were — 0.2 ± 1.3 mm (vertical), −0.1 ± 1.0 mm (longitudinal), and 0.0 ± 0.4 mm (lateral). 2) The average isocenter shifts (difference of the target positions between pre‐ and post‐treatment) were −0.2 ± 1.0 mm (1SD for all errors, vertical), −0.3 ± 1.2 mm (longitudinal), and 0.1 ± 0.5 mm (lateral). Conclusion: Both averages of post‐treatment shift and pre‐treatement deviation of the target were negligible considering the uncertainty. Both results showed lower positioning uncertainty by half in the lateral direction compared with the vertical and longitudinal directions, which suggest the lateral target motion was restricted better than the vertical and longitudinal. Same magnitude of position uncertainty of the pre‐deviation and post‐shift indicates the limit of the precision of daily ExacTrac's automatic positioning.

Modeling the impact of treatment uncertainties in radiotherapy

The aim of this thesis is to model the impact (dosimetric and radiobiological) of prostate radiotherapy treatment uncertainties and techniques for displaying potential dose errors. A Monte Carlo code is written to simulate both the treatment planning and delivery phases. Distributions of systematic organ motion, systematic positioning error, random organ motion, random positioning error, and interpatient cell sensitivity are taken from the literature. These distributions are used to sample the position and size of the prostate and rectum at each fraction of treatment. Dose to organ voxels are assigned from precalculated dose distributions (four‐field box using 6 MV photons with margins around the clinical target volume of 0.5, 1.0, and 1.5 cm). In particular, the poor estimation of prostate mean position from one planning CT series is investigated and shown to introduce large dose uncertainty. A convolution kernel is suggested that considers multiple pre‐treatment CT scans to better estimate mean prostate position. The radiobiological impact of the treatment uncertainties is evaluated in terms of tumor control probability for prostate, normal tissue complication probability for rectal bleeding and uncomplicated tumor control probability using established models. Local prostate control is highly sensitive to treatment uncertainties with inadequate margin size and to interpatient radiosensitivity. Rectal bleeding is more sensitive to margin size than to deformation. A number of ways to display the dosimetric uncertainty are presented. The display tools proposed allow the planner to consider potential variations in dose visually and may assist with margin size considerations.

907 Poster Introduction of multi-modality imaging for conformal prostate radiation treatment

907 Poster Introduction of multi-modality imaging for conformal prostate radiation treatment