Changes In Patient Anatomy Research Articles

Monitoring proton therapy with PET.

Protons are being used in radiation therapy because of typically better dose conformity and reduced total energy deposited in the patient as compared with photon techniques. Both aspects are related to the finite range of a proton beam. The finite range also allows advanced dose shaping. These benefits can only be fully utilized if the end of range can be predicted accurately in the patient. The prediction of the range in tissue is associated with considerable uncertainties owing to imaging, patient set-up, beam delivery, interfractional changes in patient anatomy and dose calculation. Consequently, a significant range (of the order of several millimetres) is added to the prescribed range in order to ensure tumour coverage. Thus, reducing range uncertainties would allow a reduction of the treatment volume and reduce dose to potential organs at risk.

Towards adaptive IMRT sequencing for the MR-linac

The MRI linear accelerator (MR-linac) that is currently being installed in the University Medical Center Utrecht (Utrecht, The Netherlands), will be able to track the patient’s target(s) and Organ(s) At Risk during radiation delivery. In this paper, we present a treatment planning system for intensity-modulated radiotherapy (IMRT). It is capable of Adaptive Radiotherapy and consists of a GPU Monte Carlo dose engine, an inverse dose optimization algorithm and a novel adaptive sequencing algorithm. The system is able to compensate for patient anatomy changes and enables radiation delivery immediately from the first calculated segment. IMRT plans meeting all clinical constraints were generated for two breast cases, one spinal bone metastasis case, two prostate cases with integrated boost regions and one head and neck case. These plans were generated by the segment weighted version of our algorithm, in a 0 T environment in order to test the feasibility of the new sequencing strategy in current clinical conditions, yielding very small differences between the fluence and sequenced distributions. All plans went through stringent experimental quality assurance on Delta4 and passed all clinical tests currently performed in our institute. A new inter-fraction adaptation scheme built on top of this algorithm is also proposed that enables convergence to the ideal dose distribution without the need of a final segment weight optimization. The first results of this method confirm that convergence is achieved within the first fractions of the treatment. These features combined will lead to a fully adaptive intra-fraction planning system able to take into account patient anatomy updates during treatment.

A Contralateral Esophagus Sparing Technique to Limit Severe Esophagitis Associated With Concurrent Chemoradiation Therapy in Lung Cancer Patients: Treatment Toxicity

Acute esophagitis is a dose limiting side effect in patients undergoing concurrent chemoradiation therapy (CRT) for lung cancer. Severe (Radiation Therapy Oncology Group [RTOG] grade 3 or greater) esophagitis generally occurs in 15%-20% of non-small cell lung cancer (NSCLC) patients, which may result in treatment breaks that compromise local tumor control and poses a barrier to dose escalation > 66 Gy. Here, we report a novel contralateral esophagus sparing technique (CEST) that employs intensity modulated radiation therapy (IMRT) to reduce the incidence of severe esophagitis. We retrospectively reviewed the radiation charts of lung cancer patients receiving concurrent CRT in curative intent in the senior author’s practice from 1/2013 to 1/2014. All patients underwent custom immobilization, 4D CT simulation, IMRT planning with multicriteria optimization, and treatments with daily cone beam CT guidance. Adaptive planning was performed for tumor regression and other changes in patient anatomy during treatment. In addition to standard organs at risk, the esophageal wall contralateral (CE) to the tumor was contoured as an avoidance structure to guide driving a steep dose fall off gradient beyond the target volume. Esophagitis was recorded using the RTOG acute toxicity grading system. We identified 20 consecutive patients treated with concurrent CRT of at least 63 Gy in whom there was gross tumor within 1 cm of the esophagus. Median age was 69 years (range, 46-83 years). The majority of patients had stage III NSCLC (75%). Only 1 patient was treated with 63 Gy, all others received ≥66 Gy and the median radiation dose was 70.2 Gy (range, 63-72.15 Gy) delivered over 35 to 40 fractions. In all patients, >99% of the planning and internal target volumes were covered by at least 90% and 99% of prescription dose, respectively. Strikingly, no patient developed RTOG grade 3 esophagitis (0/20, 95% CL 0-16%) despite the high total doses delivered. The frequency of grade 0, 1, 2 esophagitis was 25%, 55%, and 20%, respectively. The median V45 and V55 of the CE was 1.8 cc and 0.3 cc, respectively. The average max esophageal dose was 70 Gy and the mean dose was 24 Gy, indicating effective sparing of the esophagus cross section by CEST. We report a simple yet effective method to avoid exposing the entire esophagus cross section to high doses, which mirrors the concept of posterior rectum sparing in the treatment of prostate cancer. Despite a median dose of 70 Gy in our cohort, we observed not a single case of grade 3+ esophagitis and few cases of grade 2 reactions. We, therefore, hypothesize that CEST improves the esophagus toxicity profile in lung cancer patients receiving concurrent CRT even at doses above the standard 60-66 Gy levels. A prospective multi-institutional trial of IMRT with CEST and dose escalation to 74 Gy will be conducted.

A Contralateral Esophagus Sparing Technique to Limit Severe Esophagitis Associated With Concurrent Chemoradiation Therapy in Lung Cancer Patients

A Contralateral Esophagus Sparing Technique to Limit Severe Esophagitis Associated With Concurrent Chemoradiation Therapy in Lung Cancer Patients

Evaluation of Transit Dosimetry Software for Helical Tomotherapy

3D in vivo dosimetry is the only method capable of verifying the entire radiation therapy chain, including a potential machine malfunctioning, data corruption, or changes in patient anatomy. The purpose of this work was to evaluate limits and capabilities of a new transit dosimetry software for use with tomotherapy system by testing on heterogeneous phantom.

SU-E-J-254: Utility of Pinnacle Dynamic Planning Module Utilizing Deformable Image Registration in Adaptive Radiotherapy

Purpose For certain highly conformal treatment techniques, changes in patient anatomy due to weight loss and/or tumor shrinkage can result in significant changes in dose distribution. Recently, the Pinnacle treatment planning system added a Dynamic Planning module utilizing Deformable Image Registration (DIR). The objective of this study was to evaluate the effectiveness of this software in adapting to altered anatomy and adjusting treatment plans to account for it. Methods We simulated significant tumor response by changing patient thickness and altered chin positions using a commercially-available head and neck (H&N) phantom. In addition, we studied 23 CT image sets of fifteen (15) patients with H&N tumors and eight (8) patients with prostate cancer. In each case, we applied deformable image registration through Dynamic Planning module of our Pinnacle Treatment Planning System. The dose distribution of the original CT image set was compared to the newly computed dose without altering any treatment parameter. Result was a dose if we did not adjust the plan to reflect anatomical changes. Results For the H&N phantom, a tumor response of up to 3.5 cm was correctly deformed by the Pinnacle Dynamic module. Recomputed isodose contours on new anatomies were within 1 mm of the expected distribution. The Pinnacle system configuration allowed dose computations resulting from original plans on new anatomies without leaving the planning system. Original and new doses were available side-by-side with both CT image sets. Based on DIR, about 75% of H&N patients (11/15) required a re-plan using new anatomy. Among prostate patients, the DIR predicted near-correct bladder volume in 62% of the patients (5/8). Conclusions The Dynamic Planning module of the Pinnacle system proved to be an accurate and useful tool in our ability to adapt to changes in patient anatomy during a course of radiotherapy.

TH‐E‐17A‐09: High Quality and Artifact‐Free 4D Cone Beam CT and Its Application in Adaptive Treatment Planning

Purpose:To develop a high quality 4D cone beam CT (4DCBCT) method that is immune to patient/couch truncations and to investigate its application in adaptive replanning of lung XRT.Methods:In this study, IRB‐approved human subject CBCT data was acquired using a Varian on‐board imager with 1 minute rotation time. The acquired projection data was retrospectively sorted into 20 respiratory phase bins, from which 4DCBCT images with high SNR and high temporal resolution were generated using Prior Image Constrained Compressed Sensing (PICCS). Couch and patient truncations generate strong data inconsistency in the projection data and artifacts in the 4DCBCT image. They were addressed using an adaptive PICCS method. The artifact‐free PICCS‐4DCBCT images were used to generate adaptive treatment plans for the same patient at the 10th (day 21) and 30th (day 47) fractions. Dosimetric impacts with and without PICCS‐ 4DCBCT were evaluated by isodose distributions, DVHs, and other dosimetric factors.Results:The adaptive PICCS‐4DCBCT method improves image quality by removing residue truncation artifacts; measured universal image quality increased 37%. The isodose lines and DVHs with PICCS‐4DCBCT‐based adaptive replanning were significantly more conformal to PTV than without replanning due to changes in patient anatomy caused by progress of the treatment. The mean dose to PTV at the 10th fraction was 63.1Gy with replanning and 64.2Gy without replanning, where the prescribed dose was 60Gy, in 2Gy × 30 fractions. The mean dose to PTV at the 30th fraction was 61.6Gy with replanning and 64.9Gy without replanning. Lung V20 was 37.1%, 41.9% and 43.3% for original plan, 10th fraction plan and 30th fraction plan; with re‐planning, Lung V20 was 37.1%, 32%, 27.8%.Conclusion:4DCBCT imaging using adaptive PICCS is able to generate high quality, artifact‐free images that potentially can be used to create replanning for improving radiotherapy of the lung. K Niu, K Li, J Smilowitz: Nothing to Disclose.G Chen: General Electric Company Research funded, Siemens AG Research funded, Varian Medical Systems Research funded, Hologic Research funded.

SU‐E‐T‐452: Impact of Respiratory Motion On Robustly‐Optimized Intensity‐Modulated Proton Therapy to Treat Lung Cancers

Purpose:We compared conventionally optimized intensity‐modulated proton therapy (IMPT) treatment plans against the worst‐case robustly optimized treatment plans for lung cancer. The comparison of the two IMPT optimization strategies focused on the resulting plans' ability to retain dose objectives under the influence of patient set‐up, inherent proton range uncertainty, and dose perturbation caused by respiratory motion.Methods:For each of the 9 lung cancer cases two treatment plans were created accounting for treatment uncertainties in two different ways: the first used the conventional Method: delivery of prescribed dose to the planning target volume (PTV) that is geometrically expanded from the internal target volume (ITV). The second employed the worst‐case robust optimization scheme that addressed set‐up and range uncertainties through beamlet optimization. The plan optimality and plan robustness were calculated and compared. Furthermore, the effects on dose distributions of the changes in patient anatomy due to respiratory motion was investigated for both strategies by comparing the corresponding plan evaluation metrics at the end‐inspiration and end‐expiration phase and absolute differences between these phases. The mean plan evaluation metrics of the two groups were compared using two‐sided paired t‐tests.Results:Without respiratory motion considered, we affirmed that worst‐case robust optimization is superior to PTV‐based conventional optimization in terms of plan robustness and optimality. With respiratory motion considered, robust optimization still leads to more robust dose distributions to respiratory motion for targets and comparable or even better plan optimality [D95% ITV: 96.6% versus 96.1% (p=0.26), D5% – D95% ITV: 10.0% versus 12.3% (p=0.082), D1% spinal cord: 31.8% versus 36.5% (p =0.035)].Conclusion:Worst‐case robust optimization led to superior solutions for lung IMPT. Despite of the fact that robust optimization did not explicitly account for respiratory motion it produced motion‐resistant treatment plans. However, further research is needed to incorporate respiratory motion into IMPT robust optimization.

SU‐F‐BRF‐03: Quality Assurance of Deformable Image Registration in Radiotherapy

Purpose:To introduce methods to analyze Deformable Image Registration (DIR) and identify regions of potential DIR errors.Methods:DIR Deformable Vector Fields (DVFs) quantifying patient anatomic changes were evaluated using the Jacobian determinant and the magnitude of DVF curl as functions of tissue density and tissue type. These quantities represent local relative deformation and rotation, respectively. Large values in dense tissues can potentially identify non‐physical DVF errors. For multiple DVFs per patient, histograms and visualization of DVF differences were also considered. To demonstrate the capabilities of methods, we computed multiple DVFs for each of five Head and Neck (H' N) patients (P1–P5) via a Fast‐symmetric Demons (FSD) algorithm and via a Diffeomorphic Demons (DFD) algorithm, and show the potential to identify DVF errors.Results:Quantitative comparisons of the FSD and DFD registrations revealed <0.3 cm DVF differences in >99% of all voxels for P1, >96% for P2, and >90% of voxels for P3. While the FSD and DFD registrations were very similar for these patients, the Jacobian determinant was >50% in 9–15% of soft tissue and in 3–17% of bony tissue in each of these cases. The volumes of large soft tissue deformation were consistent for all five patients using the FSD algorithm (mean 15%±4% volume), whereas DFD reduced regions of large deformation by 10% volume (785 cm3) for P4 and by 14% volume (1775 cm3) for P5. The DFD registrations resulted in fewer regions of large DVF‐curl; 50% rotations in FSD registrations averaged 209±136 cm3 in soft tissue and 10±11 cm3 in bony tissue, but using DFD these values were reduced to 42±53 cm3 and 1.1±1.5 cm3, respectively.Conclusion:Analysis of Jacobian determinant and curl as functions of tissue density can identify regions of potential DVF errors by identifying non‐physical deformations and rotations.Collaboration with Phillips Healthcare, as indicated in authorship.

The VANILLA sensor as a beam monitoring device for X-ray radiation therapy

Cancer treatments such as intensity-modulated radiotherapy (IMRT) require increasingly complex methods to verify the accuracy and precision of the treatment delivery. In vivo dosimetry based on measurements made in an electronic portal imaging device (EPID) has been demonstrated. The distorting effect of the patient anatomy on the beam intensity means it is difficult to separate changes in patient anatomy from changes in the beam intensity profile. Alternatively, upstream detectors scatter and attenuate the beam, changing the energy spectrum of the beam, and generate contaminant radiation such as electrons. We used the VANILLA device, a Monolithic Active Pixel Sensor (MAPS), to measure the 2D beam profile of a 6MV X-ray beam at Bristol Hospital in real-time in an upstream position to the patient without clinically significant disturbance of the beam (0.1% attenuation). MAPSs can be made very thin (~20μm) with still a very good signal-to-noise performance. The VANILLA can reconstruct the collimated beam edge with approximately 64μm precision.

In vivo dosimetry in external beam radiotherapy

In vivo dosimetry (IVD) is in use in external beam radiotherapy (EBRT) to detect major errors, to assess clinically relevant differences between planned and delivered dose, to record dose received by individual patients, and to fulfill legal requirements. After discussing briefly the main characteristics of the most commonly applied IVD systems, the clinical experience of IVD during EBRT will be summarized. Advancement of the traditional aspects of in vivo dosimetry as well as the development of currently available and newly emerging noninterventional technologies are required for large-scale implementation of IVD in EBRT. These new technologies include the development of electronic portal imaging devices for 2D and 3D patient dosimetry during advanced treatment techniques, such as IMRT and VMAT, and the use of IVD in proton and ion radiotherapy by measuring the decay of radiation-induced radionuclides. In the final analysis, we will show in this Vision 20∕20 paper that in addition to regulatory compliance and reimbursement issues, the rationale for in vivo measurements is to provide an accurate and independent verification of the overall treatment procedure. It will enable the identification of potential errors in dose calculation, data transfer, dose delivery, patient setup, and changes in patient anatomy. It is the authors' opinion that all treatments with curative intent should be verified through in vivo dose measurements in combination with pretreatment checks.

Virtual couch shift (VCS): accounting for patient translation and rotation by online IMRT re-optimization

When delivering conventional intensity modulated radiotherapy (IMRT), discrepancies between the pre-treatment CT/MRI/PET based patient geometry and the daily patient geometry are minimized by performing couch translations and/or small rotations. However, full compensation of, in particular, rotations is usually not possible. In this paper, we introduce an online ‘virtual couch shift (VCS)’: we translate and/or rotate the pre-treatment dose distribution to compensate for the changes in patient anatomy and generate a new plan which delivers the transformed dose distribution automatically. We show for a phantom and a cervical cancer patient case that VCS accounts for both translations and large rotations equally well in terms of DVH results and 2%/2 mm γ analyses and when the various aspects of the clinical workflow can be implemented successfully, VCS can potentially outperform physical couch translations and/or rotations. This work is performed in the context of our hybrid 1.5 T MRI linear accelerator, which can provide translations and rotations but also deformations of the anatomy. The VCS is the first step toward compensating all of these anatomical changes by online re-optimization of the IMRT dose distribution.

Cumulative Dose With Organ Deformation for Cervical Cancer Treated With Multifraction High-dose-rate Brachytherapy

Three-dimensional image-based high dose rate (HDR) treatment planning for cervical cancer enables volumetric tracking of dose for organs at risk. The inter-fractional changes in patient anatomy make addition of the volumetric recto-sigmoid and bladder dose parameters from fraction to fraction inconsistent. In this study we explore the feasibility and accuracy of calculating cumulative dose considering organ deformation for cervical cancer brachytherapy (BT) in an effort to better understand dose actually delivered in multiple fractions.

Adaptive Planning of Intensity Modulated Radiation Therapy for Head-and-Neck Cancers for Improved Accuracy of Delivered Dose in Patients With Clinically Significant Anatomical Change Due to Therapy

Purpose/Objective(s): Adaptive planning of intensity-modulated radiation therapy (AP-IMRT) for head and neck cancer (HNCA) will improve conformality of delivered target dose after significant geometric changes in patient’s anatomy related to therapy response. We examined the effects of mid-course IMRT replanning using FDG PET-CT imaging to determine the relative change in delivered dose to target tissues and normal structures compared with the initial IMRT plan. Materials/Methods: Three HNCA patients with locally advanced, stage IVA/IVB disease with large nodes (4, 5.5, 7 cm) underwent one IMRT replan based upon mid-treatment PET-CT at 36 Gy of planned 72 Gy course. All patients had a partial response to definitive chemoradiation. Primary sites were oropharynx and supraglottic larynx, while one subject had an unknown primary. Adapted clinical target volume (AP-CTV), planning target volume (AP-PTV), and normal structures were recontoured on the new PET-CT for optimized IMRT planning. The change in calculated delivered dose of the new optimized IMRT plan was compared to the initial IMRT plan dose recalculated for the new CT data set and adapted contours. Using rigid image registration, we superimposed the new CT scan into the initial CT scan in order to calculate the dose delivered by the initial IMRT plan to the new CT scan. We evaluated the change in dose delivered to the new regions of interest, which were re-drawn by the treating physician, including AP-CTV, AP-PTV, salivary glands and spinal cord. Results: Mid-treatment PET-CT imaging displayed significant changes in patient anatomy that resulted in differences in both target tissue and normal tissue volumes. In one case, the left parotid volume and APCTV_58 decreased by 27% and 21%, respectively. DVH analysis demonstrated a small but meaningful underdosing to AP-CTV_58 and AP-PTV_58 due to geometric changes in targets. V99 for AP-CTV_58 was <54 Gy and V99 for AP-PTV_58 was <46 Gy with the initial prescribed dose of 58 Gy. The maximum spinal cord dose increased from 27.7 Gy to 35.2 Gy, an increase of 27.1%. The mean right parotid dose increased by 15.8% while the mean left parotid dose remained unchanged. Conclusions: The anatomical changes seen in HNCA patients undergoing definitive IMRT pose a significant challenge in providing accurate dosing during the course of treatment. AP-IMRT provides greater conformality in dose delivery and improved CTV/PTV dose coverage especially when significant clinical response is noted on physical examination or daily pretreatment imaging. AP-IMRT can also maximize sparing of radiosensitive normal structures. Adaptive planning is fast becoming an important aspect of IMRT and will require further research to increase effectiveness for individualized therapy goals. Author Disclosure: M. Bishop: None. A. Al-Basheer: None. J. Greskovich: None.

The use of exit detector sinograms to detect anatomical variations for patients extending beyond the TomoTherapy field of view: A feasibility study

This work describes an independent method to use the TomoTherapy Hi-ART megavoltage CT imaging system for daily monitoring of anatomical changes of cancer patients whose anatomy extends beyond the imaging field of view. The imaging detector response to changes in attenuating media was measured using water-equivalent plastic. Weight loss was simulated using an anthropomorphic phantom and determining the system's ability to detect the weight loss. Layers of tissue-equivalent bolus were added to an anthropomorphic pelvis phantom and CT simulations of the phantom were conducted, one in which the phantom and bolus were both within the TomoTherapy imaging field of view, and another in which the couch was raised so that the bolus was outside the field of view. Gynecological treatment plans were developed using the TomoTherapy treatment planning system, and successive fractions of the plan were then delivered to the phantom. Weight loss was simulated by removing a 0.5 cm layer of bolus following each fraction. The exit detector sinograms were obtained from each fraction, and ratios of sinograms were calculated relative to a reference sinogram for which all bolus was in place. Histograms of ratio sinograms were determined and used to correlate with simulated weight loss. Exit detector sinograms and ratio histograms were also retrospectively analyzed for five patients all of whose anatomies extended beyond the imaging field of view and all of whom experienced weight variations exceeding 10% during treatment. Exit detector signal is well correlated to changes in attenuator thickness as demonstrated in both slab and anthropomorphic phantom geometries. Measured and expected signal increases agreed to within less than 2% for simulated weight loss on the anthropomorphic phantom. Exit detector signals for pelvic patients with significant weight loss variations were consistent with phantom measurements. The analysis of the ratio sinograms for the phantom measurements and real patients indicated that exit detector sinograms can be used to detect relative changes in patient anatomy for each fraction as a means of in vivo quality assurance.

Poster - Thur Eve - 25: In vivo dosimetric verification of intensity-modulated radiation therapy.

Dosimetric verification of patient treatment plans has become increasingly important due to the widespread use of complicated delivery techniques. IMRT and VMAT treatments are typically verified prior to start of the patient's course of treatment, using a point dose and/or a film measurement. Pre-treatment verification will not detect patient or machine-related errors; therefore, in vivo dosimetric verification is the only way to determine if the patient's treatment was delivered correctly. Portal images were acquired throughout the course of five prostate and six head-and-neck patient IMRT treatments. The corresponding predicted images were calculated using a previously developed portal dose image prediction algorithm, which combines a versatile fluence model with a patient scatter and EPID dose prediction model. The prostate patient image agreement was found to vary day-to-day due to rectal gas pockets and the effect of adjustable support rails on the patient couch. The head-and-neck patient images were observed to be more consistent daily, but an increased measured dose was evident at the periphery of the patient, likely due to patient weight loss. The majority of the fields agreed within 3% and 3 mm for greater than 90% of the pixels, as established by the χ-comparison. This work demonstrates the changes in patient anatomy that are detectable with the portal dose image prediction model. Prior to clinical implementation, the effect of the couch must be incorporated into the model, the image acquisition must be automatically scheduled and routine EPID QA must be undertaken to ensure the collection of high-quality EPID images.

Accumulating daily‐varied dose distributions of prostate radiation therapy with soft‐tissue–based kV CT guidance

Even with daily image guidance based on soft tissue registration, deviations of fractional doses can be quite large due to changes in patient anatomy. It is of interest to ascertain the cumulative effect of these deviations on the total delivered dose. Daily kV CT data acquired using an in‐room CT for five prostate cancer patients were analyzed. Each daily CT was deformably registered to the planning CT using an in‐house tool. The resulting deformation field was used to map the delivered daily dose onto the planning CT, then summed to obtain the cumulative (total delivered) dose to the patient. The delivered cumulative values of prostate D100 on average were only 2.9% less than their planned values, while the PTV D95 were 3.6% less. The delivered rectum and bladder V70s can be twice what was planned. The less than 3% difference between delivered and planned prostate coverage indicates that the PTV margin of 5 mm was sufficient with the soft‐tissue–based kV CT guidance for the cases studied.PACS number: 87.55.km

SU-E-T-630: Adaptive Liver SBRT: Daily Re-Planning to Compensate for Non-Rigid Anatomy Changes Improves Dose Distributions

Purpose: In stereotactic body radiotherapy(SBRT) of livertumors, daily image guidance based on contrast enhanced CT‐scans enables precise delivery of high tumordoses with small margins. However non‐rigid patient anatomy changes could still cause OAR dose constraint violations. In this study we evaluated the impact of daily re‐planning based on contrast‐ enhanced CT‐scans to compensate for non‐rigid anatomy changes. Methods: For eight patients with twelve tumors in different liver regions, three planning strategies were simulated. In all strategies, the patient was shifted each fraction to correct for the tumor displacement derived from registration of the daily CT‐scan. In the first strategy, the patient is irradiated with the non‐coplanar treatment plan that was based on the planning CT‐scan. In the second strategy the fluence profiles of the treatment beams were re‐ optimized based on the daily CT scan (beam angles remained the same as for the planning scan). In the last strategy, for each fraction a completely new plan (beam angles and profiles) was generated based on the daily CT‐ scan. Non‐rigid registration and LQ‐corrections were used to obtain cumulative dose distributions. Results: In 50% of the patients, constraint violations occurred for the repeated plan when we look at the dose distributions of the single fractions. For 33% of those patients, severe constraint violations in a single fraction even resulted in constraint violations in the integral dose distribution. In those cases, the beamweight plan and adaptive plan were easily capable of taking those OAR deformations into account and both methods designed good treatment plans without constraint violations. The adaptive plan performed slightly better than the beamweight plan, but the latter requires only some minutes computation time while full re‐optimization would take a few hours. Conclusions: Adaptive re‐optimization of fluence profiles based on daily CT scans can significantly improve dose distributions in liverSBRT.

48 speaker IMAGE REGISTRATION IN RADIOTHERAPY

Image registration is widely recognized as an important tool in Radiotherapy (RT) to better configure RT plans and to evaluate RT efficacy. Image registration is more and more used to integrate multimodal information derived from different imaging modalities, in order to better define structural and functional characteristics of the tumor and consequently to enhance delineation of Planning and Biological Target Volumes; furthermore registration of CT images acquired before and during RT is employed to estimate patient anatomical changes during time, thus allowing the study of deformation of organs induced by RT as well as the introduction of Adaptive RT. However, the good behavior of an image registration technique is strictly related to the specific clinical application and often to the specific image acquisition protocol: each registration approach presents relative merits and limitations that should be known in order to really exploit the value of registration in RT. In this context an important point is the availability of tools able to quantitatively assess the spatial accuracy of registration. The lesson introduces the principal registration techniques from the theoretical point of view. Specifically, the models of spatial transformations (rigid and non rigid) as well as the main categories of registrations (feature-based, voxel-based) are comparatively discussed. The main proposed approaches for non rigid deformation are reviewed. Finally, the methodological framework to evaluate the performance of image registration in RT applications is presented and the main indices to quantitatively assess registration accuracy are introduced.

49 speaker ADVANCES IN REFERENCE DOSIMETRY – WHERE DO WE GO NEXT?

Image registration is widely recognized as an important tool in Radiotherapy (RT) to better configure RT plans and to evaluate RT efficacy. Image registration is more and more used to integrate multimodal information derived from different imaging modalities, in order to better define structural and functional characteristics of the tumor and consequently to enhance delineation of Planning and Biological Target Volumes; furthermore registration of CT images acquired before and during RT is employed to estimate patient anatomical changes during time, thus allowing the study of deformation of organs induced by RT as well as the introduction of Adaptive RT. However, the good behavior of an image registration technique is strictly related to the specific clinical application and often to the specific image acquisition protocol: each registration approach presents relative merits and limitations that should be known in order to really exploit the value of registration in RT. In this context an important point is the availability of tools able to quantitatively assess the spatial accuracy of registration. The lesson introduces the principal registration techniques from the theoretical point of view. Specifically, the models of spatial transformations (rigid and non rigid) as well as the main categories of registrations (feature-based, voxel-based) are comparatively discussed. The main proposed approaches for non rigid deformation are reviewed. Finally, the methodological framework to evaluate the performance of image registration in RT applications is presented and the main indices to quantitatively assess registration accuracy are introduced.