Image Guided Radiation Therapy Strategy Research Articles

A review of Image Guided Radiation Therapy in head and neck cancer from 2009-201 - Best Practice Recommendations for RTTs in the Clinic.

Radiation therapy (RT) is beneficial in Head and Neck Cancer (HNC) in both the definitive and adjuvant setting. Highly complex and conformal planning techniques are becoming standard practice in delivering increased doses in HNC. A sharp falloff in dose outside the high dose area is characteristic of highly complex techniques and geometric uncertainties must be minimised to prevent under dosage of the target volume and possible over dosage of surrounding critical structures. CTV-PTV margins are employed to account for geometric uncertainties such as set up errors and both interfraction and intrafraction motion. Robust immobilisation and Image Guided Radiation Therapy (IGRT) is also essential in this group of patients to minimise discrepancies in patient position during the treatment course. IGRT has evolved with increased 2-Dimensional (2D) and 3-Dimensional (3D) IGRT modalities available for geometric verification. 2D and 3D IGRT modalities are both beneficial in geometric verification while 3D imaging is a valuable tool in assessing volumetric changes that may have dosimetric consequences for this group of patients. IGRT if executed effectively and efficiently provides clinicians with confidence to reduce CTV-PTV margins thus limiting treatment related toxicities in patients. Accumulated exposure dose from IGRT vary considerably and may be incorporated into the treatment plan to avoid excess dose. However, there are considerable variations in the application of IGRT in RT practice. This paper aims to summarise the advances in IGRT in HNC treatment and provide clinics with recommendations for an IGRT strategy for HNC in the clinic.

Optimal image guided radiation therapy strategy for organs at risk sparing in radiotherapy of the prostate including pelvic lymph nodes

Background and purposePurpose of this study was to quantify the OAR dose for different position correction strategies, and to determine which strategy is most optimal for treating patients on the prostate and pelvic lymph nodes. Materials and methodsFor 30 patients, four different treatment plans were made reflecting different correction strategies: online correction on bony anatomy; offline correction on bony anatomy; online correction on the prostate fiducials; using 1 cm margins around both CTVs. The dose to the PTVs and OARs was quantified and a pairwise statistical analysis was performed. ResultsNo statistically significant differences were observed in the dose to the PTVs, ensuring that any OAR sparing is not caused by differences in PTV coverage. Dose to the rectum and anal canal was lowest when applying an online correction on prostate fiducials, although the total PTV volume was higher. Dose to the small bowel bag and femoral heads was slightly higher compared to online correction on bony structures, but well within clinically acceptable limits. ConclusionAlthough the total PTV volume is higher when applying an online correction on the prostate, this strategy leads to the most optimal sparing of relevant OARs, at the cost of a slightly higher dose to the femoral heads and small bowel bag.

Image Guided Radiation Therapy Strategies for Pelvic Lymph Node Irradiation in High-Risk Prostate Cancer: Motion and Margins

To quantify the relative motion of the pelvic lymph nodes (LNs), seminal vesicles (SV) and prostate and define indicative margins for image-guided radiotherapy based on bony anatomy or prostate correction strategies for a 3 or 6 degrees-of-freedom couch. Nineteen patients had a planning computed tomography (CT) scan followed by a mean of 11 repeated CT scans during radiation therapy. The prostate, SV, and external and internal iliac LN regions on the left and right were outlined on each CT scan. Systematic and random uncertainties were determined along with correlations between the motions of these regions. The clinical target volume to planning target volume margins required to take only motion into account were calculated for each guidance method. For bone guidance, motion of the prostate and LNs was largely uncorrelated. Margins to compensate for motion (left-right, superior-inferior, anterior-posterior, in cm) based on a 3-DOF couch were as follows: prostate (0.2, 0.6, 0.8), SV (0.4, 0.9, 1.0), and LNs (0.3, 0.4, 0.6). For prostate guidance, margins were calculated for correlated motion: prostate (0, 0, 0), SV (0.3, 0.5, 0.4), and LNs (0.3, 0.5, 0.9). For a 6-DOF couch, these margins were as follows: prostate (0.2, 0.6, 0.8), SV (0.3, 0.9, 1.0), and LNs (0.3, 0.4, 0.3) for bone guidance. For prostate guidance, margins were as follows: prostate (0, 0, 0), SV (0.2, 0.5, 0.4), and LNs (0.3, 0.6, 0.6). Image guided radiation therapy based on bony anatomy requires larger prostate and SV margins, and guidance on prostate requires larger LN margins. Neither guidance strategy is optimal, and a combination of the 2 or treatment adaptation after a number of fractions might be preferable. Calculation of the total margin should also include delineation uncertainties.

SU‐F‐J‐32: Do We Need KV Imaging During CBCT Based Patient Set‐Up for Lung Radiation Therapy?

Purpose:To evaluate the role of 2D kilovoltage (kV) imaging to complement cone beam CT (CBCT) imaging in a shift threshold based image guided radiation therapy (IGRT) strategy for conventional lung radiotherapy.Methods:A retrospective study was conducted by analyzing IGRT couch shift trends for 15 patients that received lung radiation therapy to evaluate the benefit of performing orthogonal kV imaging prior to CBCT imaging. Herein, a shift threshold based IGRT protocol was applied, which would mandate additional CBCT verification if the applied patient shifts exceeded 3 mm to avoid intraobserver variability in CBCT registration and to confirm table shifts. For each patient, two IGRT strategies: kV + CBCT and CBCT alone, were compared and the recorded patient shifts were categorized into whether additional CBCT acquisition would have been mandated or not. The effectiveness of either strategy was gauged by the likelihood of needing additional CBCT imaging for accurate patient set‐up.Results:The use of CBCT alone was 6 times more likely to require an additional CBCT than KV+CBCT, for a 3 mm shift threshold (88% vs 14%). The likelihood of additional CBCT verification generally increased with lower shift thresholds, and was significantly lower when kV+CBCT was used (7% with 5 mm shift threshold, 36% with 2 mm threshold), than with CBCT alone (61% with 5 mm shift threshold, 97% with 2 mm threshold). With CBCT alone, treatment time increased by 2.2 min and dose increased by 1.9 cGy per fraction on average due to additional CBCT with a 3mm shift threshold.Conclusion:The benefit of kV imaging to screen for gross misalignments led to more accurate CBCT based patient localization compared with using CBCT alone. The subsequently reduced need for additional CBCT verification will minimize treatment time and result in less overall patient imaging dose.

Comparison of Various Online Strategies to Account for Interfractional Variations for Pancreatic Cancer

To identify practical techniques to address the large interfractional variations for pancreas irradiation by comparing various used/proposed online strategies. The daily computed tomography (CT) images acquired using a respiration-gated in-room CT (CTVision; Siemens) for 10 pancreatic cancer patients treated with image guided radiation therapy (IGRT) were analyzed. The contours of the pancreas and organs at risk on each daily CT set were generated by populating from the planning CT using a deformable registration tool (ABAS; Elekta) with manual editing. Nine online strategies were considered: (1) standard IGRT (ie, IGRT with 0-mm additional margin [AM]); (2) IGRT with 2-mm AM; (3) IGRT with 5-mm AM; (4) IGRT with plan renormalized to maintain 95% planning target volume (PTV) coverage; (5) full-scale reoptimization; (6) reoptimization starting from the original plan; (7) segment aperture morphing (SAM) from the original plan, based on PTV shape change; (8) SAM plus segment weight optimization; and (9) reoptimization starting from the SAM plan. One-way analysis of variance was applied to plan qualities for the 9 strategies to assess statistical significance in difference. The 3 IGRT strategies (1-3) lead to either inadequate PTV coverage or higher doses to critical structures, indicating that the additional margins alone are not adequate to account for the changes. The full-scale reoptimization results in the best plan but requires the delineation of several structures, which is time consuming. The SAM strategy (7) was the fastest one, because it requires delineating only 1 structure (target), and its plan quality was comparable to that for the full-scale reoptimization. Online replanning strategies can lead to either reduced organs-at-risk dose and/or improved target coverage as compared with the current practice of IGRT. The SAM-based online replanning is comparable to full-scale reoptimization and is efficient for practical use.

Poster - Thur Eve - 28: Optimization of a prostate cancer IGRT protocol.

Our image-guided radiation therapy (IGRT) protocol for post-prostatectomy patients involves acquiring a kV cone beam computed tomography (CBCT) dataset at each fraction and shifting the treatment couch to align the surgical clips. This IGRT strategy requires significant resources, and delivers non-negligible dose to normal tissues. The objective of this work is to evaluate this IGRT protocol against two alternative strategies in terms of the dose-volume statistics for target and organ at risk regions. Our method involves deforming the planning CT to the CBCT dataset acquired at each fraction, computing dose on the deformed dataset, and inversely transforming the dose back onto the original planning CT dataset. The treatments of six patients were evaluated assuming three IGRT scenarios: no IGRT, daily IGRT using the clinically employed couch shifts, and alternating day IGRT. The doses delivered to the clinical target volumes are within approximately 3.2, 1.3, and 2.1% of the plan for the non-IGRT, daily, and alternating day IGRT protocols, respectively. Doses to relevant portions of the organs at risk deviate from the plan by up to 10.5, 13.1 and 10.7% for non-IGRT, daily IGRT, and alternating day IGRT protocols, respectively. Some cases do not differ significantly between IGRT and non-IGRT protocols in terms of cumulative DVHs, highlighting the difficult task of correcting prostate bed deformations via the treatment couch translations. In general, the alternating day IGRT protocol was found to result in a clinically insignificant deviation in delivered dose while providing a significant reduction in resource use and patient imaging dose.

TU-C-BRCD-01: Initiating an IGRT Program

The demands for precision of modern radiation therapy approaches (IMRT, SBRT) require a comprehensive understanding of the application of Image-Guided Radiation Therapy (IGRT) systems and their utilization. Modern IGRT systems provide unprecedented visualization of anatomy over the recent state-of-the art (MV portal imaging), with modern image guidance hardware (kV X-Ray, in-room CT, emerging external and internal imaging systems). They furthermore integrate advanced image registration methods as well as integrated couch control systems. While significantly empowering precision therapy, IGRT comes at a cost. Not only does the IGRT equipment require specific attention for commissioning and quality assurance, in many instances the physicists will need to re-learn or acquire new knowledge about imaging-specific issues. In all cases, a significant change to the operational procedures of the department will occur, and both extensive staff training, as well as implementation of new policies, procedures, and workflow models for treatment, will be necessary. The relationship between information gathered at simulation, tradeoffs made during treatment planning, and plan sensitivities will influence the use of image guidance, and uncertainties in imaging at the treatment unit may require re-consideration of margins and planning strategies. This course will help guide a physicist through the process of initiating an IGRT program. The overall concept of implementing IGRT from a departmental perspective will be briefly overviewed. A detailed discussion of IGR T technologies and the details of their commissioning and quality assurance needs will be provided. The development of IGRT strategies, policies, training, and process quality control will be described. Learning Objectives: 1. Understand the technical and procedural components involved in initiating an IGRT program 2. Understand the costs and benefits of various levels of IGRT use 3. Understand the needs for quality assurance tied to IGRT implementation 4. Understand the relationships between IGRT and decisions made in simulation and treatment planning Gain sufficient knowledge to proactively plan new IGRT utilization paradigms in the clinic

TH-C-BRA-02: Adaptive Image-Guided Radiation Therapy: A Physicianˈs PerspectiveTherapy Symposium

Image guided radiation therapy(IGRT) today is focused on the use of in‐room imaging technologies to improve patient setup and target localization. IGRT in the future, however, will also include the use of imaging to adapttreatment to changes in the tumor (and patient) during treatment. One of the most important changes which can be adapted to is in morphology of both the tumor and surrounding normal tissues. Newer in‐room volumetric imaging opens the door to adapting to such changes. Discussion of adaptive IGRT quickly turn to the technical issues involved including automated segmentation and deformable registration. However, a fundamental understanding of the variety and frequency of morphologic changes which occur is crucial to the development of adaptive IGRT strategies. This lecture reviews the morphologic changes which occur during treatment of patients with CNS, Head and Neck, lung, gastrointestinal, breast, gynecologic, genitourinary and pediatric tumors. Published data on adaptive IGRT approaches in a variety of tumor sites will also be presented.

SU‐GG‐J‐57: Comparison of the Residual Error in Treatment Set‐Up Using Different Imaging Modalities: A Phantom Study

Purpose: To assess the accuracy of three different imaging modalities (orthogonal pairs of 2D‐kV or MV images and kV cone‐beam CT (CBCT)) for image guided radiation therapy (IGRT) for four clinical treatment scenarios. Method and Materials: The accuracy of IGRT was investigated using four anatomical phantoms, each representing a specific clinical site and treatment set‐up strategy. Bony alignment was used as a surrogate for tumour location and set‐up accuracy for pelvic, head and neck and thorax phantoms, while implanted fiducials were the alignment target of a second pelvic phantom. Each phantom was subjected to the IGRT process currently used at this institution. For each imaging modality, the phantoms were set‐up on the treatment couch and imaged. These images were aligned to the reference images and a couch translation was applied to correct for the set‐up error. Verification images were then obtained to determine the residual error, which is an indication of IGRT accuracy. Results: All three image modalities resulted in a low average residual error (<1mm), with the residual error represented by the mean 3D displacement vector. The 3D CBCT resulted in the lowest overall residual error (0.35mm) as compared to the 2D MV imaging (0.7mm) and 2D kV imaging (0.5mm). This advantage, however, is site specific, and the use of volumetric imaging had the largest impact on IGRT strategies relying on bony alignment to the skull or thorax. For example, the volumetric information helped reduce the residual error to 0.24mm for the skull, compared to 1.04 and 0.63mm for the MV and kV portal imaging based techniques, respectively. Conclusion: CBCT imaging provided the lowest residual error for set‐up correction, but the magnitude of the advantage was site‐specific. More sophisticated or clinically time‐consuming techniques would be best suited to treatment sites for which they would provide the most benefit.