Medical physics challenges in clinical MR-guided radiotherapy

The integration of magnetic resonance imaging (MRI) for guidance in external beam radiotherapy has faced significant research and development efforts in recent years. The current availability of linear accelerators with an embedded MRI unit, providing volumetric imaging at excellent soft tissue contrast, is expected to provide novel possibilities in the implementation of image-guided adaptive radiotherapy (IGART) protocols. This study reviews open medical physics issues in MR-guided radiotherapy (MRgRT) implementation, with a focus on current approaches and on the potential for innovation in IGART.Daily imaging in MRgRT provides the ability to visualize the static anatomy, to capture internal tumor motion and to extract quantitative image features for treatment verification and monitoring. Those capabilities enable the use of treatment adaptation, with potential benefits in terms of personalized medicine. The use of online MRI requires dedicated efforts to perform accurate dose measurements and calculations, due to the presence of magnetic fields. Likewise, MRgRT requires dedicated quality assurance (QA) protocols for safe clinical implementation.Reaction to anatomical changes in MRgRT, as visualized on daily images, demands for treatment adaptation concepts, with stringent requirements in terms of fast and accurate validation before the treatment fraction can be delivered. This entails specific challenges in terms of treatment workflow optimization, QA, and verification of the expected delivered dose while the patient is in treatment position. Those challenges require specialized medical physics developments towards the aim of fully exploiting MRI capabilities. Conversely, the use of MRgRT allows for higher confidence in tumor targeting and organs-at-risk (OAR) sparing.The systematic use of MRgRT brings the possibility of leveraging IGART methods for the optimization of tumor targeting and quantitative treatment verification. Although several challenges exist, the intrinsic benefits of MRgRT will provide a deeper understanding of dose delivery effects on an individual basis, with the potential for further treatment personalization

Level of Anxiety in Breast Cancer Patients receiving Locoregional Radiation Therapy and Its correlation with inter-fraction variations observed during delivery of treatment

INTRODUCTION : Breast cancer is the most frequent cancer among women according to the GLOBOCON 2012 report contributing to 27% of all new cancers in women in India. Radiation therapy forms an integral part of its multi-modality treatment. Various studies have found a significant level of anxiety among patients undergoing radiation therapy due to fear of unknown. During delivery of radiation therapy for breast cancer patients, other than setup uncertainties, target motion due to breathing is also significant. High anxiety levels may lead to significant change in breathing pattern which could result in larger setup variations observed during the treatment. AIMS AND OBJECTIVES : Our study aimed to estimate the level of anxiety in non metastatic breast cancer patients undergoing radiation therapy. It also aimed to measure the inter fraction variations and to determine any correlation between the observed anxiety levels and inter fraction variations recorded during the treatment. Thus, trying to determine the need of counseling for anxious patients to reduce these errors. METHODS AND MATERIALS : The anxiety levels were measured using the Beck Anxiety Inventory (BAI) questionnaire which contains 21 items. It was administered at the time of simulation, first three days of treatment and weekly once during the course of treatment. The inter-fraction variation was measured using the electronic portal images for the same days and correlation was calculated using Pearson correlation coefficient.. RESULTS: Twenty seven patients who received chest wall or whole breast irradiation with or without nodal irradiation using field in field technique were assessed. The anxiety levels were found to be high at the time of simulation and on the first day of treatment (median anxiety score was 7) , it then declined rapidly over the subsequent days(median score was 4 and 3 respectively on second and third day of treatment). The mean systemic error observed for vertical, lateral and longitudinal direction was 3.4,3.4 and 3 mm and random error for all parameters was within 5 mm. The group systematic error for central lung distance (CLD) was 3.1 mm with a standard deviation of 1.8 mm. The CLD variation compared to baseline was found to be more on the first day of treatment and was significantly correlated (r=0.45, p=0.02) with higher level of anxiety seen at starting of treatment. CONCLUSION : Patients have high anxiety levels during planning and starting of treatment. This affects the breathing pattern and therefore the treatment delivery in breast cancer patients especially on the first day of treatment. For optimal treatment and care of patients, an effort to understand the patients' anxiety and addressing it by better communication about radiation therapy procedure and likely side effects should be undertaken by the oncology team

A method to verify sections of arc during intrafraction portal dosimetry for prostate VMAT.

This study investigates the use of a running sum of images during segment-resolved intrafraction portal dosimetry for volumetric modulated arc therapy (VMAT), so as to alert the operator to an error before it becomes irremediable. At the time of treatment planning, predicted portal images were created for each segment of the VMAT arc, and at the time of delivery, intrafraction monitoring software polled the portal imager to read new images as they became available. The predicted and measured images were compared and displayed on a segment basis. In particular, a running sum of images from ten segments (a 'section') was investigated, with mean absolute difference between predicted and measured images being quantified. Images for 13 prostate patients were used to identify appropriate tolerance values for this statistic. Errors in monitor units of 2%-10%, field size of 2-10 mm, field position of 2-10 mm and path length of 10-50 mm were deliberately introduced into the treatment plans and delivered to a water-equivalent phantom and the sensitivity of the method to these errors was investigated. Gross errors were also considered for one case. The patient images show considerable variability from segment to segment, but when using a section of the arc the variability is reduced, so that the maximum value of mean absolute difference between predicted and measured images is reduced to below 12%, after excluding the first 10% of segments. This tolerance level is also found to be applicable for delivery of the plans to a water-equivalent phantom. Using this as a tolerance level for the error plans, a 10% increase in monitor units is detected, 4 mm increase or shift in multileaf collimator settings can be detected, and an air gap of dimensions 40 mm  ×  50 mm is detected. Gross errors can also be detected instantly after the first 10% of segments. The running difference between predicted and measured images over ten segments is able to identify errors at specific regions of the arc, as well as in the overall treatment

Improving Radiotherapy Targeting for Cancer Treatment Through Space and Time

Radiotherapy is a common medical treatment in which lethal doses of ionizing radiation are preferentially delivered to cancerous tumors. In external beam radiotherapy, radiation is delivered by a remote source which sits several feet from the patient\u27s surface. Although great effort is taken in properly aligning the target to the path of the radiation beam, positional uncertainties and other errors can compromise targeting accuracy. Such errors can lead to a failure in treating the target, and inflict significant toxicity to healthy tissues which are inadvertently exposed high radiation doses. Tracking the movement of targeted anatomy between and during treatment fractions provides valuable localization information that allows for the reduction of these positional uncertainties. Inter- and intra-fraction anatomical localization data not only allows for more accurate treatment setup, but also potentially allows for 1) retrospective treatment evaluation, 2) margin reduction and modification of the dose distribution to accommodate daily anatomical changes (called `adaptive radiotherapy\u27), and 3) targeting interventions during treatment (for example, suspending radiation delivery while the target it outside the path of the beam). The research presented here investigates the use of inter- and intra-fraction localization technologies to improve radiotherapy to targets through enhanced spatial and temporal accuracy. These technologies provide significant advancements in cancer treatment compared to standard clinical technologies. Furthermore, work is presented for the use of localization data acquired from these technologies in adaptive treatment planning, an investigational technique in which the distribution of planned dose is modified during the course of treatment based on biological and/or geometrical changes of the patient\u27s anatomy. The focus of this research is directed at abdominal sites, which has historically been central to the problem of motion management in radiation therapy

Accurate Targeting of Liver Tumors in Stereotactic Radiation Therapy

This doctoral thesis concerns the treatment of liver cancer patients using external beam radiotherapy. The quality of this treatment greatly depends on delivering a high radiation dose to the tumor while keeping the dose as low as possible to surrounding healthy tissues. One of the major challenges is locating the tumor at the moment of dose delivery. In this ork, the uncertainty of locating the tumor was investigated. For this purpose, gold markers were implanted in the liver tissue and visualized on X-ray images. The markers were used to measure day-to-day tumor mobility and motion due to respiration. Furthermore, it was found that major improvements in the targeting accuracy can be achieved by using the markers for guiding the treatment procedure