Electron Portal Imaging System - Article Example

? The Role of Electronic Portal Imaging Systems in Radiation Oncology Since the emergence of electronic portal imaging (EPI) systems towards the end of the 1950s, several advancements in the field have significantly contributed towards shaping the effectiveness of portal imaging in patient positioning that must be performed accurately before the commencement of radiotherapy. This paper explores the innovations in portal imaging to assert that the course of these advancements has been largely positive in assisting the attainment of the objectives of portal imaging. The paper employs the evidence provided by research and studies into the field to arrive at plausible conclusions, the effectiveness of procedures such as the implantation of radiopaque gold markers as an alternative practice in anatomical regions whose movement is not dependent upon bony landmarks is assessed, conclusions declare that the aforementioned procedure holds substantial credibility and is a correct method that can be utilized as an on-line strategy for image-guided radiation therapy (IGRT), this outcome is supported by mathematical computations and evaluations of the results. The paper also outlines and identifies potential advancements in the field such as the implementation of aSi-based EPIDs that can play a critical role in developing and aiding EPI systems in the near future. 1. Introduction The treatment of cancer by the means of radiation or exposure to a radioactive element requires further research to allow for the achievement and enhancement of pivotal objectives behind radiotherapy. According to Kirby and Glendinning (2006: pS50) advancements in radiotherapy must occur to explore solutions through which the dose that is aimed towards the target volume is enhanced such that the impact of high radiation on adjoining tissue, which is in a healthy state is lessened to a substantial extent. Pouliot et al. (2003: p862) state that in determining the exact location of the target volume the electron beams must be pointed in a precise direction to satisfy the requirements of a consistent dosimetric exposure. Piermattei et al. (2006) have presented a procedure which allows for the in vivo determination of the focal point of radiation that is tested on the target volume of pelvic regions. With regards to the accomplishment of aforementioned objectives and aims, enhancements in electronic portal imaging systems can play a critical role. Portal imaging essentially assists the geometric substantiation of field position (Kirby and Glendinning 2006: pS50) thereby, diminishing the possibility of geometric ambiguity (Pouliot et al. 2003: p862). According to Pouliot et al. (2003), the process of portal imaging aims to employ the assistance of bony landmarks to establish the placement and setup of the patient that is relative to the focal point of radiation, however, further considerations into the subject are required to direct researchers into the application and implementation of portal imaging systems in radiotherapy when discussing target volumes in soft tissue tumors of the prostate. The paper aims to discuss the advancements in portal imaging systems over the decades by assessing comprehensive literatures presented by Kirby and Glendinning (2006), Pouliot et al. (2003), Baker et al. (2005) and Piermattie et al. (2006), the primary focus of this assessment rests upon evaluating the technological innovations and developments in electronic portal imaging devices (EPIDs). Dedicated sections in the paper also discuss the use of radiopaque markers in the accurate visualization of the prostate via portal imaging to depict the accuracy of its placement throughout the procedure of radiotherapy by the means of a study conducted on a sample size of 11 patients. In conclusion the paper reviews and appraises the advancements in the development of electronic portal imaging systems by assessing the literature under review and stating that the developments in electronic portal imaging systems over the decades have been positive and highly substantial. Reflections for further developments in portal imaging are also presented and areas of consideration in the field are also identified. 2. Initial Developments in EPI Systems Kirby and Glendinning (2006) note that initial developments in portal imaging were marked by the introduction of radiographic films that have been consistently transformed and improved upon to produce images of enhanced quality; this factor has significantly contributed towards the enhancement of electronic portal imaging (EPI) systems by assisting the attainment of fine quality images and improving the utility of cassettes, however, the usage of radiographic films in EPI systems has also led to the emergence of certain limitations which have rendered it ineffective for the purposes of on-line imaging and extensively increased the necessity of storage options. Even though, later advancements led to the development of Computed radiography (CR) the benefits and limitations of the system presented similar results to that of radiographic film with the additional advantage of reusability (Kirby and Glendinning 2006: pS51). 3. The Progress of EPIDs Since the emergence of the first framework designed for the development of EPI systems, electronic portal imaging devices (EPIDs) have intended to integrate the expertise of computer technology to generate fine quality images with further advantages such as quick generation of images. Another motive behind the development of EPIDs has been to accommodate and assist on-line image-guided radiation therapy (IGRT) strategies that are implemented as a corrective measure during the process of radiation oncology (Kirby and Glendinning 2006). Electronic portal imaging devices that are based on the assistance of cameras employ the same pattern for functioning as used by film, in doing so a metal cover is integrated with a rare earth compound ranging from 100-400 mg cm-1 in thickness (Kirby and Glendinning 2006: pS52). It should be noted however, that camera based EPIDs have also displayed certain limitations and inadequacies in their functioning that is a consequence of the camera being able to capture an extremely minute quantity of the light (Kirby and Glendinning 2006: pS52). Kirby and Glendinning (2006) note that one of the measures that can be undertaken to combat this inadequacy is to augment the thickness of rare earth compounds such as phosphor, it is important to note however that this measure must be finalized once its adverse impact on spatial resolution is taken into consideration. Another significant advancement in electronic portal imaging devices that was extensively promoted throughout the 1990s came in with the introduction of ionization chambers in EPIDs, this invention significantly reduced the time that was taken for image generation, however, the issue of poor spatial resolution which was also identified in camera based EPIDs persisted. A benefit of this technology lies in its reduction of geometric ambiguity which according to Pouliot et al. (2003: p862) helps enhance dose conformity. Considering the limitations associated with camera based EPIDs, progress in EPI systems has also been assisted by research into the development of improved camera based EPIDs that can counter the problems that were connected with previous models. In newer models the integration of optical fiber channels within the device guides the movement of the light from phosphor to the camera (Kirby and Glendinning 2006: pS53) however, certain inconsistencies with this design may impact image quality in an adverse manner. According to Baker et al. (2005) a pivotal discovery that has significantly aided the advancements of EPIDs is the incorporation of an amorphous silicon device that has the capacity to advance conventional QC by significantly reducing the time taken for measurements and examination. Kirby and Glendinning (2006) observe that an alternative design used in portal imaging employs inorganic scintillators such as crystals to generate images of enhanced quality, this technology however increases the time that is taken for image generation and scanning. Nonetheless, the model has been under use for several clinical purposes. As a possible solution to the issue of increased scanning times in the implementation of camera based EPIDs, Baker et al. (2005) note that the incorporation of an amorphous silicon EPID has produced positive results for use in clinical practice by diminishing the overall time taken for processing the film and examining it. Kirby and Glendinning (2006: pS60) claim that the precision and accuracy of in vivo as a primary dosimetric procedure stands at a range of 2% to 5%, however, this value can be improved by the employment of aSi-based EPIDs that have shown consistent outcomes in comparison with the results produced by film (Baker et al. 2005: p1391). These conclusions are also confirmed by the study conducted by Piermattei et al. (2006) with regards to the development of aSi-based EPIDs. Apart from validating the geometric field placement and diminishing any geometric ambiguity in patient positioning, successful implementation of EPI systems must also allow for the dosimetric application such that the dose conformity is enhanced, maintained and monitored. In fulfilling this objective clinical researchers have applied in vivo dosimetry by the means of an amorphous silicon based EPID, this method has been able to generate consistency and accuracy in determining the isocenter to verify patient positioning (Piermattei et al. 2006). 4. Advancements in On-Line IGRT Strategies The progress of corrective strategies with regards to image-guided radiation therapy (IGRT) has shown significant improvements and advancements that are backed by empirical data and evidence provided by related studies. As noted during the course of this discussion, the primary objectives of portal imaging are to confirm patient position before the commencement of radiation oncology (Pouliot et al. 2003) and to validate the geometric placement of the target volume that is to be exposed to high doses of radiation (Kirby and Glendinning 2006). In the treatment of certain cancers, bony landmarks or anatomy may fail to verify the accurate location of the target volume, this scenario may occur due to a number of reasons and maybe associated with respiratory or bladder functions (Kirby and Glendinning 2006: pS59) or the nature of the target volume itself thereby, freeing its movement such that it no longer depends on bony anatomy; the prostate is an example of such a target volume (Pouliot et al. 2003). For such target volumes, the implantation of radiopaque gold markers has been operational in University of California San Francisco for more than a decade to assist the image generation of the prostate by the means of portal imaging (Pouliot et al. 2003). Kirby and Glendinning (2006) note that this procedure is capable of producing adequate images of sites that are independent of bony anatomy. A critical postulation of this approach asserts that the movement of radiopaque gold markers does not take place during radiation therapy; this hypothesis declares that the process by which the insertion of the markers acts as an alternative to the absence of bony landmarks can replicate the accuracy that is observable in portal imaging procedures that are conducted on target volumes other than the prostate. Certainly this assumption needs to be verified if the desired consequences of radiotherapy and the achievement of the objectives of portal imaging are to be ensured. In a study conducted by Pouliot et al. (2003) a sample size of 11 patients who were undergoing radiotherapy was selected, each patient was implanted with 3 radiopaque gold markers and in total, the movement of 33 markers was observed during the course of the radiotherapy. The researchers employed the assistance of ISOLOC localization software to calculate the three-dimensional space between the markers, the mathematical equation that contributed towards ensuring the exactitude of the placement of the marker was developed by considering aspects such as the size of portal imaging pixels and the isocenter to reduce any irregularities or ambiguity associated with marker position (Pouliot et al. 2003). The results of the study were analyzed by stating the standard deviation (SD) of the distances that were recorded for the three markers for each patient participating in the study and the outcomes showed that the SD of the distances was a small value (Pouliot et al. 2003: p866). The calculations conducted during the course of this study reflect that the radiopaque markers implanted in the patients did not move considerably or cover a substantial distance within the site; the researchers computed a mean standard deviation of 1.3 mm that was similar to the margin of ambiguity of the framework implemented during the course of the study (Pouliot et al. 2003: p866). In conclusion, the results of the study reflect that the implantation of radiopaque gold markers is in fact a successful approach to portal imaging and has the capacity to produce satisfactory results for those target volumes that are not dependent upon bony anatomy in their movement. Another important implication of the research findings is that they strengthen the implementation of on-line corrective strategies for image-guided radiation therapy (IGRT). 5. Conclusion The objective of this paper was to document and explore the advancements in electronic portal imaging systems, by assessing how technical and technological improvements have contributed towards the field of radiation oncology and the treatment of cancers. The study conducted by Pouliot et al. (2003) confirms the contribution of physics and mathematical assessments in comprehending the credibility, viability and success rate of technological innovations in portal imaging that can assist in the treatment of regions whose cure cannot be managed by traditional approaches. The literature presented by Pouliot et al. (2003), Kirby and Glendinning (2006) is a reflection of the significant advancements that have been made with regards to electronic portal imaging (EPI) systems to enhance the quality of images and the time that is consumed for the generation of images. More importantly, the advancements in EPIDs have been aided by studies and researches that have verified the application of various techniques in in vivo dosimetry and determination of the isocenter accuracy for administering the dose, Baker et al. (2005) have been able to validate that amorphous silicon based EPIDs have the capability to reduce the inconsistencies attached with film such as issues with consistency and accuracy, the advantages of aSi-based EPIDs that have been identified by Piermattei (2006) also confirm that the implementation of the process can enhance dose administration as assessed on a target volume such as pelvic tumors. The role of portal imaging in radiation oncology is undeniable and should not be overlooked by researchers, as Kirby and Glendinning (2006: S62) state that advancements in EPI systems may still be contributed towards by promising technology such as the AMFPI to enhance patient positioning. Moreover, it should also be noted that investigators must continue to measure, check, monitor and verify the credibility of procedures such as radiopaque marker implantation to test the assumptions and hypothesis behind their implementation and administration over patients, this would help confirm the success rate of such procedures and also increase the possibility of discovering other effective procedures that can be applied to ensure that the objectives of portal imaging are successfully attained. BIBLIOGRAPHY Baker, S. J., Budgell, G. J., & Mackay, R. I. (2005). Use of an amorphous silicon electronic portal imaging device for multileaf collimator quality control and calibration. Physics in medicine and biology, 50(7), 1377. Kirby, M. C., & Glendinning, A. G. (2006). Developments in electronic portal imaging systems. British journal of radiology, 79(Special Issue 1), S50-S65. Piermattei, A., Fidanzio, A., Stimato, G., Azario, L., Grimaldi, L., D’Onofrio, G., ... & Cellini, N. (2006). In vivo dosimetry by an aSi-based EPID. Medical physics, 33, 4414. Pouliot, J., Aubin, M., Langen, K. M., Liu, Y. M., Pickett, B., Shinohara, K., & Roach III, M. (2003). (Non)-migration of radiopaque markers used for on-line localization of the prostate with an electronic portal imaging device. International Journal of Radiation Oncology* Biology* Physics, 56(3), 862-866. Read More