Dosimetry of Static and Moving Targets in IMRT and VMAT
Meaning of dosimetry
Dosimetry refers to the measurement of the dose that is absorbed and which is delivered by ionizing radiation. It is a scientific sub – specialty that is mostly applied in medical physics and health physics. In more specific terms, dosimetry in health and medical physics involves the assessment and calculation of the dose of radiation as received by the body of a human being (Jin et al. 2013).
Dosimetry in IMRT and VMAT
IMRT stands for Intensity Modulated Radiation Therapy. The implementation of both IMRT and VMAT systems has necessitated the dire need for accuracy in dosages that are delivered to patients. Comprehensive quality assurance is one vital concern in the medical fraternity and therefore, programs have been introduced to ensure that there is accurate functioning of all the components in radiotherapy. There has been an increasing need for the use of in vivo dosimetry (IVD) during the treatment of the patients (Mijnheer et al.2013).
In vivo dosimetry is used in external beam radiotherapy also incorporate the use of point detectors. These types of dosimetry offer challenges in terms of measurement especially during conformal radiotherapy that is done in 3D. Moreover, during pre – treatment IMRT delivery has the tendency of being undetected in quality assurance checks. The importance of the use of the in vivo dosimetry (IVD) is in its ability to detect many errors and the assessment of all differences that are deemed to be clinically important especially when it comes to both delivered and planned doses. Moreover, in vivo dosimetry provides detailed records for patients’ doses and goes a step further in fulfilling the requirements by the law (studentski et al. 2013).
EPID dosimetry has a set of unique possibilities for the purposes of quality assurance of both IMRT and in vivo dosimetry applications. Two approaches are mostly used with the application of the EPID in vivo dosimetry. First, a planned CT data of a patient is used to predict a portal dose image. This is done by placing the patient in the beam in alignment with the EPID. This first approach offer limitations that come as a result of the inability to determine the correlation between the patient (phantom) and the EPID dose differences. Hence most physicians prefer employing the back – projection method. The second approach is very simple and was introduced by Piermattei which was primary meant for the determination of the midplane dose using the in vivo dosimetry. This method involves the determination of the beam along the axis of the central beam and this incorporates the use of a – Si type of EPID of the central pixels.
The VMAT verification on the other hand calls for the need to modify software for the purpose of incorporating automatic correction and image acquisition of the gantry – angle. The automatic correction is meant for the EPID panel flex. The backward – projection in 3D in IMRT is adapted for the purpose of the inclusion of the primary dose calculation instead of the use of measured transmission data. Clinical VMAT plan are mostly presented as a report of EPID based dosimetry and is similar to the report of the 2D verification of IMRT (Tsai et al. 2011).
The position of the patient during and before treatment is very significant. The accuracy of dose delivery to a patient (phantom) is not dependent on the correct position of a patient. This because the correct patient positioning cannot directly account for the malfunctioning of a machine, the change in the anatomy of a patient or the corruption of the pieces of data. The most important element of patient treatment is through the determination of the dosages that are offered to the patients which is done by means of in vivo dosimetry. This is the importance of the 3D dose verification involving in vivo dosimetry.
Jin, X., Yi, J., Zhou, Y., Yan, H., Han, C., & Xie, C. (2013). Comparison of whole-field simultaneous integrated boost VMAT and IMRT in the treatment of nasopharyngeal cancer. Medical Dosimetry, 38(4), 418-423. http://dx.doi.org/10.1016/j.meddos.2013.05.004
Mijnheer, B., Olaciregui-Ruiz, I., Rozendaal, R., Sonke, J. J., Spreeuw, H., Tielenburg, R., … & Mans, A. (2013). 3D EPID-based in vivo dosimetry for IMRT and VMAT. In Journal of Physics: Conference Series (Vol. 444, No. 1, p. 012011). IOP publishing.
Studenski, M., Bar-Ad, V., Siglin, J., Cognetti, D., Curry, J., Tuluc, M., & Harrison, A. (2013). Clinical experience transitioning from IMRT to VMAT for head and neck cancer. Medical Dosimetry, 38(2), 171-175. http://dx.doi.org/10.1016/j.meddos.2012.10.009
Tsai, C., Wu, J., Chao, H., Tsai, Y., & Cheng, J. (2011). Treatment and Dosimetric Advantages Between VMAT, IMRT, and Helical TomoTherapy in Prostate Cancer. Medical Dosimetry, 36(3), 264-271. http://dx.doi.org/10.1016/j.meddos.2010.05.001