Let present things in details as they are applied in a clinical routine: in modern radiotherapy techniques such as intensity-modulated radiation therapy (IMRT) and volume modulated arc therapy (VMAT), the quality assurance (QA) process is vital; so, careful pretreatment checks with a patient‐related QA protocol is a crucial step in the pretreatment process in radiation therapy, because of treatment plan complexity and high sophisticated technology behind the radiation delivery.
Several tools are available for a two dimensional evaluation, and among these tools we can find arrays of diodes, (e.g. MapCHECK) or arrays of ionization chambers, (e.g. PTW Octavius®‐4D). For example, in a research project published in August 17, 2020, I worked with OCTAVIUS 4D system (PTW) [3]. This system consists of an ion chamber array embedded in a cylindrical phantom which, assisted by an inclinometer, rotates synchronously with the LINAC’S gantry. For VMAT plans (as it is the case for that publication), it measures planar dose distributions as a function of gantry angle in order to compute the resulting 3D dose distribution. There are two options for the ion chamber array:
Octavius 729 (general purpose) used in that paper and Octavius 1000 SRS (small field size, high resolution). With this setup, a perfect isotropic measurement geometry is achieved. The software acquires the dose inside the entire, cylindrical volume and allows dose planes to be extracted for further analysis. The measured data can be visualized in relation to patient contours and organ structures. OCTAVIUS 4D also allows the reconstruction of a volumetric dose distribution.
The (gamma) γ-metric is the standard technique used to evaluate the agreement between the planned (calculated) dose (in the treatment planning system –TPS, in the publication cited here, I employed the Pinnacle3 as TPS) and measured dose (in OCTAVIUS 4D) and can be obtained not only in a 2D array but also 3D, allowing the evaluation of the whole volumetric dose distribution. Plan dose perturbation (PDP) uses the difference between the measured and TPS calculated dose (TPS dose map) in phantom to perturb the 3D TPS calculated patient dose and create a corrected 3D dose distribution in the patient geometry. In Octavius 4D phantom, the PDP is performed using the VeriSoft software; (for comparison, another software named 3DVH is used for the same purpose for MapCHECK).
This method does not require a forward calculation algorithm. It relies on the measurement to create the perturbation matrix for correcting the TPS generated plan. This can be seen as follow: "a measurement is made on the phantom (Octavius 4D or MapCHECK for example) and compared to a TPS dose map using a software such as VeriSoft or 3DVH; and this results in a dose error map in the QA phantom in the specific plane of measurement and at the detector locations in the VeriSoft.
When the dose error is applied to the original TPS dose value (calculated or planned dose), the result constitutes the measured dose plan. The TPS dose calculation results from a dosimetry model that has been initially commissioned with the LINAC. Since the detector location in (x,y,z) coordinates is known with respect to the radiation source, the dose correction factor at each detector location can be applied to the TPS dose rays intersecting the detector and the target.
So, to summarize the above: in perturbation of a modeled dose, we have: a 3D dose function in the TPS that has been approved for treatment, i.e., unperturbed planed dose D(x,y,z), a delivery system or TPS modeling error that perturbs the planned dose function, i.e., perturbation operator “d(x,y,z)”, a QA phantom measurement of the dose delivery that results in a 3D correction technique that allows an approximate solution to the 3D perturbed dose D'(x,y,z), i.e., that which was delivered. Hence we get a good approximation to the Solution of the plan dose that has been perturbed, i.e., Plan Dose Perturbation (PDP) [4]. The presence of tissue heterogeneities, such as air cavities, lungs, bony structures, and prostheses, can greatly impact the calculated dose distribution.
The change in dose is due to the perturbation of the transport of primary and scattered photons and that of the secondary electrons set in motion from photon interactions. Depending on the energy of the photon beam and the shape, size, and constituents of the inhomogeneities, the resultant change in dose can be large. Perturbation of photon transport is more noticeable for lower-energy beams.
There is usually an increase in transmission, and therefore dose, when the beam traverses a low-density inhomogeneity. The reverse applies when the inhomogeneity has a density higher than that of water. However, the change in dose is complicated by the concomitant decrease or increase in the scatter dose [5].
With the above in mind, it appears that Monte Carlo simulations of the radiation transport and the radiation interaction with matter (human tissues in the case of radiotherapy) are extremely important and useful in medical radiation physics, particularly when the evaluation of radiation absorbed doses is considered.
This is the reason why Monte Carlo methods using Markov chain and Quantum field theory (QFT) are particularly appealing to me and constitute my primary research area of interest in Theoretical Physics.
References:
[1] McParland, B.J.. (2014). Medical Radiation Dosimetry: Theory of Charged Particle Collision Energy Loss. DOI: 10.1007/978-1-4471-5403-7. ISBN 978-1-4471-5402-0.
[2] Bouchard, H., Seuntjens, J., Duane, S., Kamio, Y., & Palmans, H. (2015). Detector dose response in megavoltage small photon beams. I. Theoretical concepts. Medical Physics, 42(10), 6033-6047. doi:10.1118/1.4930053.
[3] Bogmis, A.I., Popa, A.R., Adam, D., Ciocâltei, V., Guraliuc, N.A., Ciubotaru, F. and Chiricuță, I.-C. (2020) Complex Target Volume Delineation and Treatment Planning in Radiotherapy for Malignant Pleural Mesothelioma (MPM). International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 9, 125-140. https://doi.org/10.4236/ijmpcero.2020.93012.
[4] Nelms et al. United States Patent. Patent No.: US 7,945,022 B2. Date of Patent: May 17, 201.
[5] Halperin, Edward C, David E. Wazer, Carlos A. Perez, and Luther W. Brady. Perez & Brady's Principles and Practice of Radiation Oncology. 2019. 7th Edition. ISBN-10: 1496386795.
In medical physics, radiotherapy is one of the main treatments for cancer, used in 50% of cancer treatments, and a multi-disciplinary field of research, mostly involving medicine, physics, mathematics, and computer science.
My goal when choosing to specialize in radiotherapy for my residency training program is to help make improvement in radiotherapy treatment delivery to cancer patients.
==> My current research areas of interest are the
- Markov Chain Monte Carlo methods
- in Medical Radiation Physics,
- in Quantum field theory (QFT);
- development of new treatment planning algorithms for radiotherapy;
- study of the radiobiological mechanisms underlying the FLASH effect for FLASH radiotherapy using the Geant4-DNA code.
Below are the abstracts of my 2 MSc's Theses and 5 selected academic projects in R and MATLAB programming.