Biomedical subjects

Lee Chin

Publications and source records attributed to Lee Chin.

5 recordsLinked to original sources

PubMed2006

Automatic online adaptive radiation therapy techniques for targets with significant shape change: a feasibility study.

This work looks at the feasibility of an online adaptive radiation therapy concept that would detect the daily position and shape of the patient, and would then correct the daily treatment to account for any changes compared with planning position. In particular, it looks at the possibility of developing algorithms to correct for large complicated shape change. For co-planar beams, the dose in an axial plane is approximately associated with the positions of a single multi-leaf collimator (MLC) pair. We start with a primary plan, and automatically generate several secondary plans with gantry angles offset by regular increments. MLC sequences for each plan are calculated keeping monitor units (MUs) and number of segments constant for a given beam (fluences are different). Bulk registration (3D) of planning and daily CT images gives global shifts. Slice-by-slice (2D) registration gives local shifts and rotations about the longitudinal axis for each axial slice. The daily MLC sequence is then created for each axial slice/MLC leaf pair combination, by taking the MLC positions from the pre-calculated plan with the nearest rotation, and shifting using a beam's-eye-view calculation to account for local linear shifts. A planning study was carried out using two head and neck region MR images of a healthy volunteer which were contoured to simulate a base-of-tongue treatment: one with the head straight (used to simulate the planning image) and the other with the head tilted to the left (the daily image). Head and neck treatment was chosen to evaluate this technique because of its challenging nature, with varying internal and external contours, and multiple degrees of freedom. Shape change was significant: on a slice-by-slice basis, local rotations in the daily image varied from 2 to 31 degrees, and local shifts ranged from -0.2 to 0.5 cm and -0.4 to 0.0 cm in right-left and posterior-anterior directions, respectively. The adapted treatment gave reasonable target coverage (100%, 90% and 80% of the base-of-tongue, left nodes and right nodes, respectively, receiving the daily prescription dose), and kept the daily cord dose below the limit used in the original plan (65%, equivalent to 46 Gy over 35 fractions). Most of the loss of coverage was due to one shoulder being raised more superior relative to the other shoulder compared with the plan. This type of skew-like motion is not accounted for by the proposed ART technique. In conclusion, this technique has potential to correct for fairly extreme daily changes in patient setup, but some control of the daily position would still be necessary. Importantly, it was possible to combine treatments from different plans (MLC sequences) to correct for position and shape change.

Algorithms↗

PubMed2006

Dynamic IMRT treatments of sinus region tumors: comparison of Monte Carlo calculations with treatment planning system calculations and ion chamber measurements.

Results are presented comparing Monte Carlo (MC) calculations for dynamic IMRT treatments of tumors in the sinus region with Eclipse treatment planning system dose calculations, and ion chamber measurements. The EGS4nrc MC code, BEAMnrc, was commissioned to simulate a Varian 21Ex Linac for both open and IMRT fields. The accuracy of the simulation for IMRT plans was evaluated using a head phantom by comparing MC, Eclipse, TLD results, and ion chamber in solid water phantom measurements. The MC code was then used to simulate dose distributions for five patients who were treated using dynamic IMRT for tumors in the sinus region. The results were compared with absolute and relative dose distributions calculated using Eclipse (pencil beam, modified-Batho inhomogeneity correction). Absolute dose differences were also compared with ion chamber results. Comparison of the doses calculated on the head phantom using MC, compared with Eclipse, ion chamber, and TLD measurements showed differences of -3.9%, -1.4%, and -2.0%, respectively (MC is colder). Relative dose distributions for the patient plans calculated using MC agreed well with those calculated using Eclipse with respect to targets and critical organs, indicating the modified-Batho correction is adequate. Average agreement for mean absolute target doses between MC and Eclipse was -3.0 +/-; 2.3% (1 s.d.). Agreement between ion chamber and Eclipse for these patients was -2.2 +/- 1.9%, compared with 0.2 +/- 2.0% for all head and neck IMRT patients. When Eclipse doses were corrected based on ion chamber results, agreement between MC and Eclipse was -0.7 +/- 2.0%, indicating a small systematic uncertainty in the doses calculated using the treatment planning system for this subset of patients.

Head and Neck Neoplasms↗

PubMed2005

Maximum MLC opening effect in dynamic delivery of IMRT: leaf-positional analysis.

The analysis of dynamic multileaf collimator (MLC) positions for the delivered intensity-modulated radiotherapy (IMRT) plans is crucial in that it may capture dose delivery problems otherwise difficult to observe and quantify in the conventional dosimetric measurements with film or with an ionization chamber. In some IMRT systems, delivery of IMRT fields starts with a maximum MLC opening (roughly the shape of the target in the beam's-eye view) and then proceeds to the subsequent dynamic MLC subfields. No irradiation is required in going from the initial segment (maximum opening) to the next one, and theoretically, no dose should be delivered in that initial moment. However, due to a finite sampling time of the MLC controller, the finite speed of the MLC, and a finite leaf tolerance, there may be some dose delivered between the first and the second segment. The amount of the excess dose is higher for larger dose rates and for a smaller number of the total monitor units per IMRT field. The magnitude of the dose errors could be in the order of a few percent. Effects similar to the maximum MLC opening may occur in other situations as well, for instance, when leaves are forced to move over large distances in a short time. Confounding this are dose errors due to the uncertainty in the MLC transmission. The analysis of the actual leaf positions recorded in the dynamic MLC log file is helpful in differentiating between the two types of errors and in determining the optimal dynamic MLC delivery parameters.

Computer Simulation↗

PubMed2003

Changes in dielectric properties at 460 kHz of kidney and fat during heating: importance for radio-frequency thermal therapy.

We have developed a system to measure the changes due to heating to high temperatures in the dielectric properties of tissues in the radio-frequency range. A two-electrode arrangement was connected to a low-frequency impedance analyser and used to measure the dielectric properties of ex vivo porcine kidney and fat at 460 kHz. This frequency was selected as it is the most commonly used for radio-frequency thermal therapy of renal tumours. Tissue samples were heated to target temperatures between 48 and 78 degrees C in a hot water bath and changes in dielectric properties were measured during 30 min of heating and 15 min of cooling. Results suggest a time-temperature dependence of dielectric properties, with two separate components: one a reversible, temperature-dependent effect and the other a permanent effect due to structural events (e.g. protein coagulation, fat melting) that occur in tissues during heating. We calculated temperature coefficients of 1.3 +/- 0.1% degrees C(-1) for kidney permittivity and 1.6% degrees C(-1) for kidney conductivity, 0.9 +/- 0.1% degrees C(-1) for fat permittivity and 1.7 +/- 0.1% degrees C(-1) for fat conductivity. An Arrhenius model was employed to determine the first-order kinetic rates for the irreversible changes in dielectric properties. The following Arrhenius parameters were determined: an activation energy of 57 +/- 5 kcal mol(-1) and a frequency factor of (6 +/- 1) x 10(34) s(-1) for conductivity of kidney, an activation energy of 48 +/- 2 kcal mol(-1) and a frequency factor of 6 x 10(28) s(-1) for permittivity of kidney. A similar analysis led to an activation energy of 31 +/- 4 kcal mol(-1) and a frequency factor of (4.43 +/- 1) x 10(16) s(-1) for conductivity of fat, and an activation energy of 40 +/- 4 kcal mol(-1) and a frequency factor of 4 x 10(22) s(-1) for permittivity of fat. Structural events occurring during heating at different target temperatures as determined by histological analyses were correlated with the changes in the measured dielectric properties.

Adaptation, Physiological↗

PubMed2003

Dependence of fluence errors in dynamic IMRT on leaf-positional errors varying with time and leaf number.

In d-MLC based IMRT, leaves move along a trajectory that lies within a user-defined tolerance (TOL) about the ideal trajectory specified in a d-MLC sequence file. The MLC controller measures leaf positions multiple times per second and corrects them if they deviate from ideal positions by a value greater than TOL. The magnitude of leaf-positional errors resulting from finite mechanical precision depends on the performance of the MLC motors executing leaf motions and is generally larger if leaves are forced to move at higher speeds. The maximum value of leaf-positional errors can be limited by decreasing TOL. However, due to the inherent time delay in the MLC controller, this may not happen at all times. Furthermore, decreasing the leaf tolerance results in a larger number of beam hold-offs, which, in turn leads, to a longer delivery time and, paradoxically, to higher chances of leaf-positional errors (< or = TOL). On the other end, the magnitude of leaf-positional errors depends on the complexity of the fluence map to be delivered. Recently, it has been shown that it is possible to determine the actual distribution of leaf-positional errors either by the imaging of moving MLC apertures with a digital imager or by analysis of a MLC log file saved by a MLC controller. This leads next to an important question: What is the relation between the distribution of leaf-positional errors and fluence errors. In this work, we introduce an analytical method to determine this relation in dynamic IMRT delivery. We model MLC errors as Random-Leaf Positional (RLP) errors described by a truncated normal distribution defined by two characteristic parameters: a standard deviation sigma and a cut-off value deltax0 (deltaxo approximately TOL). We quantify fluence errors for two cases: (i) deltax0 >> sigma (unrestricted normal distribution) and (ii) deltax0 << sigma (deltax0--limited normal distribution). We show that an average fluence error of an IMRT field is proportional to (i) sigma/ALPO and (ii) deltax0/ALPO, respectively, where ALPO is an Average Leaf Pair Opening (the concept of ALPO was previously introduced by us in Med. Phys. 28, 2220-2226 (2001). Therefore, dose errors associated with RLP errors are larger for fields requiring small leaf gaps. For an N-field IMRT plan, we demonstrate that the total fluence error (if we neglect inhomogeneities and scatter) is proportional to 1/square root of N, where N is the number of fields, which slightly reduces the impact of RLP errors of individual fields on the total fluence error. We tested and applied the analytical apparatus in the context of commercial inverse treatment planning systems used in our clinics (Helios and BrainScan). We determined the actual distribution of leaf-positional errors by studying MLC controller (Varian Mark II and Brainlab Novalis MLCs) log files created by the controller after each field delivery. The analytically derived relationship between fluence error and RLP errors was confirmed by numerical simulations. The equivalence of relative fluence error to relative dose error was verified by a direct dose calculation. We also experimentally verified the truthfulness of fluences derived from the log file data by comparing them to film data.

Humans↗