Biomedical subjects

W A Tomé

Publications and source records attributed to W A Tomé.

10 recordsLinked to original sources

PubMed2006

On the possible increase in local tumour control probability for gliomas exhibiting low dose hyper-radiosensitivity using a pulsed schedule.

Using modelling, we have developed a treatment strategy for gliomas exhibiting low dose hyper-radiosensitivity (HRS) that employs both a reduced dose-rate and pulsed treatment dose delivery. The model exploits the low dose hypersensitivity observed in some glioma cell lines at low radiation doses. We show, based on in vitro data, that a pulsed delivery of external beam radiation therapy could yield significant increases in local control. We therefore propose a pulsed delivery scheme for the treatment of gliomas in which the daily treatment fraction is delivered using 0.20 Gy pulses, separated by three minutes for a time-averaged dose-rate of 0.0667 Gy/min. The dose per pulse of 0.2 Gy is near or below the transition dose observed in vitro for four of the five glioma cell lines we have studied. Using five established glioma cell lines our modelling demonstrates that our pulsed delivery scheme yields a substantial increase in tumour control probability (TCP).

Cell Line, Tumor↗

PubMed2005

Intensity-modulated radiation therapy: emerging cancer treatment technology.

The use of intensity-modulated radiation therapy (IMRT) is rapidly advancing in the field of radiation oncology. Intensity-modulated radiation therapy allows for improved dose conformality, thereby affording the potential to decrease the spectrum of normal tissue toxicities associated with IMRT. Preliminary results with IMRT are quite promising; however, the clinical data is relatively immature and overall patient numbers remain small. High-quality IMRT requires intensive physics support and detailed knowledge of three-dimensional anatomy and patterns of tumour spread. This review focuses on basic principles, and highlights the clinical implementation of IMRT in head and neck and prostate cancer.

Dose-Response Relationship, Radiation↗

PubMed2004

Quality assurance of a helical tomotherapy machine.

Helical tomotherapy has been developed at the University of Wisconsin, and 'Hi-Art II' clinical machines are now commercially manufactured. At the core of each machine lies a ring-gantry-mounted short linear accelerator which generates x-rays that are collimated into a fan beam of intensity-modulated radiation by a binary multileaf, the modulation being variable with gantry angle. Patients are treated lying on a couch which is translated continuously through the bore of the machine as the gantry rotates. Highly conformal dose-distributions can be delivered using this technique, which is the therapy equivalent of spiral computed tomography. The approach requires synchrony of gantry rotation, couch translation, accelerator pulsing and the opening and closing of the leaves of the binary multileaf collimator used to modulate the radiation beam. In the course of clinically implementing helical tomotherapy, we have developed a quality assurance (QA) system for our machine. The system is analogous to that recommended for conventional clinical linear accelerator QA by AAPM Task Group 40 but contains some novel components, reflecting differences between the Hi-Art devices and conventional clinical accelerators. Here the design and dosimetric characteristics of Hi-Art machines are summarized and the QA system is set out along with experimental details of its implementation. Connections between this machine-based QA work, pre-treatment patient-specific delivery QA and fraction-by-fraction dose verification are discussed.

Models, Theoretical↗

PubMed2001

Assessment of patient-independent intrinsic error for a noninvasive frame for fractionated stereotactic radiotherapy.

The purpose of our study was to examine the extent of patient-independent intrinsic error associated with multiple, repeat remounting of the Laitinen Stereoadapter. The Laitinen frame was repeatedly mounted on a solid water phantom and imaged using computed tomography (CT). The phantom contained five targets located in the center, anterior, right, left, and posterior orientations. The images were processed, fused, and analyzed on the Pinnacle 3-D treatment planning system. The coordinate values (in the x, y, and z directions) for each target were determined for each mounting, and an absolute mean deviation was calculated for 11 repetitions. The mean deviation in the x, y, and z direction for the central and right target, and in the x and y direction for the posterior and anterior target was less than 2.0 mm. However, the mean error in the z direction of the anterior and posterior targets was 1.79 +/- 1.02 mm and 2.20 +/- 1.32 mm, respectively. Rotational misalignment during repeat frame fixation contributed to the observed deviations and in particular affected the antero-posterior plane. With the exception of two occasions where an obvious mounting error occurred, a significant portion of error from remounting the Laitinen Stereoadapter is associated with the operator and the imaging process. The observation of an angular displacement around the axis through the earplugs suggests that a certain degree of rotational misalignment in daily remounting is possible. Targets in the antero-posterior plane are most susceptible to localization error as a consequence of rotational misalignment. In summary, the overall error is within the limits of current imaging technology but not within submillimeter accuracy. Clinical application should take these errors into consideration when designing field margins.

Biophysical Phenomena↗

PubMed2001

Optically guided intensity modulated radiotherapy.

BACKGROUND AND PURPOSE: Previously, we reported on development of an optically guided system for 3D conformal intracranial radiotherapy using multiple noncoplanar fixed fields. In this paper we report on the extension of our system for stereotactic fractionated radiotherapy to include intensity modulated static ports. METHODS AND MATERIALS: A 3D treatment plan with maximum beam separation is developed in the stereotactic space established by an optically guided system. Gantry angles are chosen such that each beam has a unique entrance and exit pathway, avoids the critical structures, and has a minimal beam's eye view projection. Once, a satisfactory treatment plan is found using this geometric approach an inverse treatment plan is developed using the beam portals established previously. The purpose of adding inverse planing is two fold, on the one hand it allows further reduction of margins around the PTV, while on the other hand it affords the possibility of conformal avoidance of critical structures that are close to or abut the PTV. RESULTS: The use of the optically guided system in conjunction with intensity modulated noncoplanar radiotherapy treatment planning using fixed fields allows the generation of highly conformal treatment plans that exhibit smaller 90, 70, and 50% of prescription dose isodose volumes, improved PITV ratios, comparable or improved EUD, smaller NTD(mean) for the critical structures, and an inhomogeneity index that is within generally accepted limits. CONCLUSION: Because optically guided technology improves the accuracy of patient localization relative to the linac isocenter and allows real-time monitoring of patient position, the planning target volume needs to be corrected only for the limitations of image resolution. Intensity modulated static beam radiotherapy planning then provides the user the ability to further reduce margins on the PTV and to conform very closely to this smaller target volume, and enhances the normal tissue sparing, and high degree of conformality possible with 3D conformal radiotherapy. In addition, since optically guided technology affords improved patient localization and online monitoring of patient position during treatment delivery it allows for safe and efficient delivery of intensity modulated radiotherapy.

Algorithms↗

PubMed2000

Selective boosting of tumor subvolumes.

PURPOSE AND BACKGROUND: It is no longer considered mandatory to deliver a uniform dose to the tumor volume in radiotherapy. Non-uniform doses are unavoidable in brachytherapy and in stereotactic radiosurgery, with often good results. Deliberately non-uniform doses may increase tumor control probability (TCP) and enable steeper dose gradients outside the treated volume to be achieved. New methods of tumor imaging might show regions of specific activity or hypoxia which could be selectively targeted. This paper investigates by modeling the effect of boosting, by dose ratios up to 2, for a range of tumor subvolumes. METHODS AND MATERIALS: A standard linear-quadratic algorithm was used to define the dose-response curve for tumors of various volumes (numbers of clonogenic cells), radiosensitivity (SF(2)), assumed slope (gamma(50)) and dose for 50% tumor control (TCD(50)). Curves of tumor control probability (TCP) were constructed to show the increase of TCP, as a function of the ratio of boost dose to the TCD(50), above the baseline 50% TCP, for a set of different proportions of tumor volume boosted. RESULTS: Calculated values of TCP increased rapidly with both boost dose ratio and with proportion of volume boosted. The increase in TCP reached a plateau after boost dose ratios of 1.2-1.3, as has been noted before, except where very large proportions of tumor volume exceeding 90% were boosted. Quite large increases of TCP, to about 75%, could be achieved if the gamma(50) slope was steep, and especially in small tumors (having fewer cells). Radiosensitivity was not an independent factor because radiosensitive tumors had a low TCD(50) and this was the baseline dose considered as unity. CONCLUSION: There were few situations where a boost dose ratio exceeding 1.3 appeared to be worthwhile or necessary. Significant increases of TCP, up from 50% to 75%, might therefore be achieved for a small increase in risk of necrosis, where a substantial proportion of tumor volume (60-80%) could be boosted.

Algorithms↗

PubMed2000

Radiotherapy for brain tumors.

Over the last 2 years, several advances have been made in the field of radiotherapy for brain tumors. Key advances are summarized in this review. Crucial technologic advances, such as radiosurgery, fractionated stereotactic radiotherapy, and intensity-modulated radiotherapy, are discussed. Better understanding of the interaction between the processes of angiogenesis, apoptosis, cell-cycle regulation, and signal transduction and the effects of ionizing radiation has made it clear that many of these "new agents" are, in fact, valuable modulators of the radiation response. Another exciting molecular discovery is the recognition of radiation-induced promoters that can be exploited to cause spatially and temporally configured expression of selected genes; this approach may represent the ideal application of conformal radiation techniques in the future, yielding well-defined genetic changes in specifically targeted tissues. The final "frontier" covered in this review is the newer categories of radiosensitizers, ranging from topoisomerase-I inhibitors, to expanded metalloporphyrins, to oxygen- dissociating agents.

Brain Neoplasms↗

PubMed2000

On the use of virtual simulation in radiotherapy of the intact breast.

In this paper a method of breast cancer treatment planning using virtual simulation implemented at the Department of Human Oncology at the University of Wisconsin is described. All patients in this procedure are placed in a custom vacuum mold in treatment position with both arms up to avoid collision with the CT scanner aperture. For all patients a CT scan of 5-mm-slice thickness is acquired. The ipsilateral and contralateral breast, the ipsilateral lung and the heart are delineated and a three-dimensional plan is generated that tries to minimize the dose to the ipsilateral lung and heart while ensuring adequate coverage of the affected breast. Digitally reconstructed radiographs are used to verify the patient setup on the treatment machine.

Breast Neoplasms↗

PubMed1999

Dosimetric characteristics of a double-focused miniature multileaf collimator.

The dosimetric characteristics of a double-focused miniature multileaf collimator (mMLC) attached to a Philips SL75/5 linear accelerator (linac) have been investigated. Output factors, percentage depth-dose, penumbra, leaf transmission, and leakage between the leaves were measured for the 6 MV x-ray beam on this accelerator. Because leakage both through and between the leaves is minimal, the linac jaws can be kept fixed while the mMLC leaf configuration is modified for different aperture shapes. This allows for accurate output prediction using the equivalent square formalism. Percent depth-dose measured for fields defined by the mMLC show little deviation from the percent depth-dose measured for fields defined by the machine jaws or Lipowitz metal blocks. Because the mMLC matches beam divergence in both directions, allows minimal beam transmission, and has a large source-to-collimator distance, the penumbra is sharper for fields defined by the mMLC than for fields defined by the linac jaws or Lipowitz metal blocks. Based on these data, dose calculations for mMLC-defined fields can be applied with no change in procedures from those used for fields defined using conventional methods.

Radiotherapy Planning, Computer-Assisted↗

PubMed1998

On the calculation of mean restricted collision stopping powers.

An analytical method for the calculation of ratios of mean restricted collision stopping powers (L/rho)(g)m averaged over the charged particle spectra and the photon spectrum that is accurate to first order has been developed, and it has been explored whether a moderate change in the photon fluence spectrum with field size has an effect on the mean restricted collision stopping power ratio in high-Z materials. The results of this study indicate that for the case of a miniphantom, moderate changes in the photon fluence spectrum have only a weak effect on the ratios of mean restricted collision stopping powers.

Calibration↗