Biomedical subjects

Peter Hoban

Publications and source records attributed to Peter Hoban.

4 recordsLinked to original sources

PubMed2006

Commissioning and quality assurance of a commercial stereotactic treatment-planning system for extracranial IMRT.

A 3D treatment-planning system (TPS) for stereotactic intensity-modulated radiotherapy (IMRT) using a micro-multileaf collimator has been made available by Radionics. In this work, we report our comprehensive quality assurance (QA) procedure for commissioning this TPS. First, the accuracy of stereotaxy established with a body frame was checked to ensure accurate determination of a target position within the planning system. Second, the CT-to-electron density conversion curve used in the TPS was fitted to our site-specific measurement data to ensure the accuracy of dose calculation and measurement verification in a QA phantom. Using the QA phantom, the radiological path lengths were verified against known geometrical depths to ensure the accuracy of the ray-tracing algorithm. We also checked inter- and intraleaf leakage/transmission for adequate jaw settings. Measurements for dose verification were performed in various head/neck and prostate IMRT treatment plans using the patient-specific optimized fluence maps. Both ion chamber and film were used for point dose and isodose distribution verifications. To ensure that adjacent organs at risk receive dose within the expectation, we used the Monte Carlo method to calculate dose distributions and dose-volume histograms (DVHs) for these organs at risk. The dosimetric accuracy satisfied the published acceptability criteria. The Monte Carlo calculations confirmed the measured dose distributions for target volumes. For organs located on the beam boundary or outside the beam, some differences in the DVHs were noticed. However, the plans calculated by both methods met our clinical criteria. We conclude that the accuracy of the XKnifetrade mark RT2 treatment-planning system is adequate for the clinical implementation of stereotactic IMRT.

Brain Neoplasms↗

PubMed2005

Dosimetric advantage and clinical implication of a micro-multileaf collimator in the treatment of prostate with intensity-modulated radiotherapy.

This paper investigates the dosimetric benefits of a micro-multileaf (4-mm leaf width) collimator (mMLC) for intensity-modulated radiation therapy (IMRT) treatment planning of the prostate cancer and its potential application for dose escalation and hypofractionation. We compared treatment plans for IMRT delivery using 2 different multileaf collimator (MLC) leaf widths (4 vs. 10 mm) for 10 patients with prostate cancer. Treatment planning was performed on the XknifeRT2 treatment planning system. All beams and optimization parameters were identical for the mMLC and MLC plans. All of the plans were normalized to ensure that 95% of the planning target volume (PTV) received 100% of the prescribed dose (74 Gy). The differences in dose distribution between the 2 groups of plans using the mMLC and the MLC were assessed by dose-volume histogram (DVH) analysis of the target and critical organs. Significant reductions in the volume of rectum receiving medium to higher doses were achieved using the mMLC. The average decrease in the volume of the rectum receiving 40, 50, and 60 Gy using the mMLC plans was 40.2%, 33.4%, and 17.7%, respectively, with p-values less than 0.0001 for V40 and V50 and 0.012 for V60. The mean dose reductions for D17 and D35 for the rectum were 20.0% (p < 0.0001) and 18.3% (p < 0.0002), respectively, when compared to those with the MLC plans. There were consistent reductions in all dose indices studied for the bladder. The target dose inhomogeneity was improved in the mMLC plans by an average of 32%. In the high-dose range, there was no significant difference in the dose deposited in the "hottest" 1 cc of the rectum between the 2 MLC plans for all cases (p > 0.78). Because of the reduction of rectal volume receiving medium to higher doses, dose to the prostate target can be escalated by about 20 Gy to over 74 Gy, while keeping the rectal dose (either denoted by D17 or D35) the same as those with the use of the MLC. The maximum achievable dose, derived when the rectum is allowed to reach the tolerance level, was found to be in the range of 113-172 Gy (using the tolerance value of D17). We conclude that the use of the mMLC for IMRT of the prostate may facilitate dose hypofractionation due to its dosimetric advantage in significantly improving the DVH parameters of the prostate and critical organs. When used for conventional fractionation scheme, mMLC for IMRT of the prostate may reduce the toxicity to the critical organs.

Equipment Design↗

PubMed2002

A method for physically based radiotherapy optimization with intelligent tissue weight determination.

An important aspect of the intensity modulated radiotherapy (IMRT) process is that of optimizing the beam intensity profiles. Most such methods use a comparison between the defined dose distribution (including prescription dose and dose limits for critical structures) and the current distribution. The comparison may involve dose differences or dose ratios. This paper investigates four different ratio-based methods, with a hypothetical U-shaped target and cylindrical phantom as the test case. All methods are shown to give satisfactory results. There are differences in the dose distributions produced and these can be related to the formulation of the respective fluence-update methods. In common with other IMRT beam optimization methods the techniques used here include tissue-weighting factors which express the relative importance of achieving the goal dose level or limit. Altering the values of these weights can improve the ability of the optimization algorithm to adhere to the goal dose levels. Typically the weights are adjusted on a "trial and error" basis until the planner is satisfied with the results. In this paper a technique for automatic updating of the weights according to the optimization results is applied to each of the four fluence update methods. With this technique the difference between the current dose level in each tissue and the specified dose constant for that tissue is used to increase the weight should the constraint not be met. It is found that the four methods yield similar results if the weights are updated in this way.

Algorithms↗

PubMed2002

A comparison of physically and radiobiologically based optimization for IMRT.

Many optimization techniques for intensity modulated radiotherapy have now been developed. The majority of these techniques including all the commercial systems that are available are based on physical dose methods of assessment. Some techniques have also been based on radiobiological models. None of the radiobiological optimization techniques however have assessed the clinically realistic situation of considering both tumor and normal cells within the target volume. This study considers a ratio-based fluence optimizing technique to compare a dose-based optimization method described previously and two biologically based models. The biologically based methods use the values of equivalent uniform dose calculated for the tumor cells and integral biological effective dose for normal cells. The first biologically based method includes only tumor cells in the target volume while the second considers both tumor and normal cells in the target volume. All three methods achieve good conformation to the target volume. The biologically based optimization without the normal tissue in the target volume shows a high dose region in the center of the target volume while this is reduced when the normal tissues are also considered in the target volume. This effect occurs because the normal tissues in the target volume require the optimization to reduce the dose and therefore limit the maximum dose to that volume.

Algorithms↗