Biomedical subjects

M L Dirkx

Publications and source records attributed to M L Dirkx.

8 recordsLinked to original sources

PubMed2001

Commissioning of a commercially available system for intensity-modulated radiotherapy dose delivery with dynamic multileaf collimation.

PURPOSE: To commission commercially available equipment for intensity-modulated radiotherapy (IMRT) using dynamic multileaf collimation (DMLC). MATERIALS AND METHODS: First, the stability in leaf positioning and in realized IMRT profiles on a Varian 2300 C/D machine were determined as a function of time and gantry angle, and as a result of treatment interrupts. Second, dose distributions calculated with the CadPlan (Varian) treatment planning system, using leaf trajectories calculated with the leaf motion calculator (LMC) algorithm, were compared with distributions realized at the 2300 C/D unit. RESULTS: Day-to-day and gantry angle variations in leaf positioning and dose delivery were very small (less than 0.1-0.2 mm and 2%). The effect of treatment interrupts on measured dose distributions was less than 2%. The agreement between the final dose distribution calculated by CadPlan and the measured dose was generally within 2%, or 2 mm at steep dose gradients, using a leaf transmission value of 1.8% and a leaf separation value of 2 mm in LMC. For narrow peaks, deviations of up to 6% were observed. LMC does not synchronize adjacent leaf trajectories resulting in tongue-and-groove underdosages of up to 29% for extreme cases. CONCLUSIONS: The 2300 C/D machine is suitable for accurate and reproducible DMLC treatments. The agreement between dose predictions with LMC and CadPlan, and realized doses at this unit is clinically acceptable for most cases. However, differences between calculated and actual dose values may exist in peaked fluences or due to tongue-and-groove effects. Therefore, pretreatment dosimetric verification for each patient is recommended.

Computer Simulation↗

PubMed2000

Beam intensity modulation for penumbra enhancement and field length reduction in lung cancer treatments: a dosimetric study.

BACKGROUND AND PURPOSE: In a recent treatment planning study, a previously published technique for superior-inferior field length reduction for prostate cancer patients, based on penumbra enhancement using static beam intensity modulation (BIM) with a multileaf collimator, was investigated for lung cancer treatments. For the patient group studied, the field lengths could be reduced by 1.4 cm and an average dose escalation of 6 Gy (maximum 16 Gy) appeared to be possible without any increase in the calculated risk of radiation pneumonitis. However, this planning study was performed with a treatment planning system that does not correctly account for the increased lateral secondary electron transport in lung tissue, resulting in too steep beam penumbrae. Therefore, prior to clinical implementation, an extensive dosimetric study was performed to evaluate and optimize BIM for penumbra enhancement and superior-inferior field length reduction in lung cancer treatments. MATERIALS AND METHODS: Film dosimetry was performed in several phantoms consisting of water equivalent and lung equivalent materials, both for a 6 and a 10 MV photon beam. Measured dose distributions were used to (i) adapt the BIM technique to properly account for increased lateral secondary electron transport, (ii) compare BIM dose distributions in lung material with dose distributions of standard treatment fields, and (iii) investigate the use of our treatment planning system for the design of BIM plans for lung cancer patients. RESULTS: Compared with our treatment planning study the superior and inferior boost fields, used in the BIM technique for penumbra enhancement, had to be longer and of a higher weight to compensate for the increased lateral secondary electron transport in lung tissue. With these modifications in the BIM technique, field lengths could indeed be reduced by 1.4 cm compared with treatment with standard fields, without the appearance of underdosages in the most superior and inferior target areas, whilst better sparing the healthy lung tissue. Practical rules were derived to use our treatment planning system for the design of BIM treatment plans. CONCLUSIONS: In spite of the increased lateral secondary electron transport in lung tissue, static BIM with a multileaf collimator may effectively be used for penumbra enhancement and superior-inferior field length reduction in lung cancer treatments.

Dose-Response Relationship, Radiation↗

PubMed2000

Testing of the stability of intensity modulated beams generated with dynamic multileaf collimation, applied to the MM50 racetrack microtron.

Recently, we have published a method for the calculation of required leaf trajectories to generate optimized intensity modulated x-ray beams by means of dynamic multileaf collimation [Phys. Med. Biol. 43, 1171-1184 (1998)]. For the MM50 Racetrack Microtron it has been demonstrated that the dosimetric accuracy of this method, in combination with the dose calculation algorithm of the Cadplan 3D treatment planning system, is adequate for a clinical application (within 2% or 0.2 cm). Prior to initiating patient treatment with dynamic multileaf collimation (DMLC), tests have been performed to investigate the stability of DMLC fields generated at the MM50, (i) in time, (ii) subject to gantry rotation and (iii) in case of treatment interrupts, e.g., caused by an error detected by the treatment machine. The stability of relative dose profiles, normalized to a reference point in a relatively flat part of the modulated beam profile, was assessed from measurements with an electronic portal imaging device (EPID), with a linear diode array attached to the collimator and with film. The dose in the reference point was monitored using an ionization chamber. Tests were performed for several intensity modulated fields using 10 and 25 MV photon beams. Based on film measurements for sweeping 0.1 cm leaf gaps it was concluded that in an 80 days period the variation in leaf positioning was within 0.05 cm, without requiring any recalibration. For a uniform 10x10 cm2 field, realized dynamically by a scanning 0.4x10 cm2 slit beam, a maximum variation in slit width of 0.01 cm was derived from ionization chamber measurements, both in time and for gantry rotation. For a clinical example, the dose in the reference point reproduced within 0.2% (1 SD) over a period of 100 days. Apart from regions with very large dose gradients, variations in the relative beam profiles measured with the EPID were generally less than 1% (1 SD). For different gantry angles the dose profiles also reproduced within 1%, showing that gravity has a negligible influence. No significant deviations between uninterrupted and interrupted treatments could be observed, indicating that the effects of acceleration and deceleration of the leaves are negligible and that a DMLC treatment can be finished correctly after a treatment interrupt. Our previous and present studies have demonstrated that the dosimetric accuracy and stability of intensity modulated beams, generated at the MM50 by means of dynamic multileaf collimation, are adequate for clinical use. Patient treatment using dynamic multileaf collimation has been started in our clinic.

Algorithms↗

PubMed1999

Beam intensity modulation for penumbra enhancement in the treatment of lung cancer.

PURPOSE: A treatment planning study was performed for patients with lung cancer in order to investigate the extent to which doses to critical structures could be reduced by penumbra enhancement at the superior and inferior field edges, using beam intensity modulation (BIM) with a multileaf collimator. By applying two independent published models for the prediction of the incidence of normal tissue complications, the potential for dose escalation without increasing the incidence of pneumonitis was estimated. METHODS AND MATERIALS: For 12 patients, the standard treatment technique was compared with the BIM technique using the Cadplan 3D planning system (Varian-Dosetek). Dose distributions in the healthy lung tissue were evaluated by considering both lungs minus the tumor as one functional unit. The following parameters were compared: (i) the average normalized total dose (NTD), (ii) the lung volume receiving an NTD of more than 20 Gy, and (iii) the calculated normal tissue complication probability (NTCP). RESULTS: Due to the applied BIM technique, the field lengths could be reduced by 1.4 cm for all patients, while achieving a minimum dose at the superior and inferior parts of the target of 95% of the isocenter dose. Compared to the standard technique, BIM reduced the patient mean of the average NTD for the healthy lung tissue from 16.5 to 15.3 Gy. The volume of healthy lung tissue receiving an NTD of 20 Gy or more was reduced by 9.7% (range 2.2 to 23.1%). The calculated NTCP reduced from 10.7% to 7.6% on average. The length of the esophagus that received a dose of 60 Gy or more could be reduced for 5 of the 6 stage III patients in this study. Based on equal lung NTCPs for the standard technique and the BIM technique, a mean dose escalation of 5.7 Gy (range 1.1 to 16.0 Gy) was possible for the 12 patients in this study. Based on equal average NTDs for the two techniques, the patient mean of the allowed dose escalation was 6.5 Gy (range 1.1 to 18.2 Gy). All dose escalations would be possible without exceeding the spinal cord tolerance dose. CONCLUSIONS: The BIM technique reduced the dose delivery to critical tissues. Two published methods for estimating the incidence of pneumonitis both pointed to a potential for dose escalation of 6 to 7 Gy on average with the BIM technique, without increasing the incidence of pneumonitis. For 2 of the 12 patients in this study the estimated allowed dose escalation even exceeded 15 Gy.

Esophagus↗

PubMed1999

Dosimetric verification of intensity modulated beams produced with dynamic multileaf collimation using an electronic portal imaging device.

Dose distributions can often be significantly improved by modulating the two-dimensional intensity profile of the individual x-ray beams. One technique for delivering intensity modulated beams is dynamic multileaf collimation (DMLC). However, DMLC is complex and requires extensive quality assurance. In this paper a new method is presented for a pretreatment dosimetric verification of these intensity modulated beams utilizing a charge-coupled device camera based fluoroscopic electronic portal imaging device (EPID). In the absence of the patient, EPID images are acquired for all beams produced with DMLC. These images are then converted into two-dimensional dose distributions and compared with the calculated dose distributions. The calculations are performed with a pencil beam algorithm as implemented in a commercially available treatment planning system using the same absolute beam fluence profiles as used for calculation of the patient dose distribution. The method allows an overall verification of (i) the leaf trajectory calculation (including the models to incorporate collimator scatter and leaf transmission), (ii) the correct transfer of the leaf sequencing file to the treatment machine, and (iii) the mechanical and dosimetrical performance of the treatment unit. The method was tested for intensity modulated 10 and 25 MV photon beams; both model cases and real clinical cases were studied. Dose profiles measured with the EPID were also compared with ionization chamber measurements. In all cases both predictions and EPID measurements and EPID and ionization chamber measurements agreed within 2% (1 sigma). The study has demonstrated that the proposed method allows fast and accurate pretreatment verification of DMLC.

Algorithms↗

PubMed1998

Leaf trajectory calculation for dynamic multileaf collimation to realize optimized fluence profiles.

An algorithm for the calculation of the required leaf trajectories to generate optimized intensity modulated beam profiles by means of dynamic multileaf collimation is presented. This algorithm iteratively accounts for leaf transmission and collimator scatter and fully avoids tongue-and-groove underdosage effects. Tests on a large number of intensity modulated fields show that only a limited number of iterations, generally less than 10, are necessary to minimize the differences between optimized and realized fluence profiles. To assess the accuracy of the algorithm in combination with the dose calculation algorithm of the Cadplan 3D treatment planning system, predicted absolute dose distributions for optimized fluence profiles were compared with dose distributions measured on the MM50 Racetrack Microtron and resulting from the calculated leaf trajectories. Both theoretical and clinical cases yield an agreement within 2%, or within 2 mm in regions with a high dose gradient, showing that the accuracy is adequate for clinical application.

Algorithms↗

PubMed1997

Field margin reduction using intensity-modulated x-ray beams formed with a multileaf collimator.

PURPOSE: In axial, coplanar treatments with multiple fields, the superior and inferior ends of a planning target volume (PTV) are at risk to get underdosed due to the overlapping penumbras of all treatment fields. We have investigated a technique using intensity modulated x-ray beams that allows the use of small margins for definition of the superior and inferior field borders while still reaching a minimum PTV-dose of 95% of the isocenter dose. METHODS AND MATERIALS: The applied intensity modulated beams, generated with a multileaf collimator, include narrow (1.1-1.6 cm) boost fields to increase the dose in the superior and inferior ends of the PTV. The benefits of this technique have been assessed using 3D treatment plans for 10 prostate cancer patients. Treatment planning was performed with the Cadplan 3D planning system (Varian-Dosetek). Dose calculations for the narrow boost fields have been compared with measurements. The application of the boost fields has been tested on the MM50 Racetrack Microtron (Scanditronix Medical AB), which allows fully computer-controlled setup of all involved treatment fields. RESULTS: Compared to our standard technique, the superior-inferior field length can be reduced by 1.6 cm, generally yielding smaller volumes of rectum and bladder in the high dose region. For the narrow boost fields, calculated relative dose distributions agree within 2% or 0.2 cm with measured dose distributions. For accurate monitor unit calculations, the phantom scatter table used in the Cadplan system had to be modified using measured data for square fields smaller than 4 x 4 cm2. The extra time needed at the MM50 for the setup and delivery of the boost fields is usually about 1 min. CONCLUSION: The proposed use of intensity modulated beams yields improved conformal dose distributions for treatment of prostate cancer patients with a superior-inferior field size reduction of 1.6 cm. Treatments of other tumor sites can also benefit from the application of the boost fields.

Humans↗

PubMed1995

Daily dosimetric quality control of the MM50 Racetrack Microtron using an electronic portal imaging device.

The MM50 Racetrack Microtron, suited for advanced three-dimensional conformal radiotherapy techniques, is a complex machine in various respects. Therefore, for a number of gantry angles, daily quality control of the absolute output and fluence profiles of the scanned beams are mandatory. For the applied photon beams, a fast method for these daily checks, based on dosimetric measurements with the Philips SRI-100 Electronic Portal Imaging Device (EPID), has been developed and tested. Open beams are checked for four different gantry angles; for gantry angle 0, a wedged field is checked as well. Performing and analyzing the measurements takes about 10 min. The applied EPID has favourable characteristics for dosimetric quality control measurements: absolute output measurements reproduce within 0.5% (1 SD) and the reproducibility of relative (2D) beam profile measurements is 0.2% (1 SD). The day-to-day sensitivity stability over a period of one month is 0.6% (1 SD). Measured grey scale values are within 0.2% linear with the applied dose. The 2D fluence profile of the 25 MV photon beam of the MM50 is very stable in time: during a period of 5 months a maximum fluctuation of 2.2% has been observed. Once, a deviation in the cGy/MU-value of 6% was detected. There is no interlock in the MM50-system that would have prevented patient treatment with this strongly deviating output. Based on the results of this study and on clinical requirements regarding acceptability of deviations of beam characteristics, a protocol has been developed including action levels for additional investigations and, if necessary, adjustment of the beam characteristics.

Calibration↗