Vibeke N. Hansen

The application of transit dosimetry to precision radiotherapy.

A method of using electronic portal imaging (EPI) for transit dosimetry is described. In this method, a portal image of the treatment field is first aligned with a digitally reconstructed radiograph (DRR) to geometrically relate the computed tomography (CT) scan, used to generate the DRR, with the EPI. Then the EPI is corrected for scatter within the patient to yield a map of primary fluence striking the detector. This is backprojected through the planning CT data set to yield a distribution of primary fluence within the patient. This distribution is then convolved with dose deposition kemels to yield a map of dose delivery within the patient. Such a distribution may be compared with the dose distribution resulting from the original treatment plan in order to evaluate the adequacy of the treatment. This method has been evaluated using a humanoid phantom. We find the transit dosimetry relative dose distribution when compared with film and thermoluminescent dosimeter (TLD) measurements and compared with our planning system to agree within 2% in the pelvic region of a humanoid phantom.

WEEKLY VOLUME AND DOSIMETRIC CHANGES DURING CHEMORADIOTHERAPY WITH INTENSITY-MODULATED RADIATION THERAPY FOR HEAD AND NECK CANCER: A PROSPECTIVE OBSERVATIONAL STUDY

PURPOSE The aim of this study was to investigate prospectively the weekly volume changes in the target volumes and organs at risk and the resulting dosimetric changes during induction chemotherapy followed by chemoradiotherapy with intensity-modulated radiation therapy (C-IMRT) for head-and-neck cancer patients. METHODS AND MATERIALS Patients receiving C-IMRT for head-and-neck cancer had repeat CT scans at weeks 2, 3, 4, and 5 during radiotherapy. The volume changes of clinical target volume 1 (CTV1) and CTV2 and the resulting dosimetric changes to planning target volume 1 (PTV1) and PTV2 and the organs at risk were measured. RESULTS The most significant volume differences were seen at week 2 for CTV1 and CTV2. The reductions in the volumes of CTV1 and CTV2 at week 2 were 3.2% and 10%, respectively (p = 0.003 and p < 0.001). The volume changes resulted in a significant reduction in the minimum dose to PTV1 and PTV2 (2 Gy, p = 0.002, and 3.9 Gy, p = 0.03, respectively) and an increased dose range across PTV1 and PTV2 (2.5 Gy, p < 0.001, and 5.1 Gy, p = 0.008, respectively). There was a 15% reduction in the parotid volumes by week 2 (p < 0.001) and 31% by week 4 (p < 0.001). There was a statistically significant increase in the mean dose to the ipsilateral parotid only at week 4 (2.7 Gy, p = 0.006). The parotid glands shifted medially by an average of 2.3 mm (p < 0.001) by week 4. CONCLUSION The most significant volumetric changes and dosimetric alterations in the tumor volumes and organs at risk during a course of C-IMRT occur by week 2 of radiotherapy. Further adaptive radiotherapy with replanning, if appropriate, is recommended.

The delivery of intensity modulated radiotherapy to the breast using multiple static fields

BACKGROUND AND PURPOSE To develop a method of using a multileaf collimator (MLC) to deliver intensity modulated radiotherapy (IMRT) for tangential breast fields, using an MLC to deliver a set of multiple static fields (MSFs). MATERIALS AND METHODS An electronic portal imaging device (EPID) is used to obtain thickness maps of medial and lateral tangential breast fields. From these IMRT deliveries are designed to minimize the volume of breast above 105% of prescribed dose. The deliveries are universally-wedged beams augmented with a set of low dose shaped irradiations. Dosimetric and planning QA of this method has been compared with the standard, wedged treatment and the corresponding treatment using physical compensators. Several options for delivering the MSF treatment are presented. RESULTS The MSF technique was found to be superior to the standard technique (P value=0.002) and comparable with the compensated technique. Both IMRT methods reduced the volume of breast above 105% dose from a mean value of 12.0% of the total breast volume to approximately 2.8% of the total breast volume. CONCLUSIONS This MSF method may be used to reduce the high dose volume in tangential breast irradiation significantly. This may have consequences for long-term side effects, particularly cosmesis.

An efficient Monte Carlo-based algorithm for scatter correction in keV cone-beam CT

A new method is proposed for scatter-correction of cone-beam CT images. A coarse reconstruction is used in initial iteration steps. Modelling of the x-ray tube spectra and detector response are included in the algorithm. Photon diffusion inside the imaging subject is calculated using the Monte Carlo method. Photon scoring at the detector is calculated using forced detection to a fixed set of node points. The scatter profiles are then obtained by linear interpolation. The algorithm is referred to as the coarse reconstruction and fixed detection (CRFD) technique. Scatter predictions are quantitatively validated against a widely used general-purpose Monte Carlo code: BEAMnrc/EGSnrc (NRCC, Canada). Agreement is excellent. The CRFD algorithm was applied to projection data acquired with a Synergy XVI CBCT unit (Elekta Limited, Crawley, UK), using RANDO and Catphan phantoms (The Phantom Laboratory, Salem NY, USA). The algorithm was shown to be effective in removing scatter-induced artefacts from CBCT images, and took as little as 2 min on a desktop PC. Image uniformity was greatly improved as was CT-number accuracy in reconstructions. This latter improvement was less marked where the expected CT-number of a material was very different to the background material in which it was embedded.

Quality assurance of the dose delivered by small radiation segments

The use of intensity modulation with multiple static fields has been suggested by many authors as a way to achieve highly conformal fields in radiotherapy. However, quality assurance of linear accelerators is generally done only for beam segments of 100 MU or higher, and by measuring beam profiles once the beam has stabilized. We propose a set of measurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose delivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either photographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the three radiotherapy centres on a set of five different Philips SL accelerators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monitor unit over the range of 1 to 100 MU was found to be accurate to within +/-5% of the nominal dose per monitor unit as defined for the delivery of 100 MU for all the energies. For four out of the five accelerators the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some cases found to be larger for small segments by a maximum of 9% of the flatness/symmetry for large segments. The result of this study provides the dosimetric evidence that the delivery of small segment doses as top-up fields for beam intensity modulation is feasible. However, it should be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement should be performed for each machine before the delivery of small dose segments is approved. In some cases it may be advisable to use a low pulse repetition frequency (PRF) to obtain more accurate dose delivery of small segments.

Design of compensators for breast radiotherapy using electronic portal imaging

A novel method of designing intensity modulated beams (IMBs) to achieve compensation in external beam radiotherapy of the breast, without the need for CT scans, is presented. The design method comprises three parts: (1) an electronic portal image is used to generate a map of radiological thickness; (2) this map is then used to obtain an estimate of the breast and lung outline; (3) a TMR-based dose calculation algorithm is then used to determine the optimum beam profile to achieve the best dose distribution. The dose distributions calculated for IMBs were compared with those calculated for the use of simple wedges. The results for two patients studied indicate that the dose inhomogeneity for IMBs is +/- 5%, compared with a value of +/- 10% for a wedged plan. The uncertainty in radiological thickness measurement corresponds to a dosimetric error of +/- 2%. Other errors associated with outline estimation are typically less than 2%, with a largest value of +5% for one of the patients who had a large and highly asymmetrical breast. The results for the two patients studied suggest that the uncertainties in the method are significantly smaller than the improvement in dose uniformity produced.

Treatment of lung cancer using volumetric modulated arc therapy and image guidance: A case study

Background. Volumetric modulated arc therapy (VMAT) is a radiotherapy technique in which the gantry rotates while the beam is on. Gantry speed, multileaf collimator (MLC) leaf position and dose rate vary continuously during the irradiation. For optimum results, this type of treatment should be subject to image guidance. The application of VMAT and image guidance to the treatment of a lung cancer patient is described. Material and methods. In-house software AutoBeam was developed to facilitate treatment planning for VMAT beams. The algorithm consisted of a fluence optimisation using the iterative least-squares technique, a segmentation and then a direct-aperture optimisation. A dose of 50 Gy in 25 fractions was planned, using a single arc with 35 control points at 10° intervals. The resulting plan was transferred to a commercial treatment planning system for final calculation. The plan was verified using a 0.6cm3 ionisation chamber and film in a rectangular phantom. The patient was treated supine on a customised lung board and imaged daily with cone-beam CT for the first three days then weekly thereafter. Results. The VMAT plan provided slightly improved coverage of the planning target volume (PTV) and slightly lower volume of lung irradiated to 20 Gy (V20) than a three-field conformal plan (PTV minimum dose 85.0 Gy vs. 81.8 Gy and lung V20 31.5% vs. 34.8%). The difference between the measured and planned dose was −1.1% (measured dose lower) and 97.6% of the film passed a gamma test of 3% and 3mm. The VMAT treatment required 90s for delivery of a single fraction of 2 Gy instead of 180s total treatment time for the conformal plan. Conclusion. VMAT provides a quality dose distribution with a short treatment time as shown in an example of a lung tumour. The technique should allow for more efficient delivery of high dose treatments, such as used for hypofractionated radiotherapy of small volume lung tumours, and the technique may also be used in conjunction with Active Breathing Control, where fewer breath holds will be required.

Dosimetric evaluation of compensation in radiotherapy of the breast: MLC intensity modulation and physical compensators

BACKGROUND AND PURPOSE Electronic portal images may be used to design the compensation required to maximise dose uniformity in the breast from opposed tangential beams. MATERIALS AND METHODS Four methods of implementing the desired compensation have been studied: a simple wedge, a physical compensator in conjunction with a wedge; one open field plus four shaped multi-leaf-collimated (MLC) fields, and one wedged field in conjunction with three shaped MLC fields. Evaluation was performed using thermoluminescent dosimeters (TLDs) placed inside a phantom which was designed to mimic the human breast. The measured results are compared with both the prediction of the in-house compensation design software and with the dose predicted by the GE Target II planning system. The implications of each method for the time taken to plan and deliver treatment were analysed. RESULTS The dose inhomogeneity, as measured at seven points in the central plane was greatest for the simple wedge (root mean square (rms) = 4.5%) compared to an open field plus four shaped MLC fields (rms = 2.2%), a wedged field plus three shaped MLC fields (rms = 3.3%), and the physical compensator (rms = 2.4%). The times required to plan and prepare these treatments varied considerably. The standard wedged treatment required under 15 min; both MLC-based and the physical compensator treatments required approximately 50 min. Differences of treatment delivery times were up to 8 min. CONCLUSIONS These results indicate that the dose inhomogeneity can be reduced by beam intensity modulation designed using EPIDs.

Direct measurement and analytical modeling of scatter in portal imaging

In this study a direct measurement of scatter in portal imaging for various air gaps and scatterer thicknesses at a beam energy of 6 MV is presented. The experimental data are compared with results from a Monte Carlo (MC) scatter model. In the regime where the air gap is larger than 9.3 cm the MC and the experiment agree. Based on this MC model an analytical model is developed, which takes all important interaction processes into account. It comprises a rigorous treatment of first order scattering and an estimation of photons scattered more than once within the phantom. This estimation is based on the assumption that higher order scattering can be considered as isotropically distributed around a certain scatter origin located in the midplane of the phantom. It is found that relative deviations between the MC model and the analytical model are of 2% to 3% in regions where scattering is very large.

Practical implementation of compensators in breast radiotherapy

BACKGROUND AND PURPOSE A method of using electronic portal imaging to design compensators for tangential breast irradiation has been developed. We describe how this has been implemented. MATERIALS AND METHODS The compensator design method generates wedged and unwedged beam weights, in conjunction with templates for multiple lead-sheet compensators and pseudo-CT outlines. The latter describe the breast and lung profiles in a set of transverse slices. The layers of the compensator and pseudo-CT outlines are transferred to a treatment planning system for verification. The accuracy of the planning system for the high transmission blocks used to describe the compensators has been verified using a plotting tank system. Dose volume histogram data and transaxial and sagittal plan slices have been compared for both standard and compensated treatments for a sample set of five patients. RESULTS The planning system predicted the dose at depths of 1.5 and 5 cm to within 2% for the compensators tested. The biggest source of discrepancy was a consequence of the planning system requiring blocks to have integer percentage transmission. For all patients studied, the compensated treatment resulted in a significant reduction in the percentage volume outside the 95-105% dose, with an average reduction of 10.2%. The percentage volume outside the 95-107% dose was also reduced by typically 3.4%. The implementation was found to yield a convenient automatic method of designing compensators using electronic portal imaging and verifying the results using a planning system. CONCLUSIONS These results indicate that this method of implementation can be used in practice. The dosimetric accuracy of the treatment planning system is limited by the requirement that blocks should be of integer transmission, but this effect is small.