S Webb

A comparison of conformal and intensity-modulated techniques for oesophageal radiotherapy

BACKGROUND AND PURPOSEnTo investigate the potential of intensity-modulated radiotherapy (IMRT) to reduce lung irradiation in the treatment of oesophageal carcinoma with radical radiotherapy.nnnMATERIALS AND METHODSnA treatment planning study was performed to compare two-phase conformal radiotherapy (CFRT) with IMRT in five patients. The CFRT plans consisted of anterior, posterior and bilateral posterior oblique fields, while the IMRT plans consisted of either nine equispaced fields (9F), or four fields (4F) with orientations equal to the CFRT plans. IMRT plans with seven, five or three equispaced fields were also investigated in one patient. Treatment plans were compared using dose-volume histograms and normal tissue complication probabilities.nnnRESULTSnThe 9F IMRT plan was unable to improve on the homogeneity of dose to the planning target volume (PTV), compared with the CFRT plan (dose range, 16.9+/-4.5 (1 SD) vs. 12.4+/-3.9%; P=0.06). Similarly, the 9F IMRT plan was unable to reduce the mean lung dose (11.7+/-3.2 vs. 11.0+/-2.9 Gy; P=0.2). Similar results were obtained for seven, five and three equispaced fields in the single patient studied. The 4F IMRT plan provided comparable PTV dose homogeneity with the CFRT plan (11.8+/-3.3 vs. 12.4+/-3.9%; P=0.6), with reduced mean lung dose (9.5+/-2.3 vs 11.0+/-2.9 Gy; P=0.001).nnnCONCLUSIONSnIMRT using nine equispaced fields provided no improvement over CFRT. This was because the larger number of fields in the IMRT plan distributed a low dose over the entire lung. In contrast, IMRT using four fields equal to the CFRT fields offered an improvement in lung sparing. Thus, IMRT with a few carefully chosen field directions may lead to a modest reduction in pneumonitis, or allow tumour dose escalation within the currently accepted lung toxicity.

Inverse planning with constraints to generate smoothed intensity-modulated beams

Highly conformal dose distributions can be generated by intensity-modulated radiotherapy. Intensity-modulated beams (IMBs) are generally determined by inverse-planning techniques designed to maximize conformality. Usually such techniques apply no constraints on the form of the IMBs which may then develop fine-scale modulation. In this paper we present a technique for generating smoother IMBs, which yields a dose distribution almost identical to that without the constraint on the form of the IMBs. The method applies various filters successively at intervals throughout the iterative inverse planning. It is shown that the IMBs so determined using a simple median window filter have desirable properties in terms of increasing the efficiency of delivery by the dynamic multileaf collimator method and may be more like conventional beams than unconstrained, highly modulated IMBs.

DOSE-VOLUME CONSTRAINTS TO REDUCE RECTAL SIDE EFFECTS FROM PROSTATE RADIOTHERAPY: EVIDENCE FROM MRC RT01 TRIAL ISRCTN 47772397

PURPOSEnRadical radiotherapy for prostate cancer is effective but dose limited because of the proximity of normal tissues. Comprehensive dose-volume analysis of the incidence of clinically relevant late rectal toxicities could indicate how the dose to the rectum should be constrained. Previous emphasis has been on constraining the mid-to-high dose range (>/=50 Gy). Evidence is emerging that lower doses could also be important.nnnMETHODS AND MATERIALSnData from a large multicenter randomized trial were used to investigate the correlation between seven clinically relevant rectal toxicity endpoints (including patient- and clinician-reported outcomes) and an absolute 5% increase in the volume of rectum receiving the specified doses. The results were quantified using odds ratios. Rectal dose-volume constraints were applied retrospectively to investigate the association of constraints with the incidence of late rectal toxicity.nnnRESULTSnA statistically significant dose-volume response was observed for six of the seven endpoints for at least one of the dose levels tested in the range of 30-70 Gy. Statistically significant reductions in the incidence of these late rectal toxicities were observed for the group of patients whose treatment plans met specific proposed dose-volume constraints. The incidence of moderate/severe toxicity (any endpoint) decreased incrementally for patients whose treatment plans met increasing numbers of dose-volume constraints from the set of V30<or=80%, V40<or=65%, V50<or=55%, V60<or=40%, V65<or=30%, V70<or=15%, and V75<or=3%.nnnCONCLUSIONnConsidering the entire dose distribution to the rectum by applying dose-volume constraints such as those tested here in the present will reduce the incidence of late rectal toxicity.

Comparison of intensity-modulated tomotherapy with stereotactically guided conformal radiotherapy for brain tumors

PURPOSEnIntensity-modulated radiotherapy (IMRT) offers the potential to more closely conform dose distributions to the target, and spare organs at risk (OAR). Its clinical value is still being defined. The present study aims to compare IMRT with stereotactically guided conformal radiotherapy (SCRT) for patients with medium size convex-shaped brain tumors.nnnMETHODS AND MATERIALSnFive patients planned with SCRT were replanned with the IMRT-tomotherapy method using the Peacock system (Nomos Corporation). The planning target volume (PTV) and relevant OAR were assessed, and compared relative to SCRT plans using dose statistics, dose-volume histograms (DVH), and the Radiation Therapy Oncology Group (RTOG) stereotactic radiosurgery criteria.nnnRESULTSnThe median and mean PTV were 78 cm3 and 85 cm3 respectively (range 62-119 cm3). The differences in PTV doses for the whole group (Peacock-SCRT +/-1 SD) were 2%+/-1.8 (minimum PTV), and 0.1%+/-1.9 (maximum PTV). The PTV homogeneity achieved by Peacock was 12.1%+/-1.7 compared to 13.9%+/-1.3 with SCRT. Using RTOG guidelines, Peacock plans provided acceptable PTV coverage for all 5/5 plans compared to minor coverage deviations in 4/5 SCRT plans; acceptable homogeneity index for both plans (Peacock = 1.1 vs. SCRT = 1.2); and comparable conformity index (1.4 each). As a consequence of the transaxial method of arc delivery, the optic nerves received mean and maximum doses that were 11.1 to 11.6%, and 10.3 to 15.2% higher respectively with Peacock plan. The maximum optic lens, and brainstem dose were 3.1 to 4.8% higher, and 0.6% lower respectively with Peacock plan. However, all doses remained below the tolerance threshold (5 Gy for lens, and 50 Gy for optic nerves) and were clinically acceptable.nnnCONCLUSIONSnThe Peacock method provided improved PTV coverage, albeit small, in this group of convex tumors. Although the OAR doses were higher using the Peacock plans, all doses remained within the clinically defined threshold and were clinically acceptable. Further improvements may be expected using other methods of IMRT planning that do not limit the treatment delivery to transaxial arcs. Each IMRT system needs to be individually assessed as the paradigm utilized may provide different outcomes.

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.

Feasibility of using ultrasound for real-time tracking during radiotherapy

This study was designed to examine the feasibility of utilizing transabdominal ultrasound for real-time monitoring of target motion during a radiotherapy fraction. A clinical Acuson 128/XP ultrasound scanner was used to image various stationary and moving phantoms while an Elekta SL25 linear accelerator radiotherapy treatment machine was operating. The ultrasound transducer was positioned to image from the outer edge of the treatment field at all times. Images were acquired to videotape and analyzed using in-house motion tracking algorithms to determine the effect of the SL25 on the quality of the displacement measurements. To determine the effect on the dosimetry of the presence of the transducer, dose distributions were examined using thermoluminescent dosimeters loaded into an Alderson Rando phantom and exposed to a 10×10cm2 treatment field with and without the ultrasound transducer mounted 2.5cm outside the field edge. The ultrasound images acquired a periodic noise that was shown to occur at the pulsing frequency of the treatment machine. Images of moving tissue were analyzed and the standard deviation on the displacement estimates within the tissue was identical with the SL25 on and off. This implies that the periodic noise did not significantly degrade the precision of the tracking algorithm (which was better than 0.01mm). The presence of the transducer at the surface of the phantom presented only a 2.6% change to the dose distribution to the volume of the phantom. The feasibility of ultrasonic motion tracking during radiotherapy treatment is demonstrated. This presents the possibility of developing a noninvasive, real-time and low-cost method of tracking target motion during a treatment fraction.

The dosimetric consequences of inter-fractional patient movement on conventional and intensity-modulated breast radiotherapy treatments.

BACKGROUND AND PURPOSEnA method has been developed to enable a comparison to be made between the effects of movement on conventional tangential breast treatments and intensity-modulated treatments delivered using compensators.nnnMATERIALS AND METHODSnThe effects of set-up error and organ motion were studied for a set of six patients. Images were taken of these patients over the course of their treatment and conventional wedged and compensated treatment plans were designed for each. Dose-volume statistics were used to evaluate each of the treatment plans by examining the volume outside the dose range 95-105%. To assess the effects of movement alone, the volume change from day 1 was also calculated.nnnRESULTSnThirty-six estimated CT-sets were available for evaluation. Measurements of breast volume showed the volume to increase to a peak between fraction 4 and 8 and then decrease back below the initial volume. The standard treatment was found to yield 29/36 plans (81%) with greater than 5% volume outside the dose range 95-105%. For the compensated plans this dropped to 11/36 plans (31%). The analysis of the volume changes from day 1 showed that for both standard and compensated treatments 7/30 plans (23%) had an increase in volume outside the dose range 95-105% of greater than 5% of the total planning target volume.nnnCONCLUSIONSnThe compensated treatment is more susceptible to patient movement. However, the actual volume of tissue outside 95-105% dose is less when compared to standard treatment implying the compensated treatment is still superior.

Quantifying the effect of respiratory motion on lung tumour dosimetry with the aid of a breathing phantom with deforming lungs

The contribution of organ and tumour motion to the degradation of planned dose distributions during radiotherapy to the breathing lung has been experimentally investigated and quantified. An anthropomorphic, tissue-equivalent breathing phantom with deformable lungs has been built, in which the lung tumour can be driven in any arbitrary 3D trajectory. The trajectory is programmed into a motion controller connected to a high-precision moving platform that is connected to the tumour. The motion controller is connected to the accelerators dose counter and the speed of motion is scaled to the dose rate. This ensures consistent delivery despite variation in either the dose rate or inter-segment timing. For this study, the phantom was made to breathe by a set of periodic equations representing respiratory motion by an asymmetric, trigonometric function. Several motion amplitudes were selected to be applied in the primary axis of motion. Five three-dimensional, geometrically conformal (3DCRT) fractions with different starting phases (spaced uniformly in the breathing cycle) were delivered to the phantom and compared to a delivery where the phantom was static at the end-expiration position. A set of intensity-modulated radiotherapy plans (IMRT) was subsequently delivered in the same manner. Bigger amplitudes of motion resulted in a higher degree of dose blurring. Severe underdosages were observed when deliberately selecting the PTV wrongly, their extent being correlated with the degree of margin error. IMRT motion-averaged dose distributions exhibited areas of high dose in the gross tumour volume (GTV) which were not present in the static irradiations, arising from booster segments that the optimizer was creating to achieve planning target volume (PTV) homogeneity during the inverse-planning process. 3DCRT, on the other hand, did not demonstrate such effects. It has been concluded that care should be taken to control the delivered fluence when delivering IMRT to the breathing lung, even when the PTV margin has been adequately chosen to include the extent of the breathing motion.

Rapid portal imaging with a high-efficiency, large field-of-view detector

The design, construction, and performance evaluation of an electronic portal imaging device (EPID) are described. The EPID has the same imaging geometry as the current mirror-based systems except for the x-ray detection stage, where a two-dimensional (2D) array of 1 cm thick CsI(Tl) detector elements are utilized. The approximately 18% x-ray quantum efficiency of the scintillation detector and its 30 x 40 cm2 field-of-view at the isocenter are greater than other area-imaging EPIDs. The imaging issues addressed are theoretical and measured signal-to-noise ratio, linearity of the imaging chain, influence of frame-summing on image quality and image calibration. Portal images of test objects and a humanoid phantom are used to measure the performance of the system. An image quality similar to the current devices is achieved but with a lower dose. With approximately 1 cGy dose delivered by a 6 MV beam, a 2 mm diam structure of 1.3% contrast and an 18 mm diam object of 0.125% contrast can be resolved without using image-enhancement methods. A spatial resolution of about 2 mm at the isocenter is demonstrated. The capability of the system to perform fast sequential imaging, synchronized with the radiation pulses, makes it suitable for patient motion studies and verification of intensity-modulated beams as well as its application in cone-beam megavoltage computed tomography.

Dosimetric investigation of lung tumor motion compensation with a robotic respiratory tracking system: An experimental study

The benefits of using Synchrony Respiratory Tracking System (RTS) in conjunction with the CyberKnife robotic treatment device to treat a breathing tumor in an anthropomorphic, tissue-equivalent, thoracic phantom have been investigated. The following have been studied: (a) Synchronys ability to allow the CyberKnife to deliver accurately a planned dose distribution to the free-breathing phantom and (b) the dosimetric implications when irregularities in the breathing cycle and phase differences between internal (tumor) and external (chest) motion exist in the course of one treatment fraction. The breathing phantom PULMONE (phantom used in lung motion experiments) has been used, which can imitate regular or irregular breathing patterns. The breathing traces from two patients with lung cancer have been selected as input. Both traces were irregular in amplitude, frequency, and base line. Patient B demonstrated a phase difference between internal and external motion, whereas patient A did not. The experiment was divided into three stages: In stage I-static, the treatment was delivered to the static phantom. In stage II-motion, the phantom was set to breathe, following the breathing trace of each of the two patients. Synchrony was switched off, so no motion compensation was made. In stage III-compensation, the phantom was set to breathe and Synchrony was switched on. A linear correspondence model was chosen to allow for phase differences between internal and external motion. Gafchromic EBT film was inserted in the phantom tumor to measure dose. To eradicate small errors in film alignment during readout, a gamma comparison with pass criteria of 3%/3 mm was selected. For a more quantitative approach, the percentage of pixels in each gamma map that exceeded the value of 1 (P1) was also used. For both breathing signals, the dose blurring caused by the respiratory motion of the tumor in stage II was degraded considerably compared with stage I (P1 = 15% for patient A and 8% for patient B). The motion compensation via the linear correspondence model was sufficient to provide a dose distribution that satisfied the set gamma criteria (P1=3% for patient A and 2% for patient B). Synchrony RTS has been found satisfactory in recovering the initial detail in dose distribution, for realistic breathing signals, even in the case where a phase delay between internal tumor motion and external chest displacement exists. For the signals applied here, a linear correspondence model provided an acceptable degree of motion compensation.