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The limited radiation tolerance of the small-bowel causes toxicity for patients receiving conventionally-fractionated radiotherapy for rectal cancer. Safe radiotherapy dose-escalation will require a better understanding of such toxicity. We conducted a systematic review and meta-analysis using published datasets of small bowel dose-volume and outcomes to analyse the relationship with acute toxicity. SCOPUS, EMBASE & MEDLINE were searched to identify twelve publications reporting small-bowel dose-volumes and toxicity data or analysis. Where suitable data were available (mean absolute volume with parametric error measures), fixed-effects inverse-variance meta-analysis was used to compare cohorts of patients according to Grade ≥3 toxicity. For other data, non-parametric examinations of irradiated small-bowel dose-volume and incidence of toxicity were conducted, and a univariate logistic regression model was fitted. On fixed-effects meta-analysis of three studies (203 patients), each of the dose-volume measures V5Gy-V40Gy were significantly greater (p

Background and purpose A recent update of the RTOG 9811, reported differing relapse rates for early and late anal squamous cell carcinoma following chemoradiotherapy (CRT). There may be a role for dose-individualization, however the dose–response relationship for anal cancer is not currently known. Intensity-modulated radiotherapy (IMRT) has been widely adopted with multiple series published. The aim is to fit a tumor control probability (TCP) model to the published IMRT data. Materials and methods We performed a systematic review of PubMed and Embase databases to identify thirteen appropriate papers, including 625 patients. Predefined data fields were collected. A standard linear quadratic TCP model, which included repopulation, was fit by least squares minimization. Results The fitted TCP curve demonstrated a dose–response relationship with α = 0.196 Gy–1. The curve suggests: in early stage tumours, a dose reduction from 50 Gy to 45 Gy reduces 2 year local control from 98% to 95%; in late stage tumours, a dose escalation from 50 Gy to 55 Gy improves the 2 year local control rate from approximately 50% to 80%. Conclusions The published data are broadly consistent with a linear quadratic dose–response model. Dose-individualization in anal cancer should be further investigated in the context of clinical trials.

Background An unexpected finding from the phase III parotid sparing radiotherapy trial, PARSPORT (ISRCTN48243537, CRUK/03/005), was a statistically significant increase in acute fatigue for those patients who were treated with intensity-modulated radiotherapy (IMRT) compared to standard conventional radiotherapy (CRT). One possible explanation was the difference in dose to central nervous system (CNS) structures due to differing beam portals. Using data from the trial, a dosimetric analysis of individual CNS structures was performed. Method Dosimetric and toxicity data were available for 67 patients (27 CRT, 40 IMRT). Retrospective delineation of the posterior fossa, brainstem, cerebellum, pituitary gland, pineal gland, hypothalamus, hippocampus and basal ganglia was performed. Dosimetry was reviewed using summary statistics and dose–volume atlases. Results A statistically significant increase in maximum and mean doses to each structure was observed for patients who received IMRT compared to those who received CRT. Both maximum and mean doses were significantly higher for the posterior fossa, brainstem and cerebellum for the 42 patients who reported acute fatigue of Grade 2 or higher ( p ⩽0.01) compared to the 25 who did not. Dose–volume atlases of the same structures indicated that regions representing larger volumes and higher doses to each structure were consistent with a higher incidence of acute fatigue. There was no association between the dose distribution and acute fatigue for the other structures tested. Conclusions The excess fatigue reported in the IMRT arm of the trial may, at least in part, be attributed to the dose distribution to the posterior fossa, cerebellum and brainstem. Future studies that modify dose delivery to these structures may allow us to test the hypothesis that radiation-induced fatigue is avoidable.

Pancreatic adenocarcinoma patients have a poor prognosis, with the fi ve-year overall survival rate 5% [1]. Treatment options for these patients may include pre-operative or defi nitive chemo-radiotherapy (CRT). Patterns of failure suggest that inclusion of elective lymph nodes in the treatment volume may improve local control [2]. These extended volumes, in conjunction with large margins to account for breathing motion, generate substantial planning treatment volumes (PTV) which may increase the risk of gastro-intestinal (GI) toxicity. Several publications have investigated the use of intensity-modulated radiotherapy (IMRT) [3 – 7] and intensity-modulated arc therapy (IMAT) [8 – 11] for treatment of pancreatic cancer, where the improved dose conformation may reduce dose to surrounding normal tissue, and allow dose escalation for improved local control. Despite studies showing a correlation between dosimetric parameters and GI toxicity for three-dimensional (3D)-RT and IMRT [12] there is some debate over dose constraints for stomach, duodenum and small bowel. Differences in organ delineation and prescribed dose may limit the comparison of dose-volume histogram (DVH) parameters, and a standard dose-volume analysis is often limited to only a few points in the DVH data, which may not always correspond directly to a clinical outcome. However, radiobiological modelling evaluates treatment plans by analysing the entire DVH and reducing this multifactorial comparison into a single clinically relevant parameter. As the parameters used for modelling normal tissue complication probability (NTCP) are derived from observed rates of toxicity in clinical trials, these parameters should be cited as a range of values (e.g. covering a 95% confi dence interval). Each parameter set is specifi c for a selected endpoint and is also dependent on the patient cohort and the treatment technique used. Careful comparison of the predicted complications with observed clinical toxicity is required to validate each set of NTCP parameters which may be found in the literature [13]. Nonetheless, radiobiological modelling may be useful for assessing different planning techniques and dose prescriptions, and has been applied to compare predicted toxicity to stomach and duodenum for a dose escalation study using tomotherapy plans for pancreatic cancer [5]. The current study compares the use of NTCP models and dose-volume metrics to analyse RapidArc (IMAT) and three-dimensional conformal treatment (3D-RT) plans for locally-advanced pancreatic cancer (LAPC). Using commercially available biological evaluation software module (Eclipse, Varian, Palo Alto

Purpose To review the published clinical data on non-small cell lung cancer treated with radical radiotherapy to confirm a dose–response relationship as a basis for further dose-escalation trials. Methods Twenty-four published clinical trials were identified, 16 of which – with 29 different standard, hyper- and hypofractionated treatment schedules – were analysed. Prescription doses were converted to biologically-equivalent dose (BED), with a correction for repopulation. Disease-free survival data were corrected for the stage profile of each cohort to allow better comparison of results. We also analysed moderate (grade II and III) lung and oesophageal acute toxicity related to the corrected BED delivered to the tumour. Results The clinical data analysed showed good agreement between the observed and modelled disease-free survival at 2years when compared to the published models of Fenwick (correlation coefficient 0.525, p =0.003) and Martel (correlation coefficient 0.492, p =0.007), indicating a clear tumour dose–response. In the normally fractionated treatments (∼2Gy per fraction), improved disease-free survival was generally observed in the shorter schedules (maximum around 6weeks). However, the best outcomes were obtained for the hypofractionated schedules. No systematic relationship was seen between prescribed dose and lung or oesophageal acute toxicity, possibly due to dose selection depending on V 20 or MLD in some studies and the diversity of the patients analysed. Conclusions We have demonstrated a dose–response relationship for NSCLC based on clinical data. The clinical data provide a rational basis for selection of dose escalation schedules to be tested in future randomised trials.

There is growing clinical evidence that functional imaging is useful for target volume definition and early assessment of tumour response to external beam radiotherapy. A subject that has perhaps received less attention, but is no less promising, is the application of functional imaging to the prediction or measurement of radiation adverse effects in normal tissues. In this manuscript, we review the current published literature describing the use of positron emission tomography (PET), four-dimensional computed tomography (4D-CT), single photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI) to study normal tissue function in the context of radiotherapy to the lung, liver and head & neck. Published results to date demonstrate that functional imaging can be used to preferentially avoid normal tissues not easily identifiable on solely anatomical images. It is also a potentially very powerful tool for the early detection of radiotherapy-induced normal tissue adverse effects and could provide valuable data for building predictive models of outcome. However, one of the major challenges to building useful predictive models is that, to date, there are very little data available with combined images of normal function, 3D delivered radiation dose and clinical outcomes. Prospective data collection through well-constructed studies which use established morbidity scores is clearly a priority if significant progress is to be made in this area.

Background and purpose The study aimed to examine specific avoidance of functional lung (FL) defined by a single photon emission computerized tomography (SPECT) lung perfusion scan, using intensity modulated radiotherapy (IMRT) and three-dimensional conformal radiotherapy (3-DCRT) in patients with non-small cell lung cancer (NSCLC). Materials and methods Patients with NSCLC underwent planning computerized tomography (CT) and lung perfusion SPECT scan in the treatment position using fiducial markers to allow co-registration in the treatment planning system. Radiotherapy (RT) volumes were delineated on the CT scan. FL was defined using co-registered SPECT images. Two inverse coplanar RT plans were generated for each patient: 4-field 3-DCRT and 5-field step-and-shoot IMRT. 3-DCRT plans were created using automated AutoPlan optimisation software, and IMRT plans were generated employing Pinnacle 3 treatment planning system (Philips Radiation Oncology Systems). All plans were prescribed to 64Gy in 32 fractions using data for the 6MV beam from an Elekta linear accelerator. The objectives for both plans were to minimize the volume of FL irradiated to 20Gy (fV 20 ) and dose variation within the planning target volume (PTV). A spinal cord dose was constrained to 46Gy. Volume of PTV receiving 90% of the prescribed dose (PTV 90 ), fV 20 , and functional mean lung dose (fMLD) were recorded. The PTV 90 /fV 20 ratio was used to account for variations in both measures, where a higher value represented a better plan. Results Thirty-four RT plans of 17 patients with stage I–IIIB NSCLC suitable for radical RT were analysed. In 6 patients with stage I–II disease there was no improvement in PTV 90 , fV 20 , PTV/fV 20 ratio and fMLD using IMRT compared to 3-DCRT. In 11 patients with stage IIIA–B disease, the PTV was equally well covered with IMRT and 3-DCRT plans, with IMRT producing better PTV 90 /fV 20 ratio (mean ratio – 7.2 vs. 5.3, respectively, p =0.001) and reduced fMLD figures compared to 3-DCRT (mean value – 11.5 vs. 14.3Gy, p =0.001). This was due to reduction in fV 20 while maintaining PTV coverage. Conclusion The use of IMRT compared to 3-DCRT improves the avoidance of FL defined by perfusion SPECT scan in selected patients with locally advanced NSCLC. If the dose to FL is shown to be the primary determinant of lung toxicity, IMRT would allow for effective dose escalation by specific avoidance of FL.

Background and purpose Patients with non-small cell lung cancer (NSCLC) often have inhomogeneous lung perfusion. Radiotherapy planning computed tomography (CT) scans have been accurately co-registered with lung perfusion single photon emission computed tomography (SPECT) scans to design radiotherapy treatments which limit dose to healthy ‘perfused' lung. Patients and methods Patients with localised NSCLC had CT and SPECT scans accurately co-registered in the planning system. The SPECT images were used to define a volume of perfused ‘functioning' lung (FL). Inverse planning software was used to create 3D-conformal plans, the planning objective being either to minimise the dose to whole lungs (WL) or to minimise the dose to FL. Results Four plans were created for each of six patients. The mean difference in volume between WL and FL was 1011.7cm 3 (range 596.2–1581.1cm 3 ). One patient with bilateral upper lobe perfusion deficits had a 16% reduction in FLV 20 (the percentage volume of functioning lung receiving ≥20Gy). The remaining patients had inhomogeneous perfusion deficits such that inverse planning was not able to sufficiently optimise beam angles to avoid functioning lung. Conclusion SPECT perfusion images can be accurately co-registered with radiotherapy planning CT scans and may be helpful in creating treatment plans for patients with large perfusion deficits.

Background and purpose To investigate the feasibility and the advantages of a portal-imaging mode on a medical accelerator, consisting of a thin low-Z bremsstrahlung target and a thin Gd 2 O 2 S/film detector, for patient imaging. Patients and methods The international code of practice for high-energy photon dosimetry was used to calibrate dosimetry instruments for the imaging beam produced by 4.75MeV electrons hitting a 6mm thick aluminium target. Images of the head and neck of a humanoid phantom were taken with a mammography film system and the dose in the phantom was measured with TLDs calibrated for this beam. The first head and neck patient images are compared with conventional images (taken with the treatment beam on a film radiotherapy verification detector). Visibility of structures for six patients was evaluated. Results Images of the head and neck of a humanoid phantom, taken with both imaging systems showed that the contrast increased dramatically for the new system while the dose required to form an image was less than 10 −2 Gy. The patient images taken with the new and the conventional systems showed that air–tissue interfaces were better defined in the new system image. Anatomical structures, visible on both films, are clearer with the new system. Additionally, bony structures, such as vertebrae, were clearly visible only with the new system. The system under evaluation was significantly better for all features in lateral images and most features in anterior images. Conclusions This pilot study of the new portal imaging system showed the image quality is significantly improved.

Purpose/objective: To analyse outcome and toxicity of stereotactic body radiotherapy (SBRT) in pancreatic cancer (PDAC). Material/methods: We systematically reviewed full reports on outcome and toxicity transforming prescription doses to equivalent doses of 2Gy (EQD2) and biological equivalent doses (BED). Pearson product-moment correlation coefficient, regression analysis and Lyman–Kutcher–Burman modelling were used. Results: Sixteen trials (572 patients) were identified. Local control correlated with dose. Additionally 4 upper gastrointestinal-SBRT trials (149 patients) were included for toxicity analysis. Acute toxicity was mild but late toxicity ⩾G2 was substantial and predominantly gastrointestinal. Late toxicity ⩾G2 and ⩾G3 correlated highly with EQD2/BED after linear ( R 2 =0.85 and 0.77, respectively) and Lyman–Kutcher–Burman modelling. Linear regression lines indicated ⩾G2 and ⩾G3 toxicity frequencies of 5% at 65Gy and 80Gy EQD2-α/β=3, respectively. A comparison of toxicity with dose constraints for duodenum revealed partly inadequate dose constraints. Conclusion: Results from multiple fraction regimens could be successfully interpreted to estimate toxicity according to EQD2/BED prescription doses, and dose constraints for the duodenum were derived, whereas local control appeared to be less dose-dependent. This analysis may be useful to plan clinical trials for SBRT and hypofractionated radiotherapy in pancreatic cancer.

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.

Purpose Most studies investigating the dose–response of the rectum focus on rectal bleeding. However, it has been reported that other symptoms such as urgency or sphincter control have a large impact on quality-of-life and that different symptoms are related to the dose to different parts of the anorectal wall. In this study correlations between the 3D dose distribution to the anal-sphincter region and radiation-induced side-effects were quantified. Materials and methods Dose–surface maps of the anal canal were generated. Next, longitudinal and lateral extent and eccentricity were calculated at different dose levels; DSHs and DVHs were also determined. Correlations between these dosimetric measures and seven clinically relevant endpoints were determined by assessing dosimetric constraints. Furthermore, an LKB model was generated. The study was performed using the data of 388 prostate patients from the RT01 trial (ISRCTN 47772397). Results Subjective sphincter control was significantly correlated with the dose to the anal surface. The strongest correlations were found for lateral extent at 53Gy ( p =0.01). Outcome was also significantly correlated with the DSH and the mean dose to the anal surface. Conclusions The dose to the anal sphincter region should be taken into account when generating treatment-plans. This could be done using shape-based tools, DSH/DVH-based tools or an NTCP model.

BACKGROUND: When induction chemotherapy (IC) is used prior to chemoradiotherapy (CRT) in head and neck cancer (HNC), functional imaging (FI) may inform adaptation of treatment plans with the aim of optimising outcomes. Understanding the impact of IC on FI parameters is, therefore, essential. PURPOSE: To prospectively evaluate the feasibility of acquiring serial FI ((18)F-FDG-PET, diffusion-weighted (DW) and dynamic contrast-enhanced (DCE) MRI) and its role in defining individualised treatment regimens following IC in HNC. METHODS AND MATERIALS: Ten patients with stage III and IV HNC underwent conventional (CT and MRI) and functional (DW, DCE-MRI and (18)F-FDG-PET/CT) imaging at baseline and following two cycles of IC prior to definitive CRT. RESULTS: One patient withdrew due to claustrophobia. Seven out of nine patients had a complete metabolic response to IC on (18)F-FDG-PET imaging. DCE-MRI showed a significant fall in transfer constant (K(trans)) (0.209 vs 0.129 min(-1)P

Background and purpose The Normal Tissue Complication Probability (NTCP) for rectum is usually defined for late rectal bleeding. This study calculates NTCP parameter values for additional rectal toxicity endpoints observed in clinical practise. Materials and methods 388 patients from the multicentre MRC-RT01 prostate conformal radiotherapy trial (ISRCTN 47772397) were used to derive independent Lyman Kutcher Burman model (LKB) parameters for five late rectal toxicity endpoints: rectal bleeding, proctitis, stool frequency, loose stools and rectal urgency. The parameters were derived using maximum likelihood estimation. Bootstrap and leave-one-out methods were employed to test the generalisability of the results for use in a general population. Results A consistent pattern of increasing value of TD50(1) for Grade 2 toxicity only compared to Grades 1 and 2 toxicity was observed for all endpoints. Parameter values varied between endpoints (particularly for the volume parameter n). TD50(1), m and n were 68.5 Gy (95% CI)(66.8–70.8), 0.15 (0.13–0.17) and 0.13 (0.10–0.17), respectively, for G2 rectal bleeding. Bootstrap and leave-one-out results showed that the rectal bleeding and proctitis parameter fits were extremely robust. Conclusions The variation between the values derived for different endpoints may indicate different patho-physiological responses of the rectum to radiation. Therefore different parameter sets would be required to predict specific rectal toxicity endpoints.

Background and purpose Accurate reporting of complications following radiotherapy is an important part of the feedback loop to improve radiotherapy techniques. The definition of toxicity is normally regarded as the maximum or peak (P) grade of toxicity reported over the follow-up period. An alternative definition (integrated longitudinal toxicity (ILT)) is proposed which takes into account both the severity and the duration of the complication. Methods and materials In this work, both definitions of toxicity were used to derive dose–volume constraints for six specific endpoints of late rectal toxicity from a cohort of patients who received prostate radiotherapy in the MRC RT01 trial. The dose–volume constraints were derived using ROC analysis for 30, 40, 50, 60, 65 and 70 Gy. Results Statistically significant dose–volume constraints were not derived for all dose levels tested for each endpoint and toxicity definition. However, where both definitions produced constraints, there was generally good agreement. Variation in the derived dose–volume constraints was observed to be larger between endpoints than between the two definitions of toxicity. For one endpoint (stool frequency (LENT/SOM)) statistically significant dose–volume constraints were only derived using ILT. Conclusions The longitudinal definition of toxicity (ILT) produced results consistent with those derived using peak toxicity and in some cases provided additional information which was not seen by analysing peak toxicity alone.

INTRODUCTION: The prognosis from non-small cell lung cancer remains poor, even in those patients suitable for radical radiotherapy. The ability of radiotherapy to achieve local control is hampered by the sensitivity of normal structures to irradiation at the high tumour doses needed. This study aimed to look at the potential gain in tumour control probability from dose escalation facilitated by moderate deep inspiration breath-hold. METHOD: The data from 28 patients, recruited into two separate studies were used. These patients underwent planning with and without the use of moderate deep inspiration breath-hold with an active breathing control (ABC) device. Whilst maintaining the mean lung dose (MLD) at the level of the conventional plan, the ABC plan dose was theoretically escalated to a maximum of 84 Gy, constrained by usual normal tissue tolerances. Calculations were performed using data for both lungs and for the ipsilateral lung only. Resulting local progression-free survival at 30 months was calculated using a standard logistic model. RESULTS: The prescription dose could be escalated from 64 Gy to a mean of 73.7+/-6.5 Gy without margin reduction, which represents a statistically significant increase in tumour control probability from 0.15+/-0.01 to 0.29+/-0.11 (p

BACKGROUND AND PURPOSE: As techniques for radiotherapy delivery have developed, increasingly accurate localisation of disease is demanded. Functional imaging, particularly PET and its fusion with anatomical modalities, such as PET/CT, promises to improve detection and characterisation of disease. This study evaluated the impact of (18)FDG-PET/CT on radiotherapy target volume definition in head and neck cancer (HNC). MATERIALS AND METHODS: The PET/CT scans of patients with HNC were used in a radiotherapy planning (RTP) study. The gross tumour volume (GTV), clinical target volume (CTV) and planning target volume (PTV) were defined conventionally and compared to those defined using the PET/CT. Data were reported as the median value with 95% confidence intervals. RESULTS: Eighteen patients were consented, 9 had known primary tumour site, 9 presented as unknown primary. In nine cases where the primary site was known, the combined primary and nodal GTV (GTVp+n) increased by a median of 6.1cm(3) (2.6, 12.2) or 78% (18, 313), p=0.008 with CTV increasing by a median of 10.1cm(3) (1.3, 30.6) or 4% (0, 13) p=0.012. In 9 cases of unknown primary the GTVp+n increased by a median 6.3 cm(3) (0.2, 15.7) or 61% (4, 210), p=0.012, with CTV increasing by a median 155.4 cm(3) (2.7, 281.7) or 95% (1, 137), p=0.008. CONCLUSION: (18)FDG-PET revealed disease lying outside the conventional target volume, either extending a known area or highlighting a previously unknown area of disease, including the primary tumour in 5 cases. We recommend PET/CT in the RTP of all cases of unknown primary. In patients with a known primary, although the change in volume was statistically significant the clinical impact is less clear. (18)FDG-PET can also show areas within the conventional target volume that are hypermetabolic which may be possible biological target volumes for dose escalation studies in the future.

The additional benefit of positron emission tomography (PET) in the initial staging of non-small cell lung cancer (NSCLC) has generated interest in 18 F-fluorodeoxyglucose (FDG) PET as a means of defining the extent of primary lung tumour for radiotherapy treatment planning (RTP). A review of published data suggests that PET results in a reduction in the CT-derived GTV for NSCLC primary target volume in 15% of the patients. This is principally due to the ability of PET to distinguish tumour from atelectasis. However, the difficulty of tumour edge definition, limited spatial resolution and tumour motion during image acquisition currently limits the accuracy of PET in target volume delineation in NSCLC without adjacent lung consolidation. This is compounded by the lack of data correlating PET with spatial pathology at the primary tumour site. With the current technical limitations, it is not established that PET can add accuracy to the CT-defined primary target delineation in RTP of NSCLC. It is hoped that advances in PET and combined PET/CT imaging may overcome some of the technical limitations. Future use of PET for primary tumour delineation in NSCLC will also be critically dependent on the detailed studies of imaging–pathology correlation.

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 ∼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.

Purpose: A prototype scanner for large-volume megavoltage computed tomography (MVCT) in a clinical set-up is described. The ultimate aim is to improve treatment accuracy in conformal radiotherapy through patient set-up error reduction and transit dosimetry. Materials and methods: The scanner consists of a custom-built 2D CsI(Tl) crystal array viewed by a lens and a CCD camera. Image acquisition is synchronized with radiation pulses. The 2D projections resulting from a single continuous 360 ° gantry rotation are reconstructed using a cone-beam tomography algorithm. Prior to reconstruction, the raw projections are calibrated and corrected for centre of rotation movement and accelerator output fluctuation. The performance of the system has been evaluated by reconstructing projections of open fields, test objects and a humanoid phantom. Results: Hundreds of 2D projections can be acquired with a clinically-acceptable data collection time (about 2 min) and dose (approximately 40 cGy, with a possible four-fold reduction). A maximum density resolution of about 2% is achieved offering some soft tissue discrimination without using image enhancement tools. A spatial resolution of 2.5 mm is obtained. The reconstructed image intensity is linear with electron density over the range of interest. Coronal or sagittal slices through the 3D reconstruction of the humanoid phantom show a better delineation of structures than the corresponding portal images taken at the same orientation. Conclusions: A similar image quality to our current single-slice MVCT scanner is achieved with the advantage of providing tens of tomographic slices for a single gantry rotation. This work demonstrates the feasibility of clinical cone-beam MVCT and indicates how this prototype can be improved. © 1998 Elsevier Science Ireland Ltd. All rights reserved