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Image-guided Radiotherapy Based on Kilovoltage Cone-beam Computed Tomography


transferred into an absolute survival benefit of 6% compared with sequential radiochemotherapy.33


progression was observed, indicating that an increase in local control transfers directly to increased overall survival (OS).


Escalation of the irradiation dose beyond 70Gy has been shown to increase local control in both early and advanced-stage NSCLC.34,35 However, large safety margins associated with conventional RT planning and delivery techniques do not allow for the safe delivery of such escalated irradiation doses in a substantial proportion of the patients. Improving the accuracy of RT with smaller safety margins is consequently considered to be a safe approach to dose escalation with the potential to increase OS.36


Clinical Results of Image-guided Radiotherapy Clinical results of image-guided stereotactic body radiotherapy (SBRT) for stage I NSCLC are highly consistent. A multi-institutional analysis of over 400 patients treated with CBCT-based SBRT reported a local control rate of 92% at two years; local tumour control was even higher, at 95%, if the biological equitant planned target volume (PTV) prescription dose (BED10) was >106Gy.37


These excellent local control


rates in SBRT are achieved with minimal toxicity due to high accuracy of treatment planning and delivery, which allows small safety margins and confinement of the hypo-fractionated high-doses to small volumes. OS approaching best surgical results are achievable if SBRT is practised in operable patients, who refuse surgery.38


There are few


data on practice and results of IGRT in advanced-stage NSCLC. Liao et al. performed a retrospective analysis in 496 patients treated with either 3D-CRT (n=318) or 4D-CT imaging and IMRT planning (n=91).39 The authors reported significantly decreased rates of severe radiation-induced pneumonitis in the group treated with modern advanced technologies and OS survival was also significantly improved. This can be seen as a proof of principle that more accurate RT improves outcome; however, more and especially prospective data are required to better define the role of IGRT and adaptive RT in advanced-stage NSCLC.


Image Guidance for Intracranial and Head-and-neck Target Volumes Rationale for Image-guided Radiotherapy Frame-based stereotactic patient set-up for the treatment of intracranial lesions has been considered to be the most precise technique in RT. This accuracy allowed the application of high doses in a radiosurgical fashion, which achieved excellent clinical result in multiple benign and malignant intra-cranical tumours. However, fractionated treatment is not possible with invasive frame-based systems and treatment planning and delivery have to be performed within one day limiting the possibility of multimodality imaging. Non-invasive approaches using mask or bite- block systems allow fractionated treatment regimens but the accuracy of patient set-up is decreased compared with the invasive stereotactic approach. In RT of head-and-neck tumours, where patient set-up and immobilisation is usually performed with relocatable mask systems, there are no data clearly demonstrating that RT without IGRT negatively affects outcome. Nevertheless, more precise patient set-up and


1. Jaffray DA, Siewerdsen JH, Cone-beam computed tomography with a flat-panel imager: initial performance characterization, Med Phys, 2000;27(6):1311–23.


2. Jaffray DA, Siewerdsen JH, Wong JW, Martinez AA, Flat-panel cone-beam computed tomography for image-guided radiation therapy, Int J Radiat Oncol Biol Phys, 2002;53(5):1337–49.


3. Boda-Heggemann J, Walter C, Rahn A, et al., Repositioning


A: Misalignment of pulmonary tumour and bony structures after frame-based stereotactic patient positioning; B: Image registration based on the spine with misalignment of the pulmonary tumour due to a baseline shift; C: Image registration based on the pulmonary tumour with misalignment of the spine.


adaptation of the treatment to systematic changes during the fractionated treatment course may allow a reduction of safety margins with improved sparing of organs at risk.


Clinical Results of Image-guided Radiotherapy There are limited data on outcomes after image-guided stereotactic RT/radiosurgery. Breneman et al. reported a one-year local control rate of 80% after frameless image-guided radiosurgery of brain metastases, which is similar to results using the frame-based approach.40


For head-and-neck cancer, IGRT without IMRT has the potential to reduce safety margins. In a prospective study, Den et al.41 found that daily CBCT for head-and-neck cancer enabled a reduction in the CTV-to-PTV margins by about 50% (except for mobile targets such as the tongue), which could facilitate future studies of dose escalation and/or improved toxicity reduction.


By contrast, IMRT without IGRT is not recommended because the sharp dose gradients in IMRT planning require precise patient set-up. Consequently, most studies now combine conformal IMRT planning techniques with precise patient set-up using IGRT and preliminary clinical results e.g. in the re-irradiation situation are promising.42


Conclusion


CBCT volume imaging has streamlined and facilitated precision RT. Required imaging time is short, its percentage compared with currently quickly shortening beam-on times is, however, increasing, requiring further acceleration of the procedure. In many clinical situations, matching of planning CT and CBCT can be performed automatically based on the bony anatomy with a pre-set alignment clip box while manual matching by physicians or radiotherapists is still the procedure of choice for several situations, encouraging the assessment of not only target position but also OAR geometry. CBCT in combination with large- bore planning CT and good-quality digital radiographies obviates conventional simulation and also frame/fiducial-based stereotaxy. It will likely be the basis for all precision radiotherapy, such as IMRT/VMAT, ablative hypofractionated RT and proton therapy. n


accuracy of two different mask systems-3D revisited: comparison using true 3D/3D matching with cone-beam CT, Int J Radiat Oncol Biol Phys, 2006;66(5):1568–75.


4. Masi L, Casamassima F, Polli C, et al., Cone beam CT image guidance for intracranial stereotactic treatments: comparison with a frame guided set-up, Int J Radiat Oncol Biol Phys, 2008;71(3):926–33.


5. Purdie TG, Bissonnette JP, Franks K, et al., Cone-beam computed tomography for on-line image guidance of lung stereotactic radiotherapy: localization, verification, and intrafraction tumour position, Int J Radiat Oncol Biol Phys, 2007;68(1):243–52.


6. Guckenberger M, Meyer J, Wilbert J, et al., Cone-beam CT based image-guidance for extracranial stereotactic radiotherapy of intrapulmonary tumours, Acta Oncol, 2006;45(7):897–906.


No difference in systemic


Figure 3: Image Guidance in Stereotactic Body Radiotherapy for Early-stage Non-small-cell Lung Cancer


EUROPEAN ONCOLOGY & HAEMATOLOGY


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