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K.J. Carson
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MS01 - Radiation as a Systemic Therapy (ID 18)
- Event: WCLC 2013
- Type: Mini Symposia
- Track: Radiation Oncology + Radiotherapy
- Presentations: 1
- Moderators:B. Lu, P. Van Houtte
- Coordinates: 10/28/2013, 14:00 - 15:30, Bayside 201 - 203, Level 2
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MS01.4 - Integration of Functional and Molecular Imaging in Radiotherapy Planning (ID 460)
15:05 - 15:25 | Author(s): K.J. Carson
- Abstract
Abstract
Radiotherapy planning and target volume delineation in lung cancer is largely based on x-ray based imaging such as CT scanning or fluoroscopy. The most widely used functional imaging technique in the diagnosis and characterisation of NSCLC is [18]Fluoro-deoxy-glucose (FDG) position emission tomography (PET). PET acquired with CT on the same scanner (PET/CT) has been shown to be superior to CT alone in the staging of NSCLC. When PET is used to select patients for curative therapy an improvement in overall survival is seen. Many clinical studies describe an impact on the use of PET for target volume delineation (TVD) in NSCLC but none describe an improvement in clinical outcomes. Several staging studies clearly demonstrated the superiority of PET/CT over CT for identification of involved mediastinal nodes. PET based TVD was also shown to improve the identification of involved mediastinal lymph nodes. In areas of atelectasis, PET can help discriminate between areas of collapsed lung and areas of tumour. A number of studies have sought to measure the impact of PET/CT based TVD on inter-observer variation or against a gold standard. When PET/CT is used for target volume delineation alone, and the baseline staging issues are removed, PET/CT reduces the undesirable impact of inter-observer variation. Considering the impact on the resultant radiotherapy plan, PET/CT based target volume delineation has been shown to reduce the dose to normal structures and this may open the possibility of dose escalation. When used in its most basic form, images from the staging PET/CT scan can be visually correlated with the radiotherapy planning (RTP) CT image to identify areas of disease for inclusion within the treatment volume. To improve the accuracy of correlation a staging PET/CT scan can be registered with the planning CT and rigid registration is recommended to undertake this. One option is to acquire a PET/CT exclusively for the purpose of RTP after a staging PET has been acquired and the patient is deemed suitable for radical radiotherapy, requiring a separate PET scan, but removing any staging or patient selection issues. Another approach is to acquire a PET/CT in the radiotherapy treatment position both for the purposes of staging and TVD and this represents a more cost effective approach. Given the nature of PET images a number of investigations have examined the use of automated methods to define the edge of the tumour. These methods include: i) Fixed thresholding based on absolute values were areas of disease above a given value are included within the target volume (e.g. SUVmax >2.5); ii) A contour based on the a percentage of the SUVmax (e.g.40% of SUVmax); iii) The use of the ratio of the SUVmax to the average SUV within a background structure to define the SUV level to generate the auto-contour; iv) More complicated analytical methods such as the watershed method. Auto contours provide consistent contours, but have difficulty dealing with normal tissue adjacent to the tumour with high SUV uptake such as the heart. There is no clear consensus on which method most closely approximates to the tumour position and tumour edge and pathological correlation has proven difficult. Another difficulty with PET based auto-contouring is the variability of SUV values due to factors other than tumour activity such patient biological factors and scanning technical factors. At present it is recommended that any PET based contouring outside of a clinical trial should be based on a visual assessment method. As PET images are acquired over a number of minutes at each table position, it has been suggested that PET could define the entire motion trajectory of a lung tumour also known as the internal target volume (ITV) of a moving lung tumour. It has been demonstrated that a 4D PET/CT generated ITV based on the 4D PET image approximates to a 4DCT ITV. A number of studies have shown sizeable differences in SUV calculation between 3D PET/CT and 4D PET/CT and it is suggested that 4DCT provides a more accurate SUV quantification. This has implications for auto-contouring and may lead to new exciting new methods of PET based TVD based on 4DCT. FDG is the most commonly used tracer owing to its high tumour specificity and the relatively long half-life of [18]F. Other fluorine based tracers have been used to quantify tumour proliferation uisng 18 deoxy-fluoro-l-thymidine (FLT) and tumour hypoxia using Fluoroazomycin Arabinoside (F-FAZA). It has been suggested that these may be used for IMRT based dose painting Work is on-going to optimise dual tracer PET acquisition. A number of recent publications have examined the utility of PET for predicting outcome after stereotactic ablative radiotherapy (SABR) and demonstrated a clear association with SUVmax and poorer outcome. This might help identify those patients who might benefit from adjuvant therapy after SABR treatment. PET may have increased accuracy in detecting recurrence following SABR and should be used in the re-staging process. There is growing evidence of a similar clinical utility for PET in the management of patients with SCLC. PET has been shown to select patients with SCLC appropriately for radical therapy, to be predictive for outcome following therapy and for TVD. In conclusion, functional imaging is an essential part of the radiotherapy planning process for both NSCLC and SCLC. For both disease sites PET is critical for baseline staging and patient selection for radical therapy. PET should be used to inform TVD in NSCLC and to guide TVD in SCLC. PET is useful for identify relapse in patients treated with radiotherapy. It may also be useful as predictor of response and for adaptive radiotherapy. On-going research is still required particularly in the era of 4D PET/CT, given the promise 4D PET/CT has for improved accuracy in quantification and volume delineation.
