Obtaining accurate (metabolic) tumour boundaries may be important for treatment planning in radiotherapy and/or for use as prognostic factor and/or to monitor response during therapy, and may therefore have a direct impact on clinical outcome.
Tumour delineation methods are only suited for radiotherapy planning purposes if they correspond well with pathology. The present results indicate that VOI50, VOIA41 and VOISchaefer show good agreement with pathology after removing two outliers (R2: 0.82, slope: 1.00-1.06, Figure
1). These outliers were located closely to high uptake regions, e.g. mediastinum and heart. Only those tumour delineation methods should be selected for radiotherapy planning purposes if they are able to distinguish between these adjacent normal tissues and the tumour. VOIA50, VOIRTL and GradWT2 did not show any outliers, provided only small, non-significant differences in maximum tumour diameter (<4.7 mm, p>0.10), and showed fair correlation (R2>0.62, slope 0.88-0.97) with pathology (Tables
2). These results correspond with those of previous studies
[4, 13], in which small differences in maximum tumour diameter between an adaptive threshold method and pathology were reported
, as well as good correlation between maximum diameters obtained from a percentage threshold-based method and pathology
. The latter study also showed a reduction in inter-observer variability when PET-based (semi-)automatic tumour delineation methods were used. GradWT1 showed only moderate agreement with pathology (R2: 0.43) and overestimated the maximum diameter. In addition, a poor agreement obtained with pathology and GradWT1 was found for small tumours (≤2.5 cm diameter). As the sizes of the small tumours are less than three times the full-width-at-half-maximum, the influence of partial volume effects may be relatively high, causing this poor agreement with pathology. By applying modifications to the algorithm (i.e. GradWT2), the method showed more accurate tumour sizes (R2: 0.62). These findings are in line with a previous study that compared volumes derived with a gradient-based method to pathology
. SUV2.5 showed a good agreement with pathology after the removal of 5 outliers (R2: 0.79), but showed a significant overestimation in the maximum diameter (19.8 mm, p<0.001). Due to the large number of outliers and large overestimation in diameter size, it is not recommended to use this method for radiotherapy planning purposes.
Assessment of change in tumour size is important when monitoring tumour response during therapy. Although VOIA70 showed a large systematic underestimation (−36%) of diameter, this method was the most precise (smallest SD and coefficient of variation (COV,
) of 14 and 40%, respectively (Figure
1). For all other methods, SD and COV ranged from 19 to 188% and from 77 to 448%, respectively). In addition, good correlation (R2=0.81) between maximum diameters obtained from this method and pathology was observed (Table
2). Therefore, VOIA70 may be a good method for response monitoring in which relative changes in (more active part of) metabolic volume are considered.
Wu et al.
 showed that CT-based delineation provided better correlation (R2=0.87) with pathology than PET-based percentage threshold methods that did not correct for background activity (R2=0.77). This is in contrast to the present study, where CT-based delineation provided a slightly lower correlation of maximum diameter with pathology than VOI50 (R2: 0.77 and 0.82, respectively). In addition, CT-based delineation showed a moderate overestimation of maximum diameter compared with pathology (slope: 1.25 and 1.00 for CT and VOI50, respectively). Despite the good correlation, a drawback of manual CT delineation is the requirement of both a high resolution image (i.e. not a low dose CT) and an experienced observer. Even if delineation is performed by an experienced observer, manual CT delineation suffers from substantial interobserver variation
. In addition, accuracy of manual CT delineation was shown to be dependent on the colour window settings used
[15, 16]. Moreover, several conditions including chronic obstructive lung disease (COPD), cavitation, pleural fluid, necrosis, atelectasis and mucus plugs, which all occur frequently in lung cancer patients, obscure the exact boundaries of the tumour on CT inducing errors in measured tumour volume and/or diameter size. In the present study, CT delineation has been performed by only one expert physician. Although this may weaken the strength of any correlation with pathology, the results of this study were consistent with the results from another study where manual CT delineation was performed by two experienced observers
For the patient shown in Figure
3, large differences in diameter were observed between CT- and PET-based methods. In this case, the primary tumour was located close to another suspicious mass within the lymph node. In addition, the primary tumour showed heterogeneous FDG uptake, and both air-containing cavitation and fluid level on the CT scan. Note that, for this typical example, the measured tumour volume obtained using PET-based delineation methods excluded this (non-metabolic) necrotic and cystic centre that was included in manual CT delineation. However, also note that this (non-metabolic) necrotic and cystic centre was included in all maximum diameter calculations. For this tumour, the maximum diameter obtained from PET-based methods was closer to that of pathology than for corresponding CT delineation, further illustrating the conceptual differences between anatomical (CT) and metabolic (PET) volumes. As previously suggested
, CT-based delineation is unable to differentiate between high and low activity regions, and the use of PET can assist in quantifying and visualising heterogeneous tracer uptake across the tumour and PET-based delineation may be useful to define the most active part of the tumour.
Some factors might limit accurate delineation of tumour volumes and corresponding maximum diameters using the commonly available PET-based (semi-)automatic tumour delineation described in this article. First, primary lesions could be surrounded by high uptake regions, e.g. from suspected locoregional metastases, heart and spine. Therefore, application of tumour delineation methods might be more valid for peripheral tumours and less valid for more centrally located tumours, unless a (manually adjustable) bounding box around the tumour is used to prevent delineation of surrounding high uptake regions. Second, the metabolic volume of tumours could show heterogeneous tracer uptake that has been shown to have an impact on threshold-based delineation methods
. Moreover, tumours located in the thorax could be affected by respiratory motion. However, a good correlation between pathology and PET data is observed in the present study, which might indicate that lung tumors might not be strongly affected by these effects (at least not in this study). Nevertheless, a slight mismatch between PET and CT data can be observed in Figure
3. Fourth, it should be noted that trends observed in the present study may only be valid for primary lung tumours. For other locations, the local background surrounding the tumour is different, which could have an effect on the performance of the tumour volume delineation methods evaluated
. Finally, tumour delineation methods are affected by several factors, such as scanner type, radiotracer, image noise and tumour characteristics
[10, 11]. So, additional evaluations with pathology, and/or optimisation of systems or tumour delineation methods may be required for other PET/CT systems.
The present study showed some potential methodological limitations that might have influenced the results. First, it should be noted that deformations could occur between in-vivo CT imaging and ex-vivo pathology due to the softness of lung tissue
. The method used in the present study involved no inflation of the tissue after resection nor other deformation compensation techniques. All tumours were measured directly after surgery, without using preservation. Inflation is required to find the exact position of the lung tumour inside of the lung. However, inflation is expected to influence mostly the surrounding lung tissue, as the tumours imaged in this study showed a relatively solid mass. The purpose of the current study was not to determine the exact position, but to measure the maximum diameter of the tumour. In addition, the results of the present study are in line with Siedschlag et al.
 where inflation was used. Therefore, deformations of the tumour after resection are presumed to be negligible. However, ideally, a CT scan of the excised tumour should have been made to confirm that no deformations occurred. Second, no pathological data on the volume of the primary tumour was available. Therefore, only a comparison with maximum tumour diameter was made rather than with volume. Finally, it should be noted that in this study pathological correlation is available only for resectable lung tumours. However, the majority of patients that will receive radiotherapy suffer from unresectable lung tumours for which accurate tumour volume delineation is critical for treatment. However, obtaining the true volumes for this kind of tumour will remain a challenge yet to be solved.