Tumor biology-guided radiotherapy treatment planning: gross tumor volume versus functional tumor volume

Demonstration of real-time positron emission tomography biology-guided radiotherapy delivery to targets

BACKGROUND

Biology-guided radiotherapy (BgRT) is a novel technology that uses positron emission tomography (PET) data to direct radiotherapy delivery in real-time. BgRT enables the precise delivery of radiation doses based on the PET signals emanating from PET-avid tumors on the fly. In this way, BgRT uniquely utilizes radiotracer uptake as a biological beacon for controlling and adjusting dose delivery in real-time to account for target motion.

PURPOSE

To demonstrate using real-time PET for BgRT delivery on the RefleXion X1 radiotherapy machine. The X1 radiotherapy machine is a rotating ring-gantry radiotherapy system that generates a nominal 6MV photon beam, PET, and computed tomography (CT) components. The system utilizes emitted photons from PET-avid targets to deliver effective radiation beamlets or pulses to the tumor in real-time.

METHODS

This study demonstrated a real-time PET BgRT delivery experiment under three scenarios. These scenarios included BgRT delivering to (S1) a static target in a homogeneous and heterogeneous environment, (S2) a static target with a hot avoidance structure and partial PET-avid target, and (S3) a moving target. The first step was to create stereotactic body radiotherapy (SBRT) and BgRT plans (offline PET data supported) using RefleXion's custom-built treatment planning system (TPS). Additionally, to create a BgRT plan using PET-guided delivery, the targets were filled with 18F-Fluorodeoxyglucose (FDG), which represents a tumor/target, that is, PET-avid. The background materials were created in the insert with homogeneous water medium (for S1) and heterogeneous water with styrofoam mesh medium. A heterogeneous background medium simulated soft tissue surrounding the tumor. The treatment plan was then delivered to the experimental setups using a pre-commercial version of the X1 machine. As a final step, the dosimetric accuracy for S1 and S2 was assessed using the ArcCheck analysis tool-the gamma criteria of 3%/3 mm. For S3, the delivery dose was quantified using EBT-XD radiochromic film. The accuracy criteria were based on coverage, where 100% of the clinical target volume (CTV) receives at least 97% of the prescription dose, and the maximum dose in the CTV was ≤130% of the maximum planned dose (97 % ≤ CTV ≤ 130%).

RESULTS

For the S1, both SBRT and BgRT deliveries had gamma pass rates greater than 95% (SBRT range: 96.9%-100%, BgRT range: 95.2%-98.9%), while in S2, the gamma pass rate was 98% for SBRT and between 95.2% and 98.9% for BgRT plan delivering. For S3, both SBRT and BgRT motion deliveries met CTV dose coverage requirements, with BgRT plans delivering a very high dose to the target. The CTV dose ranges were (a) SBRT:100.4%-120.4%, and (b) BgRT: 121.3%-139.9%.

CONCLUSIONS

This phantom-based study demonstrated that PET signals from PET-avid tumors can be utilized to direct real-time dose delivery to the tumor accurately, which is comparable to the dosimetric accuracy of SBRT. Furthermore, BgRT delivered a PET-signal controlled dose to the moving target, equivalent to the dose distribution to the static target. A future study will compare the performance of BgRT with conventional image-guided radiotherapy.

Quantitative Bioluminescence Tomography-guided Conformal Irradiation for Preclinical Radiation Research

PURPOSE

Widely used cone beam computed tomography (CBCT)-guided irradiators in preclinical radiation research are limited to localize soft tissue target because of low imaging contrast. Knowledge of target volume is a fundamental need for radiation therapy (RT). Without such information to guide radiation, normal tissue can be overirradiated, introducing experimental uncertainties. This led us to develop high-contrast quantitative bioluminescence tomography (QBLT) for guidance. The use of a 3-dimensional bioluminescence signal, related to cell viability, for preclinical radiation research is one step toward biology-guided RT.

METHODS AND MATERIALS

Our QBLT system enables multiprojection and multispectral bioluminescence imaging to maximize input data for the tomographic reconstruction. Accurate quantification of spectrum and dynamic change of in vivo signal were also accounted for the QBLT. A spectral-derivative method was implemented to eliminate the modeling of the light propagation from animal surface to detector. We demonstrated the QBLT capability of guiding conformal RT using a bioluminescent glioblastoma (GBM) model in vivo. A threshold was determined to delineate QBLT reconstructed gross target volume (GTV_QBLT), which provides the best overlap between the GTV_QBLT and CBCT contrast labeled GBM (GTV), used as the ground truth for GBM volume. To account for the uncertainty of GTV_QBLT in target positioning and volume delineation, a margin was determined and added to the GTV_QBLT to form a QBLT planning target volume (PTV_QBLT) for guidance.

RESULTS

The QBLT can reconstruct in vivo GBM with localization accuracy within 1 mm. A 0.5-mm margin was determined and added to GTV_QBLT to form PTV_QBLT, largely improving tumor coverage from 75.0% (0 mm margin) to 97.9% in average, while minimizing normal tissue toxicity. With the goal of prescribed dose 5 Gy covering 95% of PTV_QBLT, QBLT-guided 7-field conformal RT can effectively irradiate 99.4 ± 1.0% of GTV.

CONCLUSIONS

The QBLT provides a unique opportunity for investigators to use biologic information for target delineation, guiding conformal irradiation, and reducing normal tissue involvement, which is expected to increase reproducibility of scientific discovery.

Uptake of [18F]EF5 as a Tracer for Hypoxic and Aggressive Phenotype in Experimental Head and Neck Squamous Cell Carcinoma

PURPOSE

This study aims to investigate whether the uptake of 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide ([18F]EF5) and 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) is associated with a hypoxia-driven adverse phenotype in head and neck squamous cell carcinoma cell lines and tumor xenografts.

METHODS

Xenografts were imaged in vivo, and tumor sections were stained for hypoxia-inducible factor 1α (Hif-1α), carbonic anhydrase IX (CA IX), and glucose transporter 1 (Glut-1). Tracer uptakes and the expression of Hif-1α were determined in cell lines under 1% hypoxia.

RESULTS

High [18F]EF5 uptake was seen in xenografts expressing high levels of CA IX, Glut-1, and Hif-1α, whereas low [18F]EF5 uptake was detected in xenografts expressing low amounts of CA IX and Hif-1α. The uptake of [18F]EF5 between cell lines varied extensively under normoxic conditions. A clear correlation was found between the expression of Hif-1α and the uptake of [18F]FDG during hypoxia.

CONCLUSIONS

The UT-SCC cell lines studied differed with respect to their hypoxic phenotypes, and these variations were detectable with [18F]EF5. Acute hypoxia increases [18F]FDG uptake in vitro, whereas a high [18F]EF5 uptake reflects a more complex phenotype associated with hypoxia and an aggressive growth pattern.

Prognostic significance of 18-FDG PET/CT and EUS-defined tumour characteristics in patients with oesophageal cancer

AIM

To determine the correlation between 2-[(18)F]-fluoro-2-deoxy-d-glucose (FDG) positron-emission tomography/computed tomography (PET/CT) defined maximum standardized uptake value (SUVmax) and endoluminal ultrasound-defined tumour volume (EDTV) in patients with oesophageal cancer (OC) and their relative prognostic significance.

MATERIALS AND METHODS

One hundred and eighty-five consecutive patients with OC were staged using CT, endoscopic ultrasound (EUS), and PET/CT. The maximum potential EDTV was calculated (πr(2)L, where r = tumour thickness and L = total length of disease including proximal and distal lymph node metastases). Primary outcome measure was survival from diagnosis.

RESULTS

Ninety-one percent of patients (168/185) had FDG-avid tumours on PET/CT. SUVmax correlated positively and significantly with EDTV (Spearman's rho = 0.339, p = 0.001). On univariate analysis, survival was inversely related to the PET/CT lymph node metastasis count (LNMC, p = 0.015), EUS N stage (p = 0.002), EDTV (<48 cm(3), p = 0.001), EUS total length of disease (p = 0.001), SUVmax (p = 0.002), PET/CT N stage (p < 0.0001), and EUS LNMC (p < 0.0001). On multivariate analysis two factors were significantly and independently associated with survival: EDTV (HR, 3.118; 95% CI: 1.357-7.167; p = 0.007), and PET/CT N stage (HR, 0.496; 95% CI: 0.084-1.577; p = 0.022).

CONCLUSION

EDTV and PET/CT N stage were important predictors of survival and further research is needed to identify critical prognostic values.

18F-FDG-PET-based tumor delineation in cervical cancer: threshold contouring and lesion volumes

OBJECTIVE

To evaluate a semi-automated PET-image tumor segmentation algorithm for gross tumor volume (GTV) delineation in patients with locally advanced cervical cancer.

MATERIAL AND METHODS

Thirty-two patients with locally advanced cervical cancer were retrospectively evaluated. Semi-automated PET-image-based GTV delineation was applied using a previous established algorithm (GTV2SD) and 2 fixed threshold-based methods (GTV40% and GTV50%). GTV2SD was determined as the pixel with the mean value plus 2-standard deviation of the liver intensity, and GTV40% and GTV50% with 40% and 50% of the maximum tumor intensity (Tmax), respectively. The derived volumes were then compared with the GTVs generated manually using MR (GTVMR).

RESULTS

The mean value of GTV2SD, GTV40% and GTV50% was 85.3cc, 16.2cc and 24.1cc, respectively. Good agreement was noticed between GTV2SD and GTVMR (ρ=0.88). GTV40% and GTV50% showed weaker correlation with GTVMR (ρ=0.68 and ρ=0.71, respectively).

CONCLUSIONS

This study provides preliminary evidence that metabolic tumor volume delineation is feasible using computer-generated measurements in (18)F-FDG PET images. Generation of PET-based tumor volumes is affected by the choice of threshold level used. Metabolic tumor bulk calculated using the pixel with the mean value plus 2-standard deviations of the liver intensity (GTV2SD) correlates better with the MR-derived tumor volumes. The method is a simple and clinically applicable approach to generate PET-derived GTV for radiation therapy planning of cervical cancer.

Strategies to optimize radiotherapy based on biological responses of tumor and normal tissue

Rapid developments in radiation oncology are currently taking place. Radiation-induced responses are being increasingly used for radiotherapy modification based on advancements in radiobiology. In the process of radiation treatment, radiobiological responses of tumor and normal tissue in patients are monitored non-invasively by a variety of techniques including imaging, biological methods and biochemical assays. Information collected using these methods and data on responses are further incorporated into radiotherapy optimization approaches, which not only include the optimization of radiation treatment planning, such as dose distributions in targets and treatment delivery, but also include radiation sensitivity modification and gene radiotherapy of the tumor and normal tissue. Hence, the highest tumor control rate is obtained with the utmost protection being afforded to normal tissue under this treatment modality.

Functional and molecular image guidance in radiotherapy treatment planning optimization

Functional and molecular imaging techniques are increasingly being developed and used to quantitatively map the spatial distribution of parameters, such as metabolism, proliferation, hypoxia, perfusion, and ventilation, onto anatomically imaged normal organs and tumor. In radiotherapy optimization, these imaging modalities offer the promise of increased dose sparing to high-functioning subregions of normal organs or dose escalation to selected subregions of the tumor as well as the potential to adapt radiotherapy to functional changes that occur during the course of treatment. The practical use of functional/molecular imaging in radiotherapy optimization must take into cautious consideration several factors whose influences are still not clearly quantified or well understood including patient positioning differences between the planning computed tomography and functional/molecular imaging sessions, image reconstruction parameters and techniques, image registration, target/normal organ functional segmentation, the relationship governing the dose escalation/sparing warranted by the functional/molecular image intensity map, and radiotherapy-induced changes in the image intensity map over the course of treatment. The clinical benefit of functional/molecular image guidance in the form of improved local control or decreased normal organ toxicity has yet to be shown and awaits prospective clinical trials addressing this issue.

Positron emission tomography imaging approaches for external beam radiation therapies: current status and future developments

In an era in which it is possible to deliver radiation with high precision, there is a heightened need for enhanced imaging capabilities to improve tumour localisation for diagnostic, planning and delivery purposes. This is necessary to increase the accuracy and overall efficacy of all types of external beam radiotherapy (RT), including particle therapies. Positron emission tomography (PET) has the potential to fulfil this need by imaging fundamental aspects of tumour biology. The key areas in which PET may support the RT process include improving disease diagnosis and staging; assisting tumour volume delineation; defining tumour phenotype or biological tumour volume; assessment of treatment response; and in-beam monitoring of radiation dosimetry. The role of PET and its current developmental status in these key areas are overviewed in this review, highlighting the advantages and drawbacks.

Prognostic significance of endoluminal ultrasound-defined disease length and tumor volume (EDTV) for patients with the diagnosis of esophageal cancer

BACKGROUND

This study aimed to assess the prognostic significance of endoluminal ultrasound-defined total length of disease and endoluminal ultrasound defined tumor volume (EDTV) in esophageal cancer. The hypothesis was that endoscopic ultrasound (EUS)-defined total length of disease and EDTV are both significant prognostic indicators and better predictors of outcome than endoscopic tumor length.

METHODS

In this study, 174 consecutive patients (median age, 64 years and 128 months) underwent specialist EUS, and the maximum potential EDTV was calculated (pir(2) L, where r is the tumor thickness and L is the total length of disease) including proximal and distal lymph node metastases. Of the 174 patients, 104 underwent surgery (70 had neoadjuvant chemotherapy), 60 underwent definitive chemoradiotherapy, and 10 had palliative therapy.

RESULTS

Survival was related to EUS T stage (p = 0.013), EUS N stage (p = 0.001), EUS M1a stage (p = 0.004), EUS disease length (<8 cm; p = 0.001), and EDTV (all patients <25 cm(3), p = 0.001; surgical patients <40 cm(3), p = 0.036). Forward conditional multivariate analysis showed three factors to be associated with survival: EUS N stage (hazard ratio [HR], 1.646; 95% confidence interval [CI], 1.041-2.602; p = 0.033), EUS M1a stage (HR, 2.702; 95% CI, 1.069-6.830; p = 0.036), and EDTV (HR, 2.702; 95% CI, 1.069-6.830; p = 0.025). Median and 2-year survival for EDTV <25 cm(3) versus >25 cm(3) was 43.4 months and 56%, respectively, compared with 23.5 months and 35%.

CONCLUSIONS

In this study, EDTV based on total EUS-defined length of disease emerged as a new and important prognostic indicator for patients with esophageal cancer.

Utilization of advanced imaging technologies for target delineation in radiation oncology

PURPOSE

The aim of this study was to evaluate the utilization of advanced imaging technologies for target delineation among radiation oncologists in the United States.

METHODS

A random sample of 1,600 radiation oncologists was contacted by Internet, e-mail, and fax and questioned regarding the use of advanced imaging technologies, clinical applications, and future plans for use. Advanced imaging technologies were defined as any of the following that were directly incorporated into radiation therapy planning: MRI, PET, single-photon emission CT, 4-D CT, functional MRI, and MR spectroscopy.

RESULTS

Of 1,089 contactable physicians, 394 (36%) responded. Of respondents, 65% were in private practice and 35% were in academic practice. The proportion using any advanced imaging technology for target delineation was 95%. However, the majority reported only rare (in <25% of their patients; 46.6%) or infrequent (in 25%-50% of their patients; 26.0%) utilization. The most commonly used technologies were 2-[(18)F]fluoro-2-deoxyglucose PET (76%), MRI (72%), and 4-D CT (44%). The most common cancers treated using image-guided target delineation were those of the lung (83%), central nervous system (79%), and head and neck (79%). Among users of advanced imaging technologies, 66% planned to increase use; 30% of nonusers planned to adopt these technologies in the future.

CONCLUSIONS

Advanced imaging technologies are widely used by US radiation oncologists for target delineation. Although the majority of respondents used them in <50% of their patients, the frequency of utilization is expected to increase. Studies determining the optimal application of these technologies in radiation therapy planning are needed.