Biology-guided radiotherapy: redefining the role of radiotherapy in metastatic cancer

PET/SPECT/Spectral-CT/CBCT imaging in a small-animal radiation therapy platform: A Monte Carlo study-Part II: Biologically guided radiotherapy

BACKGROUND

This study addresses the technical gap between clinical radiation therapy (RT) and preclinical small-animal RT, hindering the comprehensive validation of innovative clinical RT approaches in small-animal models of cancer and the translation of preclinical RT studies into clinical practices.

PURPOSE

The main aim was to explore the feasibility of biologically guided RT implemented within a small-animal radiation therapy (SART) platform, with integrated quad-modal on-board positron emission tomography (PET), single-photon emission computed tomography, photon-counting spectral CT, and cone-beam CT (CBCT) imaging, in a Monte Carlo model as a proof-of-concept.

METHODS

We developed a SART workflow employing quad-modal imaging guidance, integrating multimodal image-guided RT and emission-guided RT (EGRT). The EGRT algorithm was outlined using positron signals from a PET radiotracer, enabling near real-time adjustments to radiation treatment beams for precise targeting in the presence of a 2-mm setup error. Molecular image-guided RT, incorporating a dose escalation/de-escalation scheme, was demonstrated using a simulated phantom with a dose painting plan. The plan involved delivering a low dose to the CBCT-delineated planning target volume (PTV) and a high dose boosted to the highly active biological target volume (hBTV) identified by the 18F-PET image. Additionally, the Bayesian eigentissue decomposition method illustrated the quantitative decomposition of radiotherapy-related parameters, specifically iodine uptake fraction and virtual noncontrast (VNC) electron density, using a simulated phantom with Kidney1 and Liver2 inserts mixed with an iodine contrast agent at electron fractions of 0.01-0.02.

RESULTS

EGRT simulations generated over 4,000 beamlet responses in dose slice deliveries and illustrated superior dose coverage and distribution with significantly lower doses delivered to normal tissues, even with a 2-mm setup error introduced, demonstrating the robustness of the novel EGRT scheme compared to conventional image-guided RT. In the dose-painting plan, doubling the dose to the hBTV while maintaining a low dose for the PTV resulted in an organ-at-risk (OAR) dose comparable to the low-dose treatment for the PTV alone. Furthermore, the decomposition of radiotherapy-related parameters in Kidney1 and Liver2 inserts, including iodine uptake fractions and VNC electron densities, exhibited average relative errors of less than 1.0% and 2.5%, respectively.

CONCLUSIONS

The results demonstrated the successful implementation of biologically guided RT within the proposed quad-model image-guided SART platform, with potential applications in preclinical RT and adaptive RT studies.

Leveraging Programmatic Collaboration for a Radiopharmaceutical Clinic

Radiation oncologists, radiopharmacists, nuclear medicine physicians, and medical oncologists have seen a renewed clinical interest in radiopharmaceuticals for the curative or the palliative treatment of cancer. To allow for the discovery and the clinical advancement of targeted radiopharmaceuticals, these stakeholders have reformed their trial efforts and remodeled their facilities to accommodate the obligations of a program centered upon radioactive investigational drug products. Now considered informally as drugs and not beam radiotherapy, radiopharmaceuticals can be more easily studied in the traditional clinical trial enterprise ranging from phase 0-I to phase III studies. Resources and physical facilities allocated to radiopharmaceuticals have brought forth new logistics and patient experience for safe and satisfactory drug delivery. The clinical use of theranostic agents-that is, diagnostic and therapeutic radionuclide pairs-has accelerated radiopharmaceutical development.

Clinical Positron Emission Tomography/Computed Tomography: Quarter-Century Transformation of Prostate Cancer Molecular Imaging

Although positron emission tomography/computed tomography (PET/CT) underwent rapid growth during the last quarter-century, becoming a new standard-of-care for imaging most cancer types, CT and bone scan remained the gold standard for patients with prostate cancer. This occurred as 2-fluorine-18-fluoro-2-deoxy-d-glucose was perceived to have a limited role owing to low sensitivity in many patients. A resurgence of interest occurred with the use of fluorine-18-sodium-fluoride PET/CT as a replacement for bone scintigraphy, and then choline, fluciclovine, and dihydrotestosterone (DHT) PET/CT as prostate "specific" radiotracers. The last decade, however, has seen a true revolution with the meteoric rise of prostate-specific membrane antigen PET/CT.

BIOGUIDE-X: A First-in-Human Study of the Performance of Positron Emission Tomography-Guided Radiation Therapy

PURPOSE

Positron emission tomography (PET)-guided radiation therapy is a novel tracked dose delivery modality that uses real-time PET to guide radiation therapy beamlets. The BIOGUIDE-X study was performed with sequential cohorts of participants to (1) identify the fluorodeoxyglucose (FDG) dose for PET-guided therapy and (2) confirm that the emulated dose distribution was consistent with a physician-approved radiation therapy plan.

METHODS AND MATERIALS

This prospective study included participants with at least 1 FDG-avid targetable primary or metastatic tumor (2-5 cm) in the lung or bone. For cohort I, a modified 3 + 3 design was used to determine the FDG dose that would result in adequate signal for PET-guided therapy. For cohort II, PET imaging data were collected on the X1 system before the first and last fractions among patients undergoing conventional stereotactic body radiation therapy. PET-guided therapy dose distributions were modeled on the patient's computed tomography anatomy using the collected PET data at each fraction as input to an "emulated delivery" and compared with the physician-approved plan.

RESULTS

Cohort I demonstrated adequate FDG activity in 6 of 6 evaluable participants (100.0%) with the first injected dose level of 15 mCi FDG. In cohort II, 4 patients with lung tumors and 5 with bone tumors were enrolled, and evaluable emulated delivery data points were collected for 17 treatment fractions. Sixteen of the 17 emulated deliveries resulted in dose distributions that were accurate with respect to the approved PET-guided therapy plan. The 17th data point was just below the 95% threshold for accuracy (dose-volume histogram score = 94.6%). All emulated fluences were physically deliverable. No toxicities were attributed to multiple FDG administrations.

CONCLUSIONS

PET-guided therapy is a novel radiation therapy modality in which a radiolabeled tumor can act as its own fiducial for radiation therapy targeting. Emulated therapy dose distributions calculated from continuously acquired real-time PET data were accurate and machine-deliverable in tumors that were 2 to 5 cm in size with adequate FDG signal characteristics.

Hypoxia-informed RBE-weighted beam orientation optimization for intensity modulated proton therapy

BACKGROUND

Variable relative biological effectiveness (RBE) models in treatment planning have been proposed to optimize the therapeutic ratio of proton therapy. It has been reported that proton RBE decreases with increasing tumor oxygen level, offering an opportunity to address hypoxia-related radioresistance with RBE-weighted optimization.

PURPOSE

Here, we obtain a voxel-level estimation of partial oxygen pressure to weigh RBE values in a single biologically informed beam orientation optimization (BOO) algorithm.

METHODS

Three glioblastoma patients with [18 F]-fluoromisonidazole (FMISO)-PET/CT images were selected from the institutional database. Oxygen values were derived from tracer uptake using a nonlinear least squares curve fitting. McNamara RBE, calculated from proton dose, was then weighed using oxygen enhancement ratios (OER) for each voxel and incorporated into the dose fidelity term of the BOO algorithm. The nonlinear optimization problem was solved using a split-Bregman approach, with FISTA as the solver. The proposed hypoxia informed RBE-weighted method (HypRBE) was compared to dose fidelity terms using the constant RBE of 1.1 (cRBE) and the normoxic McNamara RBE model (RegRBE). Tumor homogeneity index (HI), maximum biological dose (Dmax), and D95%, as well as OAR therapeutic index (TI = gEUDCTV /gEUDOAR ) were evaluated along with worst-case statistics after normalization to normal tissue isotoxicity.

RESULTS

Compared to [cRBE, RegRBE], HypRBE increased tumor HI, Dmax, and D95% across all plans by on average [31.3%, 31.8%], [48.6%, 27.1%], and [50.4%, 23.8%], respectively. In the worst-case scenario, the parameters increase on average by [12.5%, 14.7%], [7.3%,-8.9%], and [22.3%, 2.1%]. Despite increased OAR Dmean and Dmax by [8.0%, 3.0%] and [13.1%, -0.1%], HypRBE increased average TI by [22.0%, 21.1%]. Worst-case OAR Dmean, Dmax, and TI worsened by [17.9%, 4.3%], [24.5%, -1.2%], and [9.6%, 10.5%], but in the best cases, HypRBE escalates tumor coverage significantly without compromising OAR dose, increasing the therapeutic ratio.

CONCLUSIONS

We have developed an optimization algorithm whose dose fidelity term accounts for hypoxia-informed RBE values. We have shown that HypRBE selects bE:\Alok\aaeams better suited to deliver high physical dose to low RBE, hypoxic tumor regions while sparing the radiosensitive normal tissue.

A time- and space-saving Monte Carlo simulation method using post-collimation generative adversarial network for dose calculation of an O-ring gantry Linac

PURPOSE

This study explores the feasibility of employing Generative Adversarial Networks (GANs) to model the RefleXion X1 Linac. The aim is to investigate the accuracy of dose simulation and assess the potential computational benefits.

METHODS

The X1 Linac is a new radiotherapy machine with a binary multi-leaf collimation (MLC) system, facilitating innovative biology-guided radiotherapy. A total of 34 GAN generators, each representing a desired MLC aperture, were developed. Each generator was trained using a phase space file generated underneath the corresponding aperture, enabling the generation of particles and serving as a beam source for Monte Carlo simulation. Dose distributions in water were simulated for each aperture using both the GAN and phase space sources. The agreement between dose distributions was evaluated. The computational time reduction from bypassing the collimation simulation and storage space savings were estimated.

RESULTS

The percentage depth dose at 10 cm, penumbra, and full-width half maximum of the GAN simulation agree with the phase space simulation, with differences of 0.4 % ± 0.2 %, 0.32 ± 0.66 mm, and 0.26 ± 0.44 mm, respectively. The gamma passing rate (1 %/1mm) for the planar dose exceeded 90 % for all apertures. The estimated time-saving for simulating an plan using 5766 beamlets was 530 CPU hours. The storage usage was reduced by a factor of 102.

CONCLUSION

The utilization of the GAN in simulating the X1 Linac demonstrated remarkable accuracy and efficiency. The reductions in both computational time and storage requirements make this approach highly valuable for future dosimetry studies and beam modeling.

First-Year Experience of Stereotactic Body Radiation Therapy/Intensity Modulated Radiation Therapy Treatment Using a Novel Biology-Guided Radiation Therapy Machine

Purpose

The aim of this study was to present the first-year experience of treating patients using intensity modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) with a biology-guided radiation therapy machine, the RefleXion X1 system, installed in a clinical setting.

Methods and Materials

A total of 78 patients were treated on the X1 system using IMRT and SBRT from May 2021 to May 2022. Clinical and technical data including treatment sites, number of pretreatment kilovoltage computed tomography (kVCT) scans, beam-on time, patient setup time, and imaging time were collected and analyzed. Machine quality assurance (QA) results, machine performance, and user satisfactory survey were also collected and reported.

Results

The most commonly treated site was the head and neck (63%), followed by the pelvis (23%), abdomen (8%), and thorax (6%). Except for 5 patients (6%) who received SBRT treatments for bony metastases in the pelvis, all treatments were conventionally fractionated IMRT. The number of kVCT scans per fraction was 1.2 ± 0.5 (mean ± standard deviation). The beam-on time was 9.2 ± 3.5 minutes. The patient setup time and imaging time per kVCT was 4.8 ± 2.6 minutes and 4.6 ± 1.5 minutes, respectively. The daily machine output deviation was 0.4 ± 1.2% from the baseline. The patient QA had a passing rate of 97.4 ± 2.8% at 3%/2 mm gamma criteria. The machine uptime was 92% of the total treatment time. The daily QA and kVCT image quality received the highest level of satisfaction. The treatment workflow for therapists received the lowest level of satisfaction.

Conclusions

One year after the installation, 78 patients were successfully treated with the X1 system using IMRT and/or SBRT. With the recent Food and Drug Administration clearance of biology-guided radiation therapy, our department is preparing to treat patients using positron emission tomography-guidance via a new product release, which will address deficiencies in the current image-guided radiation therapy workflow.

Aggressive Local Ablative Radiotherapy Mitigates Progression Risk in Oligometastatic Lung Adenocarcinoma

PURPOSE

This study aimed to determine the role of local ablative radiotherapy (LART) in oligometastatic/oligoprogressive lung adenocarcinoma.

MATERIALS AND METHODS

Patients (n=176) with oligometastatic lung adenocarcinoma treated with LART were identified, and those treated with LART at the initial diagnosis of synchronous oligometastatic disease (OMD group) or treated with LART when they presented with repeat oligoprogression (OPD group) were included.

RESULTS

In the OMD group (n=54), the 1- and 3-year progression-free survival (PFS) were 50.9% and 22.5%, respectively, whereas the 1- and 3-year overall survival in the OPD group were 75.9% and 58.1%, respectively. Forty-one patients (75.9%) received LART at all gross disease sites. Tyrosine kinase inhibitor (TKI) use and all-metastatic site LART were significant predictors of higher PFS (p=0.018 and p=0.046, respectively). In patients treated with TKIs at the time of LART (n=23) and those treated with all-metastatic site LART, the 1-year PFS was 86.7%, while that of patients not treated with all-metastatic site LART was 37.5% (p=0.006). In the OPD group (n=122), 67.2% of the patients (n=82) maintained a systemic therapy regimen after LART. The cumulative incidence of changing systemic therapy was 39.6%, 62.9%, and 78.5% at 6 months, 1 year, and 2 years after LART, respectively.

CONCLUSION

Aggressive LART can be an option to improve survival in patients with oligometastatic disease. Patients with synchronous oligometastatic disease receiving TKI and all-metastatic site LART may have improved PFS. In patients with repeat oligoprogression, LART might potentially extend survival by delaying the need to change the systemic treatment regimen.

Camouflaged Nanoreactors Mediated Radiotherapy-Adjuvant Chemodynamic Synergistic Therapy

Chemodynamic therapy based on the Fenton-like catalysis ability of Fe3O4 has the advantages of no involvement of chemical drugs and minimal adverse effects as well as the limitation of depletable efficacy. Radiotherapy based on high-energy radiation offers the convenience of treatment and cost-effectiveness but lacks precision and cellular adaptation of tumor cells. Approaching such dilemmas from a nanoscale materials perspective, we aim to bridge the weaknesses of both treatment methods by combining the principles of two therapeutics reciprocally. We have designed a camouflaged Fe3O4@HfO2 composite nanoreactor (FHCM), which combines a chemodynamic therapeutic agent Fe3O4 and a radiosensitizer HfO2 that both has passed clinical trials and was inspired by a cell membrane biomimetic technique. FHCM is employed as conceived radiotherapy-adjuvant chemodynamic synergistic therapy of malignant tumors, which has undergone dual scrutiny from both the physical and biological aspects. Experimental results obtained at different levels, including theory, material characterizations, and in vitro and in vivo verifications, suggest that FHCM effectively impaired tumor cells through physical and molecular biological mechanisms involving a HfO2-Fe3O4 photoelectron-electron transfer chain and DNA damage-ferroptosis-immunity chain. It is worth noting that compared to single therapies such as only chemodynamic therapy or radiotherapy, FHCM-mediated radiotherapy-adjuvant chemodynamic synergistic therapy exhibits stronger tumor inhibition efficacy. It significantly addresses the inherent limitations of chemodynamic therapy and radiotherapy and underscores the feasibility and importance of using existing clinical weapons, such as radiotherapy, as auxiliary strategies to overcome certain flaws of emerging antitumor therapeutics like chemodynamic therapy.

Effect of radiotherapy on the DNA cargo and cellular uptake mechanisms of extracellular vesicles

In the past decades, plenty of evidence has gathered pointing to the role of extracellular vesicles (EVs) secreted by irradiated cells in the development of radiation-induced non-targeted effects. EVs are complex natural structures composed of a phospholipid bilayer which are secreted by virtually all cells and carry bioactive molecules. They can travel certain distances in the body before being taken up by recipient cells. In this review we discuss the role and fate of EVs in tumor cells and highlight the importance of DNA specimens in EVs cargo in the context of radiotherapy. The effect of EVs depends on their cargo, which reflects physiological and pathological conditions of donor cell types, but also depends on the mode of EV uptake and mechanisms involved in the route of EV internalization. While the secretion and cargo of EVs from irradiated cells has been extensively studied in recent years, their uptake is much less understood. In this review, we will focus on recent knowledge regarding the EV uptake of cancer cells and the effect of radiation in this process.

Patient-Specific Auto-segmentation on Daily kVCT Images for Adaptive Radiation Therapy

PURPOSE

This study explored deep-learning-based patient-specific auto-segmentation using transfer learning on daily RefleXion kilovoltage computed tomography (kVCT) images to facilitate adaptive radiation therapy, based on data from the first group of patients treated with the innovative RefleXion system.

METHODS AND MATERIALS

For head and neck (HaN) and pelvic cancers, a deep convolutional segmentation network was initially trained on a population data set that contained 67 and 56 patient cases, respectively. Then the pretrained population network was adapted to the specific RefleXion patient by fine-tuning the network weights with a transfer learning method. For each of the 6 collected RefleXion HaN cases and 4 pelvic cases, initial planning computed tomography (CT) scans and 5 to 26 sets of daily kVCT images were used for the patient-specific learning and evaluation separately. The performance of the patient-specific network was compared with the population network and the clinical rigid registration method and evaluated by the Dice similarity coefficient (DSC) with manual contours being the reference. The corresponding dosimetric effects resulting from different auto-segmentation and registration methods were also investigated.

RESULTS

The proposed patient-specific network achieved mean DSC results of 0.88 for 3 HaN organs at risk (OARs) of interest and 0.90 for 8 pelvic target and OARs, outperforming the population network (0.70 and 0.63) and the registration method (0.72 and 0.72). The DSC of the patient-specific network gradually increased with the increment of longitudinal training cases and approached saturation with more than 6 training cases. Compared with using the registration contour, the target and OAR mean doses and dose-volume histograms obtained using the patient-specific auto-segmentation were closer to the results using the manual contour.

CONCLUSIONS

Auto-segmentation of RefleXion kVCT images based on the patient-specific transfer learning could achieve higher accuracy, outperforming a common population network and clinical registration-based method. This approach shows promise in improving dose evaluation accuracy in RefleXion adaptive radiation therapy.

Harnessing progress in radiotherapy for global cancer control

The pace of technological innovation over the past three decades has transformed the field of radiotherapy into one of the most technologically intense disciplines in medicine. However, the global barriers to access this highly effective treatment are complex and extend beyond technological limitations. Here, we review the technological advancement and current status of radiotherapy and discuss the efforts of the global radiation oncology community to formulate a more integrative 'diagonal approach' in which the agendas of science-driven advances in individual outcomes and the sociotechnological task of global cancer control can be aligned to bring the benefit of this proven therapy to patients with cancer everywhere.

Radiation Therapy Changed the Second Malignancy Pattern in Rectal Cancer Survivors

Background and Objectives: Radiotherapy (RT) plays an important role in the treatment for locally advanced rectal cancer patients. It can bring radio exposure together with the survival benefit. Cancer survivors are generally at an increased risk for second malignancies, and survivors receiving RT may have higher risks than survivors not receiving RT. Whether the risk of an all-site second malignancy may increase after RT is still debated. This study aims to compare the second malignancy pattern in rectal cancer survivors after RT. Materials and Methods: The Surveillance, Epidemiology, and End Results (SEER) database was used for analysis. In total, 49,961 rectal cancer patients (20-84 years of age) were identified between 2000 and 2012 from 18 SEER registries. All patients underwent surgery. The occurrence of second malignancies diagnosed after rectal cancer diagnosis was compared in patients who received and did not receive RT. The standardized incidence ratios (SIRs) with 95% confidence intervals (CIs) were used. SEER*Stat was used to generate the 95% CIs for the SIR statistics using the exact method. Results: Of the total 49,961 patients, 5582 developed second malignancies. For all-site second primary malignancies, the age-adjusted SIRs were 1.14 (95% CI 1.1-1.18) and 1.00 (95% CI 0.96-1.04) in the no RT and RT groups, respectively. In 23,192 patients from the surgery-only group, 2604 had second malignancies, and in 26,769 patients who received RT, 2978 developed second malignancies. With respect to every site, the risk of secondary prostate cancer was significantly lower in the RT group (SIR = 0.39, 95% CI 0.33-0.46) than that in the surgery-only group (SIR = 1.04, 95% CI 0.96-1.12). Moreover, the risk of thyroid cancer was significantly higher in the RT group (SIR = 2.80, 95% CI 2.2-3.51) than that in the surgery-only group (SIR = 1.29, 95% CI 0.99-1.66). Conclusions: RT may change the second malignancy pattern in rectal cancer survivors; the risk of prostate cancer decreased, and the risk of thyroid cancer increased most significantly.

Feasibility of biology-guided radiotherapy for metastatic renal cell carcinoma driven by PSMA PET imaging

Background

Biology-guided radiotherapy (BgRT) is a novel treatment where the detection of positron emission originating from a volume called the biological tracking zone (BTZ) initiates dose delivery. Prostate-specific membrane antigen (PSMA) positron emission tomography (PET) is a novel imaging technique that may improve patient selection for metastasis-directed therapy in renal cell carcinoma (RCC). This study aims to determine the feasibility of BgRT treatment for RCC.

Material and methods

All consecutive patients that underwent PSMA PET/CT scan for RCC staging at our institution between 2014 and 2020 were retrospectively considered for inclusion. GTVs were contoured on the CT component of the PET/CT scan. The tumor-to-background ratio was quantified from the normalized standardized uptake value (nSUV), defined as the ratio between SUVmax inside the GTV and SUVmean inside the margin expansion. Tumors were classified suitable for BgRT if (1) nSUV was greater or equal to an nSUV threshold and (2) if the BTZ was free of any PET-avid region other than the tumor.

Results

Out of this cohort of 83 patients, 47 had metastatic RCC and were included in this study. In total, 136 tumors were delineated, 1 to 22 tumors per patient, mostly in lung (40%). Using a margin expansion of 5 mm/10 mm/20 mm and nSUV threshold = 3, 66%/63%/41% of tumors were suitable for BgRT treatment. Uptake originating from another tumor, the kidney, or the liver was typically inside the BTZ in tumors judged unsuitable for BgRT.

Conclusions

More than 60% of tumors were found to be suitable for BgRT in this cohort of patients with RCC. However, the proximity of PET-avid organs such as the liver or the kidney may affect BgRT delivery.

Combined biology-guided radiotherapy and Lutetium PSMA theranostics treatment in metastatic castrate-resistant prostate cancer

Background

Lutetium-177 [177Lu]-PSMA-617 is a targeted radioligand that binds to prostate-specific membrane antigen (PSMA) and delivers radiation to metastatic prostate cancer. The presence of PSMA-negative/FDG-positive metastases can preclude patients from being eligible for this treatment. Biology-guided radiotherapy (BgRT) is a treatment modality that utilises tumour PET emissions to guide external beam radiotherapy. The feasibility of combining BgRT and Lutetium-177 [177Lu]-PSMA-617 for patients with PSMA-negative/FDG-positive metastatic prostate cancer was explored.

Materials and methods

All patients excluded from the LuPSMA clinical trial (ID: ANZCTR12615000912583) due to PSMA/FDG discordance were retrospectively reviewed. A hypothetical workflow where PSMA-negative/FDG-positive metastases would be treated with BgRT whilst PSMA-positive metastases would be treated with Lutetium-177 [177Lu]-PSMA-617 was considered. Gross tumour volume (GTV) of PSMA-negative/FDG-positive tumours were delineated on the CT component of the FDG PET/CT scan. Tumours were deemed suitable for BgRT if (1) normalised SUV (nSUV), defined as the ratio of maximum SUV (SUVmax) inside the GTV to mean SUV inside a 5 mm/10 mm/20 mm margin expansion of the GTV, was larger than a pre-specified nSUV threshold and (2) there was no PET avidity inside the margin expansion.

Results

In 75 patients screened for Lutetium-177 [177Lu]-PSMA-617 treatment, 6 patients were excluded due to PSMA/FDG discordance and 89 PSMA-negative/FDG-positive targets were identified. GTV volumes ranged from 0.3 cm3 to 186 cm3 (median GTV volume = 4.3 cm3, IQR = 2.2 cm3 – 7.4 cm3). SUVmax inside GTVs ranged between 3 and 12 (median SUVmax = 4.8, IQR = 3.9 – 6.2). With nSUV ≥ 3, 67%/54%/39% of all GTVs were suitable for BgRT within 5 mm/10 mm/20 mm from the tumour. Bone and lung metastases were the best candidates for BgRT (40%/27% of all tumours suitable for BgRT with nSUV ≥ 3 within 5 mm from the GTV were bone/lung GTVs).

Conclusions

Combined BgRT/Lutetium-177 [177Lu]-PSMA-617 therapy is feasible for patients with PSMA/FDG discordant metastases.

Tomographic detection of photon pairs produced from high-energy X-rays for the monitoring of radiotherapy dosing

Measuring the radiation dose reaching a patient's body is difficult. Here we report a technique for the tomographic reconstruction of the location of photon pairs originating from the annihilation of positron-electron pairs produced by high-energy X-rays travelling through tissue. We used Monte Carlo simulations on pre-recorded data from tissue-mimicking phantoms and from a patient with a brain tumour to show the feasibility of this imaging modality, which we named 'pair-production tomography', for the monitoring of radiotherapy dosing. We simulated three image-reconstruction methods, one applicable to a pencil X-ray beam scanning through a region of interest, and two applicable to the excitation of tissue volumes via broad beams (with temporal resolution sufficient to identify coincident photon pairs via filtered back projection, or with higher temporal resolution sufficient for the estimation of a photon's time-of-flight). In addition to the monitoring of radiotherapy dosing, we show that image contrast resulting from pair-production tomography is highly proportional to the material's atomic number. The technique may thus also allow for element mapping and for soft-tissue differentiation.

Preclinical Evaluation of 89Zr-Panitumumab for Biology-Guided Radiation Therapy

PURPOSE

Biology-guided radiation therapy (BgRT) uses real-time line-of-response data from on-board positron emission tomography (PET) detectors to guide beamlet delivery during therapeutic radiation. The current workflow requires ¹⁸F-fluorodeoxyglucose (FDG) administration daily before each treatment fraction. However, there are advantages to reducing the number of tracer injections by using a PET tracer with a longer decay time. In this context, we investigated ⁸⁹Zr-panitumumab (⁸⁹Zr-Pan), an antibody PET tracer with a half-life of 78 hours that can be imaged for up to 9 days using PET.

METHODS AND MATERIALS

The BgRT workflow was evaluated preclinically in mouse colorectal cancer xenografts (HCT116) using small-animal positron emission tomography/computed tomography (PET/CT) for imaging and image-guided kilovoltage conformal irradiation for therapy. Mice (n = 5 per group) received 7 MBq of ⁸⁹Zr-Pan as a single dose 2 weeks after tumor induction, with or without fractionated radiation therapy (RT; 6 × 6.6 Gy) to the tumor region. The mice were imaged longitudinally to assess the kinetics of the tracer over 9 days. PET images were then analyzed to determine the stability of the PET signal in irradiated tumors over time.

RESULTS

Mice in the treatment group experienced complete tumor regression, whereas those in the control group were killed because of tumor burden. PET imaging of ⁸⁹Zr-Pan showed well-delineated tumors with minimal background in both groups. On day 9 postinjection, tumor uptake of ⁸⁹Zr-Pan was 7.2 ± 1.7 in the control group versus 5.2 ± 0.5 in the treatment group (mean percentage of injected dose per gram of tissue [%ID/g] ± SD; P = .07), both significantly higher than FDG uptake (1.1 ± 0.5 %ID/g) 1 hour postinjection. To assess BgRT feasibility, the clinical eligibility criteria was computed using human-equivalent uptake values that were extrapolated from preclinical PET data. Based on this semiquantitative analysis, BgRT may be feasible for 5 consecutive days after a single 740-MBq injection of ⁸⁹Zr-Pan.

CONCLUSIONS

This study indicates the potential of long-lived antibody-based PET tracers for guiding clinical BgRT.

Image-mode performance characterisation of a positron emission tomography subsystem designed for Biology-guided radiotherapy (BgRT)

OBJECTIVES

In this study, we characterise the imaging-mode performance of the positron emission tomography (PET) subsystem of the RefleXion X1 machine using the NEMA NU-2 2018 standard.

METHODS

The X1 machine consists of two symmetrically opposing 900 arcs of PET detectors incorporated into the architecture of a ring-gantry linear accelerator rotating up to 60 RPM. PET emissions from a tumour are detected by the PET detectors and used to guide the delivery of radiation beam. Imaging performance of the PET subsystem on X1 machine was evaluated based on sensitivity of the PET detectors, spatial resolution, count-loss performance, image quality, and daily system performance check.

RESULTS

PET subsystem sensitivity was measured as 0.183 and 0.161 cps/kBq at the center and off-center positions, respectively. Spatial resolution: average FWHM values of 4.3, 5.1, and 6.7 mm for the point sources at 1, 10, and 20 cm off center, respectively were recorded. For count loss, max NECR: 2.63 kcps, max true coincidence rate: 5.56 kcps, and scatter fraction: 39.8%. The 10 mm sphere was not visible. Image-quality contrast values were: 29.6%, 64.9%, 66.5%, 81.8%, 81.2%, and background variability: 14.8%, 12.4%, 10.3%, 8.8%, 8.3%, for the 13, 17, 22, 28, 37 mm sphere sizes, respectively.

CONCLUSIONS

When operating in an imaging mode, the spatial resolution and image contrast of the X1 PET subsystem were comparable to those of typical diagnostic imaging systems for large spheres, while the sensitivity and count rate were lower due to the significantly smaller PET detector area in the X1 system. Clinical efficacy when used in BgRT remains to be validated.

ADVANCES IN KNOWLEDGE

This is the first performance evaluation of the PET subsystem on the novel BgRT machine. The dual arcs rotating PET subsystem on RefleXion X1 machine performance is comparable to those of the typical diagnostic PET system based on the spatial resolution and image contrast for larger spheres.

The Added Value of [18F]Choline PET/CT in Low-Risk Prostate Cancer Staging: A Case Report

In the management of prostate cancer (PCa), correct staging is crucial in order to assess the right therapeutic approach. [18F]Choline PET/CT has been shown to provide more accurate staging information than conventional imaging approaches. The aim of this paper is to provide a real practice demonstration of the impact of [18F]Choline PET/CT on low-risk prostate cancer staging and clinical management. We report a 64-year-old man with biochemical PCa recurrence diagnosis after transurethral resection of the prostate. The patient, after the detection of an increased level of PSA, underwent multi-parametric prostate magnetic resonance imaging (mpMRI) that did not show evidence of disease. The patient was admitted to perform [18F]Choline PET/CT that showed a macroscopic prostate recurrence. Patient underwent photon external beam radiation therapy (EBRT) treatment, and [18F]Choline PET/CT was also used to define treatment volumes. At 3- and 6-month clinical follow-up evaluations, no late toxicity was detected and a significant reduction in PSA value was shown. Therefore, our case highlights the potential usefulness of [18F]Choline PET/CT for the staging of low-risk prostate cancer and its impact on the management and quality of life of such patients. The presented case should urge the scientific community to enhance larger and multicentric studies, assessing more extensively the potential impact of [18F]Choline PET/CT in this clinical scenario.

Radiomics-guided radiation therapy: opportunities and challenges

Radiomics is an advanced image-processing framework, which extracts image features and considers them as biomarkers towards personalized medicine. Applications include disease detection, diagnosis, prognosis, and therapy response assessment/prediction. As radiation therapy aims for further individualized treatments, radiomics could play a critical role in various steps before, during and after treatment. Elucidation of the concept of radiomics-guided radiation therapy (RGRT) is the aim of this review, attempting to highlight opportunities and challenges underlying the use of radiomics to guide clinicians and physicists towards more effective radiation treatments. This work identifies the value of RGRT in various steps of radiotherapy from patient selection to follow-up, and subsequently provides recommendations to improve future radiotherapy using quantitative imaging features.

Real-time marker-less tumor tracking with TOF PET: in silico feasibility study

Purpose. Although positron emission tomography (PET) can provide a functional image of static tumors for RT guidance, it’s conventionally very challenging for PET to track a moving tumor in real-time with a multiple frame/s sampling rate. In this study, we developed a novel method to enable PET based three-dimension (3D) real-time marker-less tumor tracking (RMTT) and demonstrated its feasibility with a simulation study. Methods. For each line-of-response (LOR) acquired, its positron-electron annihilation position is calculated based on the time difference between the two gamma interactions detected by the TOF PET detectors. The accumulation of these annihilation positions from data acquired within a single sampling frame forms a coarsely measured 3D distribution of positron-emitter radiotracer uptakes of the lung tumor and other organs and tissues (background). With clinically relevant tumor size and sufficient differential radiotracer uptake concentrations between the tumor and background, the high-uptake tumor can be differentiated from the surrounding low-uptake background in the measured distribution of radiotracer uptakes. With a volume-of-interest (VOI) that closely encloses the tumor, the count-weighted centroid of the annihilation positions within the VOI can be calculated as the tumor position. All these data processes can be conducted online. The feasibility of the new method was investigated with a simulated cardiac-torso digital phantom and stationary dual-panel TOF PET detectors to track a 28 mm diameter lung tumor with a 4:1 tumor-to-background 18FDG activity concentration ratio. Results. The initial study shows TOF PET based RMTT can achieve <2.0 mm tumor tracking accuracy with 5 frame s−1 sampling rate under the simulated conditions. In comparison, using reconstructed PET images to track a similar size tumor would require >30 s acquisition time to achieve the same tracking accuracy. Conclusion. With the demonstrated feasibility, the new method may enable TOF PET based RMTT for practical RT applications.

Radiotherapy on-chip: microfluidics for translational radiation oncology

The clinical importance of radiotherapy in the treatment of cancer patients justifies the development and use of research tools at the fundamental, pre-clinical, and ultimately clinical levels, to investigate their toxicities and synergies with systemic agents on relevant biological samples. Although microfluidics has prompted a paradigm shift in drug discovery in the past two decades, it appears to have yet to translate to radiotherapy research. However, the materials, dimensions, design versatility and multiplexing capabilities of microfluidic devices make them well-suited to a variety of studies involving radiation physics, radiobiology and radiotherapy. This review will present the state-of-the-art applications of microfluidics in these fields and specifically highlight the perspectives offered by radiotherapy on-a-chip in the field of translational radiobiology and precision medicine. This body of knowledge can serve both the microfluidics and radiotherapy communities by identifying potential collaboration avenues to improve patient care.

A detailed process map for clinical workflow of a new biology-guided radiotherapy (BgRT) machine

Purpose

Biology‐guided radiotherapy (BgRT) is a new external beam radiation therapy modality combining PET‐CT with a linear accelerator that has the potential to track and treat one or more tumors in real‐time. The use of PET and radiopharmaceutical tracers introduces new processes that are different from the existing treatment processes. In this study, we have developed a process map for the clinical implementation of a prototype BgRT machine.

Methods

A team of 13 members from various radiation therapy disciplines at our institution participated in developing a prospective process map for a prototype BgRT machine. The methodology provided by the AAPM TG 100 report was followed. In particular, the steps unique to the BgRT workflow, using hypofractionated stereotactic body radiation therapy with fluorodeoxyglucose radiolabeled with fluorine‐18 (FDG) to guide beam delivery, were analyzed.

Results

The multi‐disciplinary team in the department of radiation oncology at our institution developed a prospective process map for the clinical BgRT workflow. By focusing on the appropriate level of detail, 15 major subprocesses, 133 steps, and 248 substeps were identified and the process map was agreed upon as being useful, implementable, and manageable. Seventy‐four steps from nine subprocesses, 55.6% of the whole process, were analyzed to be the BgRT unique steps. They originate mainly from: (1) acquiring multiple PET images at the BgRT machine with separate patient visits, (2) creating a unique biological treatment volume for BgRT plan (PTV_BgRT), and (3) BgRT plan optimization and treatment delivery using PET images.

Conclusion

Using BgRT to irradiate multiple metastases in the same session will impact clinical workflow, thus a graphical process map depicting the new clinical workflow with an appropriate level of detail is critical for efficient, safe, and high‐quality care. The prospective process map will guide the successful setup and use of the new BgRT system.

Utility of Biology-Guided Radiotherapy to De Novo Metastases Diagnosed During Staging of High-Risk Biopsy-Proven Prostate Cancer

Background

Biology-guided radiotherapy (BgRT) uses real-time functional imaging to guide radiation therapy treatment. Positron emission tomography (PET) tracers targeting prostate-specific membrane antigen (PSMA) are superior for prostate cancer detection than conventional imaging. This study aims at describing nodal and distant metastasis distribution from prostate cancer and at determining the proportion of metastatic lesions suitable for BgRT.

Methods

A single-institution patient subset from the ProPSMA trial (ID ACTRN12617000005358) was analysed. Gross tumour volumes (GTV) were delineated on the CT component of a PSMA PET/CT scan. To determine the suitability of BgRT tracking zones, the normalized SUV (nSUV) was calculated as the ratio of SUVmax inside the GTV to the SUVmean of adjacent three-dimensional shells of thickness 5 mm/10 mm/20 mm as a measure of signal to background contrast. Targets were suitable for BgRT if (1) nSUV was larger than an nSUV threshold and (2) non-tumour tissue inside adjacent shell was free of PET-avid uptake.

Results

Of this cohort of 84 patients, 24 had at least one pelvic node or metastatic site disease, 1 to 13 lesions per patient, with a total of 98 lesions (60 pelvic nodes/38 extra-pelvic nodal diseases and haematogenous metastases). Target volumes ranged from 0.08 to 9.6 cm³ while SUVmax ranged from 2.1 to 55.0. nSUV ranged from 1.9 to 15.7/2.4 to 25.7/2.5 to 34.5 for the 5 mm/10 mm/20 mm shell expansion. Furthermore, 74%/68%/34% of the lesions had nSUV ≥ 3 and were free of PSMA PET uptake inside the GTV outer shell margin expansion of 5 mm/10 mm/20 mm. Adjacent avid organs were another lesion, bladder, bowel, ureter, prostate, and liver.

Conclusions

The majority of PSMA PET/CT-defined radiotherapy targets would be suitable for BgRT by using a 10-mm tracking zone in prostate cancer. A subset of lesions had adjacent non-tumour uptake, mainly due to the proximity of ureter or bladder, and may require exclusion from emission tracking during BgRT.

Feasibility of biology-guided radiotherapy using PSMA-PET to boost to dominant intraprostatic tumour

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Biology-guided radiation therapy (BGRT) uses PET imaging for online image guidance.

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PSMA PET uptake is abundant in the dominant intraprostatic lesion (DIL).

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BgRT boost to PSMA-avid subvolume in the prostate region may be feasible.

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Suitable targets for BgRT were identified in the ProPSMA clinical trial.

Background

Methods

Results

Conclusions

Quality management in radiotherapy treatment delivery

Radiation Oncology continues to rely on accurate delivery of radiation, in particular where patients can benefit from more modulated and hypofractioned treatments that can deliver higher dose to the target while optimising dose to normal structures. These deliveries are more complex, and the treatment units are more computerised, leading to a re-evaluation of quality assurance (QA) to test a larger range of options with more stringent criteria without becoming too time and resource consuming. This review explores how modern approaches of risk management and automation can be used to develop and maintain an effective and efficient QA programme. It considers various tools to control and guide radiation delivery including image guidance and motion management. Links with typical maintenance and repair activities are discussed, as well as patient-specific quality control activities. It is demonstrated that a quality management programme applied to treatment delivery can have an impact on individual patients but also on the quality of treatment techniques and future planning. Developing and customising a QA programme for treatment delivery is an important part of radiotherapy. Using modern multidisciplinary approaches can make this also a useful tool for department management.

The Potential of Photoacoustic Imaging in Radiation Oncology

Radiotherapy is recognized globally as a mainstay of treatment in most solid tumors and is essential in both curative and palliative settings. Ionizing radiation is frequently combined with surgery, either preoperatively or postoperatively, and with systemic chemotherapy. Recent advances in imaging have enabled precise targeting of solid lesions yet substantial intratumoral heterogeneity means that treatment planning and monitoring remains a clinical challenge as therapy response can take weeks to manifest on conventional imaging and early indications of progression can be misleading. Photoacoustic imaging (PAI) is an emerging modality for molecular imaging of cancer, enabling non-invasive assessment of endogenous tissue chromophores with optical contrast at unprecedented spatio-temporal resolution. Preclinical studies in mouse models have shown that PAI could be used to assess response to radiotherapy and chemoradiotherapy based on changes in the tumor vascular architecture and blood oxygen saturation, which are closely linked to tumor hypoxia. Given the strong relationship between hypoxia and radio-resistance, PAI assessment of the tumor microenvironment has the potential to be applied longitudinally during radiotherapy to detect resistance at much earlier time-points than currently achieved by size measurements and tailor treatments based on tumor oxygen availability and vascular heterogeneity. Here, we review the current state-of-the-art in PAI in the context of radiotherapy research. Based on these studies, we identify promising applications of PAI in radiation oncology and discuss the future potential and outstanding challenges in the development of translational PAI biomarkers of early response to radiotherapy.

A Century of Fractionated Radiotherapy: How Mathematical Oncology Can Break the Rules

Radiotherapy is involved in 50% of all cancer treatments and 40% of cancer cures. Most of these treatments are delivered in fractions of equal doses of radiation (Fractional Equivalent Dosing (FED)) in days to weeks. This treatment paradigm has remained unchanged in the past century and does not account for the development of radioresistance during treatment. Even if under-optimized, deviating from a century of successful therapy delivered in FED can be difficult. One way of exploring the infinite space of fraction size and scheduling to identify optimal fractionation schedules is through mathematical oncology simulations that allow for in silico evaluation. This review article explores the evidence that current fractionation promotes the development of radioresistance, summarizes mathematical solutions to account for radioresistance, both in the curative and non-curative setting, and reviews current clinical data investigating non-FED fractionated radiotherapy.

Small-field measurement and Monte Carlo model validation of a novel image-guided radiotherapy system

PURPOSE

The RefleXion™ X1 is a novel radiotherapy system that is designed for image-guided radiotherapy, and eventually, biology-guided radiotherapy (BgRT). BgRT is a treatment paradigm that tracks tumor motion using real-time positron emission signals. This study reports the small-field measurement results and the validation of a Monte Carlo (MC) model of the first clinical RefleXion unit.

METHODS

The RefleXion linear accelerator (linac) produces a 6 MV flattening filter free (FFF) photon beam and consists of a binary multileaf collimator (MLC) system with 64 leaves and two pairs of y-jaws. The maximum clinical field size achievable is 400 × 20 mm² . The y-jaws provide either a 10 or 20 mm opening at source-to-axis distance (SAD) of 850 mm. The width of each MLC leaf at SAD is 6.25 mm. Percentage depth doses (PDDs) and relative beam profiles were acquired using an Edge diode detector in a water tank for field sizes from 12.5 × 10 to 100 × 20 mm² . Beam profiles were also measured using films. Output factors of fields ranging from 6.25 × 10 to 100 × 20 mm² were measured using W2 scintillator detector, Edge detector, and films. Output correction factors k of the Edge detector for RefleXion were calculated. An MC model of the linac including pre-MLC beam sources and detailed structures of MLC and lower y-jaws was validated against the measurements. Simulation codes BEAMnrc and GATE were utilized.

RESULTS

The diode measured PDD at 10 cm depth (PDD10) increases from 53.6% to 56.9% as the field opens from 12.5 × 10 to 100 × 20 mm² . The W2-measured output factor increases from 0.706 to 1 as the field opens from 6.25 × 10 to 100 × 20 mm² (reference field size). The output factors acquired by diode and film differ from the W2 results by 1.65% (std = 1.49%) and 2.09% (std = 1.41%) on average, respectively. The profile penumbra and full-width half-maximum (FWHM) measured by diode agree well with the film results with a deviation of 0.60 mm and 0.73% on average, respectively. The averaged beam profile consistency calculated between the diode- and film-measured profiles among different depths is within 1.72%. By taking the W2 measurements as the ground truth, the output correction factors k for Edge detector ranging from 0.958 to 1 were reported. For the MC model validation, the simulated PDD10 agreed within 0.6% to the diode measurement. The MC-simulated output factor differed from the W2 results by 2.3% on average (std = 3.7%), while the MC simulated beam penumbra differed from the diode results by 0.67 mm on average (std = 0.42 mm). The MC FWHM agreed with the diode results to within 1.40% on average. The averaged beam profile consistency calculated between the diode and MC profiles among different depths is less than 1.29%.

CONCLUSIONS

This study represents the first small-field dosimetry of a clinical RefleXion system. A complete and accurate MC model of the RefleXion linac has been validated.

The technical design and concept of a PET/CT linac for biology-guided radiotherapy

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This article summarizes the chief technology and concept of the world's first PET/CT Linac for BgRT.

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BgRT delivery uses annihilation photons emanating from the PET-avid tumor to guide the delivery of beamlets in real-time.

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BgRT treatment technique opens the avenue to debulking advanced and metastatic disease.

This is a summary of the design and concept of the RefleXion X1, a system for biology-guided radiotherapy (BgRT). This system is a multi-modal tomography (PET, fan-beam kVCT, and MVD) treatment machine that utilizes imaging and therapy planes for optimized beam delivery of IMRT, SBRT, SRS, and BgRT radiotherapy regimens. For BgRT delivery specifically, annihilation photons emanating outward from a PET-avid tumor are used to guide the delivery of beamlets of radiation to the tumor at sub-second latency. With the integration of PET detectors, rapid beam-station delivery, real-time tracking, and high-frequency multi-leaf collimation, the BgRT system has the potential to deliver a highly conformal treatment to malignant lesions while minimizing dose to surrounding healthy tissues. Furthermore, the potential use of a single radiotracer injection to guide radiotherapy to multiple targets opens avenues for debulking in advanced and metastatic disease states.