Molecular Imaging in the selection and evaluation of response in patients treated with Radium-223 in six different solid tumors: A single center experience

Objective: Evaluate impact after 223 Ra therapy and 18 F-NaF (sodium fluoride) PET/CT in the selection and evaluation of response in patients treated with 223 Ra in six different solid tumors. Material and Methods: Twenty patients with metastatic castration-resistant prostate cancer (mCRPC), seven metastatic castration-sensitive prostate cancer (mCSPC), three osteosarcoma, two breast cancer, two non-small cell lung cancer (NSCLC), one chondrosarcoma, one chordoma and one patient lung neuroendocrine carcinoma. Three groups of study were defined according total skeletal tumor-burden obtained by 18 F-NaF PET/CT, group 1 <1000cm 3 , group 2 1001–2999cm 3 and group 3 >3000cm 3 VOI´s. A semi-quantitative comparison was performed measuring the SUVmax values of VOIs values in all bone metastases in each patient previous to receive the first cycle of 223 Ra, after 3 and 6 cycles. Results: 30 patients non-progress disease was documented after 24±4 weeks. 8 patients progress disease was presented after three cycles of 223 Ra, two patients with osteosarcoma, four patients with mCRPC, one patient with chondrosarcoma and one patient with NSCLC. Group 1 patients showed better response rates compared to group 3 (p<0.05). Group 2 patients who showed improvement clinical and radiological, had prostate malignancies compared to those in the same group, but non-prostatic malignancies (p<0.05). No significant difference in group 2 patients compared to group 3 (p<0.67). Symptomatic skeletal-related event was observed in 7 patients. Conclusion: 18 F-NaF PET/CT allows to identify patients who show osteoblastic bone activity and discard or confirm progression in the interval PET/CT image, allowing change of treatment, reducing costs. High tumor-burden strongly suggests a poor response to treatment


Introduction
Radium-223 ( 223 Ra) was the first bone-targeting agent to demonstrate improved overall survival in patients with Castration Resistant Prostate Cancer and bone metastases (mCRPC), in addition to bone pain relief, improvement in quality of life (QoL), and prolonging time to skeletal-related events (SREs); in addition is the first therapeutic alpha-emitting radionuclide approved for treating metastatic skeletal disease in prostate cancer [1,2].
Although there are currently multiple publications about the benefits of 223 Ra in patients with mCRPC; currently 223 Ra is being tested in 2 phase 3 trials in combination with novel antihormonal agents in the chemotherapy-naive setting of asymptomatic or mildly symptomatic mCRPC, the radiological response during 223 Ra therapy is at present poorly defined [3,4].
Recently interest has grown to demonstrate its usefulness in other types of solid tumors, such as breast cancer (BC), osteosarcoma, lung cancer (LC) and metastatic castration-sensitive prostate cancer (mCSPC) [5][6][7][8] For decades, the evaluation of metastatic bone disease in PC has been limited to the use of the bone scintigraphy, a method with high sensitivity but limited specificity. 18 F-Fluoride PET/CT ( 18 F-NaF) bone imaging is ideal for staging and restaging patients with PC because of greater sensitivity, specificity, and accuracy than conventional bone scintigraphy [9]. Moreover, the project named Focus 1 to develop consensus statements in prostate cancer, recommend 68 Ga-prostate specific membrane antigen (PSMA), 18 F-Choline and recently 18 F-Flucciclovine, cancer-targeted PET at biochemical recurrence to replace conventional imaging methods (bone scintigraphy or CT), and the preferred imaging method was PET-CT with a PSMA-targeting tracer. [10] In clinical practice, bone scintigraphy with 99m Tc-MDP has been most commonly used to assess osteoblastic metastases after 6 cycles of 223 Ra and sometimes interim (after 3 cycles) to evaluate to response; in some centers in Europe, PET/CT with 18 F-Choline or 68 Ga-PSMA are the preferred radiotracers; however, these radiotracers are not yet approved for the evaluation of metastatic bone disease in other types of solid tumors; in addition, there are no criteria for response of bone metastases, but only criteria for disease progression, as evident by the emergence of new lesions [10][11]. Unfortunately, there are many limitations in evaluate response to metastatic osteoblastic lesions because is considerate non-measurable lesion, unless it is a lytic lesion associated with soft tissue component. In addition, there is a poor evidence in the role of 18 F-NaF PET/CT in staging, restaging or evaluate to response of another solid tumors such as osteosarcoma or lung cancer, for this reason we hypothesized that 18 F-NaF PET/CT is a better modality in selecting candidate, evaluate quantitative and qualitative response in the bone metastasis tumors treated with bone-targeted alpha particle therapy.
The objective of this study were to evaluate the impact after 223 Ra therapy and the potential use of 18 F-NaF PET/CT in the selection and evaluation of response in patients treated with 223 Ra in six different solid tumors.

Materials And Methods
The present retrospective clinical study was conducted at nuclear medicine and molecular Imaging department in Instituto Nacional de Cancerologia, Mexico, between February 2015 and January 2019.
All PET and PET/CT examinations were performed in compliance with 1964 Declaration of Helsinki, and the responsible regulatory bodies in Mexico. All patients (except mCRPC) received 223 Ra under the ''compassionate use'' clause of the Mexico. Written informed consent was obtained from each subject Patients Thirty-seven patients with a mean age of 55 years (29 men and 3 women, age range 20-69 years) were included in this retrospective study. Twenty patients had mCRPC, seven had mCSPC, three patients had osteosarcoma, two patients had breast cancer, two patients had non-small cell lung cancer (NSCLC), one patient had chondrosarcoma, one patient with chordoma and one patient with lung neuroendocrine carcinoma. Inclusion criteria for 18 F-NaF PET/CT was diagnosis of metastatic bone disease and progression disease after conventional therapy, with previous 18 F-FDG-PET/CT imaging as usually imaging in the evaluation of different disease, and before to initiate therapy with 223 Ra. The most representative characteristics of the population study are summarized in Table 1. In addition, patients with non-prostatic malignances experienced progression under first or second-line chemotherapy (platinum agents, doxorubicin, ifosfamide, docetaxel and everolimus) or were not eligible for chemotherapy were included. In patients with mCRPC next-generation androgendeprivation therapy (abiraterone or enzalutamide) or first-or second-line chemotherapy (docetaxel or cabazitaxel) were included. Patients with evidence of visceral metastases, lymph nodes metastases > 3 cm in long-axis diameter, confirmed by a CT scan, were excluded. Other exclusion criteria included recent or complicated nonhealing fracture and use of concomitant radiotherapy. WB emission scans were acquired at 60 minutes post-injection. Low-dose CT (from the vertex of the skull to the feet) correction was performed for both attenuation correction and topographic localization. The CT parameters used for acquisition were 140 kV, 80 mA, and 0.5 s per rotation, with a pitch of 6:1 and a slice thickness of 5 mm. After completion of the CT scan, PET data were acquired for 3 min per bed position. Emission data were corrected for randoms, dead time, scatter, and attenuation and were reconstructed iteratively by an ordered-subsets expectation maximization algorithm (4 iterations, 8 subsets) followed by a post-reconstruction smoothing gaussian filter (5 mm in full width at one-half maximum).

Timing imaging
All patients had documented disease progression before the initiation of 223 Ra therapy. Before to begun therapy all patients underwent baseline 18 F-NaF PET/CT and after 3 and 6 cycles of 223 Ra.
Same activity, biodistribution time and parameters of acquisition was applied in subsequent imaging in each patient.

Imaging analysis
PET/CT images in all standard planes were reviewed by use of the dedicated software (syngo by SIEMENS). Images were analyzed visually and quantitatively by two nuclear medicine physicians with more than 5 years of experience. Maximum standardized uptake values (SUVmax) were obtained by drawing circular regions of interest, which were automatically adapted (40% isocontour) to a 3D VOI using syngo software. Volumes of interest (VOIs) with edges around 1 to 3 cm were drawn on regions of interest, the uptake in the VOIs was classified as malignant on the basis of the radiopharmaceutical distribution pattern with match CT images (Fig. 1).
Three groups of study was defined according total skeletal burden obtained by VOIs and SUVmax values (TLSB), group 1 had less 1000 cm 3 , group 2 had 1001-2999 cm 3 and group 3 more than 3000 cm 3 , these classification was established by consensus between two nuclear medicine physicians (κ = 0.85). (Fig. 2

Safety
Previous to each cycle, absolute blood count (hemoglobin, erythrocytes, leukocytes, platelets, neutrophils and lymphocytes) was evaluated. Treatment was continued as long as absolute blood count was ≥ 1.0 g/L for neutrophils and ≥ 50 g/L for platelets. Written informed consent was obtained from each patient before administration of 223 Ra. On the basis of blood levels, toxicity was categorized using the Common Toxicity Criteria for Adverse Events (CTCAE version 4.03).

Results
All patients had bone metastases without soft-tissue lesions or visceral metastases; they all had baseline 18 F-NaF PET/CT. None of the patients were pretreated with 223 Ra or another bone seeking radiopharmaceutical. Five patients had a history of localized radiotherapy (3-4 months before starting treatment with 223 Ra) to bone, due to intense pain. All patients had pain at least Visual Analog Scale for Pain (VAS) 7, despite to receive painkillers such as NAIDS and opioids. Baseline characteristics before starting treatment with 223 Ra are presented in Table 2-4.     Improvement of bone pain was observed in 32 patients at the end of treatment compared to baseline.
In three patients a significant decrease in pain was observed after the first two cycles of Ra-223; in two patients after four cycles.
Treatment-related adverse events were observed in 10 patients (6 patients with superscan); such as fatigue, diarrhea and nausea; meanwhile did not affect continuation of therapy. We found slight to moderate decreases in neutrophils and hemoglobin in 14 (38%) patients at the end of entire therapy.
During treatment and at term, 8 patients required transfusion. Four patients (two with superscan) required prior to the last cycle administration of colony stimulating factor. None patient presented severe adverse event´s Grade III or IV according to CTCAE.
in 30 patients no progress disease was documented after 24 ± 4 weeks. In 8 patients progress disease was presented after three cycles of 223 Ra (two presented visceral metastases), two patients with osteosarcoma, four patients with mCRPC, one patient with chondrosarcoma and one patient with

NSCLC (Figs. 3 and 4)
Symptomatic skeletal-related event (SSE) was observed in 7 patients, 3 patients with mCRPC (two development pathologic fracture, and one patient required radiation therapy for worsening pain), one patient with NSCLC (pathologic fracture), one patient with osteosarcoma, one patient with chondrosarcoma and one patient with chordoma (the three patients for worsening pain) Those patients in group 1 showed better response rates compared to those in group 3 (p < 0.05).
Patients in group 2 who showed improvement clinical and radiological, had prostate malignancies compared to those in the same group, but non-prostatic malignancies (p < 0.05). While there was no significant difference in the group of patients in group 2 compared to group 3 (p < 0.67). Response rates were established according to the percentage of decreasing TLSB values, biochemical markers and improvement in pain (Fig. 5) Nine patients progressed, five patients with mCRPC, two patients with osteosarcoma, one patient with chondrosarcoma and one patient with NSCLC. All patients had high tumor burden because corresponding to group 2 and 3, but only 7 patients developed SSE as previously mentioned. In addition, the mean TLSB doesn´t improve after three cycles of 223Ra.
Discussion 223 Ra is a bone-seeking radiopharmaceutical that emits α-particles that deposit high linear energy within a short penetration range to areas of increased bone turnover, as radioactive decay occurs, near osseous metastatic sites, it selectively kills cancer cells. 223 Ra has a complex decay scheme in which 4 alpha particles resulting in high energy deposition (28.2 MeV), the high linear energy transfer of radiation results in generation of double-strand DNA breaks, and gives rise to cytotoxicity that is independent of dose rate, cell cycle growth phase, and oxygen concentration. The range of the α particles (< 100 µm) results in less hematologic toxicity than expected from β emitters [12,13].
Currently 223 Ra is recommended as a first-line treatment in mCRPC symptomatic or mildly symptomatic without visceral metastases [14] Maybe one of the doubts that does not end with clearing the ALSYMPCA trial is the poor evaluation of response with bone scan or another molecular imaging method, only was performed a baseline bone scan [1]. In recent years several studies have been published as a result of research conducted with different radiotracers, due to the need to assess the response and toxicity associated with this therapy [15,16].
However, the current recommendations of the Prostate Cancer Working Group 3 (PCWG3), European Association of Urology (EAU), NCCN, and many another guidelines, are unclear in defining the evaluation of response to any treatment of a non-measurable disease such as bone disease of the blastic type; for these reason numerous investigations have suggested the use of nuclear medicine techniques for patient selection and response evaluation, in addition similar to other therapies for mPC, a flare phenomenon with increase of bone metastases-related pain, or "increase" in apparent number of bone metastases on bone scan, may be noted during the first treatment cycles, and should not be interpreted as disease progression [14][15][16][17][18][19] The present study showed that besides that 223 Ra do not approved in another neoplasm such as BC, LC, mCSPC, and bone tumors the osteoblastic activity of the bone metastases may represent a therapeutic target due to calcium-mimetic characteristic of 223 Ra.
Skeletal evaluation with 18 F-NaF PET/CT is better to bone scintigraphy in mPC patients because of greater sensitivity, specificity, and accuracy; also, in another neoplasm such as breast cancer or lung cancer when the osteoblastic component predominates, for this reason molecular imaging always should precede therapy with 223 Ra to determine active osteoblastic lesions, because the focal uptake with 18 F-NaF PET/CT or MDP-bone scan correlates with the intensity of bone metabolism and eventually the uptake of 223 Ra [20][21][22]. In patients with both types of lung cancer, the 6 cycles were allowed to conclude; however, in one of them, the progression occurred 8 weeks after the end of the treatment. Taber  More studies dedicated to each subgroup of patients are needed to obtain important data such as progression-free survival and overall survival.

Conclusion
The molecular imaging with 18 F-NaF allows identify patients who show osteoblastic bone activity and discard or confirm progression in the interval PET/CT image, allowing an opportune change of the treatment reducing the costs for the patient and the institution. In addition, a high tumor burden strongly suggests a poor response to treatment, which in all cases is not synonymous with progression. 223 Ra is an agent with good tolerability with low SSEs rate and good pain response after completing 6 treatment cycles.

Declarations ETHICAL APPROVAL
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

INFORMED CONSENT
The institutional review board of our institute approved this retrospective study, and the requirement to obtain informed consent was waived.