Ovarian cancer is one of the most common malignant tumors of the female reproductive system, ranking third in incidence among gynecological malignancies (1). Approximately 70% of patients are diagnosed at an advanced stage (2). Bone metastasis in ovarian cancer is relatively rare, with an incidence of about 2-8% (3). The treatment of giant bone metastatic lesions has always been a clinical challenge, especially when tumor volume limits conventional radiotherapy dosing.

In recent years, stereotactic radiotherapy technology has developed rapidly, further extending to non-uniform irradiation design concepts. SCART (Stereotactic Centralized Ablative Radiation Therapy), as an emerging precision radiotherapy technology, has shown unique advantages in treating large-volume tumor lesions (4). Meanwhile, the Lattice technique, as a typical representative of spatial fractionation, creates multiple small dose hotspots within the tumor, forming a “lattice-like” dose distribution that can increase irradiation dose to cold spot areas while ensuring safe dose at target margins (5). SCART and Lattice radiotherapy represent novel approaches capable of addressing the limitations of traditional treatments by delivering highly heterogeneous intratumoral dose distributions.

Treatment efficacy was evaluated according to RECIST 1.1 criteria:

Baseline evaluation: Right iliac giant bone metastatic lesion approximately 89×78×58mm, calculated volume approximately 399.0cc, estimated SLD: 9.13 cm (Figure 1a).

Post-SCART(1 month): Lesion volume evaluated as 308cc, estimated volume change: -22.8%, SLD: 8.38 cm (change: -8.3%), efficacy evaluation: SD (stable disease) (Figure 1b).

Post-Lattice (6 months): Follow-up MRI showed tumor volume reduced to 136.1cc, GTV: 136.1 cc (change: -65.9%), estimated SLD: 6.38 cm (change: -30.1%), efficacy evaluation: PR (partial response) (Figures 1c, d).

Throughout the treatment process, patient PS score was 0-1, indicating good performance status and ability to tolerate subsequent treatment. NRS score decreased from pre-treatment 4–5 to 2.

This case presents successful treatment of giant pelvic bone metastasis using sequential spatial fractionation techniques. Initial SCART therapy provided significant pain relief but insufficient tumor shrinkage, prompting Lattice dose-escalation that achieved substantial tumor regression. The sequential approach proved effective for this radiotherapy-resistant metastasis.

Clinically, 2% of ovarian cancer patients are diagnosed with bone metastasis (6). However, actual bone metastasis incidence is higher than this data. According to autopsy data, actual bone metastasis incidence in ovarian cancer is approximately 10% (7), with most common bone metastasis sites being lumbar spine and pelvis (8). Studies show that late clinical stage, poor differentiation, and lymph node metastasis are associated with the development of bone metastasis in ovarian cancer (9). The relationship between ovarian cancer histological type and bone metastasis remains controversial, with some studies suggesting ovarian cancer histological type is unrelated to bone metastasis occurrence and development (10). However, other studies suggest ovarian clear cell carcinoma patients are more prone to bone metastasis (11), and this case’s pathological type and bone metastasis characteristics are consistent with these patterns.

SCART technique proposed by Dr. Yang Jun, as an emerging precision radiotherapy technology, has advantages of highly conformal dose distribution and small normal tissue damage (12, 13) (Figure 2). In this case, we first used SCART combined with SBRT technique for treatment. One month after treatment, patient’s pain symptoms were significantly alleviated, but imaging examination showed tumor volume reduced from initial volume to 308 cc, with magnitude of reduction did not meet PR criteria, suggesting potential insensitivity of the lesion to initial radiotherapy.

For bulky tumor lesions, especially those with poor response to conventional radiotherapy, Lattice technique provides an effective therapeutic strategy (14). Lattice technique, as an important advancement in spatial fractionated radiotherapy, creates dose peak areas within the tumor, forming a “lattice-like” dose distribution that can increase irradiation dose to cold spot areas while ensuring safe dose at target margins. Wu et al. (15) demonstrated that Lattice technique can effectively improve local control rate of large-volume tumors while reducing surrounding normal tissue damage (Figure 3).

In this case, after using Lattice technique for dose-escalation radiotherapy, tumor volume further reduced to 136.1cc at 6-month follow-up. The combination of two spatial fractionation techniques promoted tumor regression in radiotherapy-resistant tumor types without increasing Grade ≥3 acute adverse events. The pain relief after the first course is rapidly reduction, While imaging changes may not be apparent early after high-dose radiotherapy. But 1-month follow-up enables to assessment of acute toxicity reactions and establishment of baseline data for subsequent treatment decisions.

SCART represents a significant technological advancement beyond conventional SBRT, primarily distinguished by its unique capability to achieve peak-to-valley ratios exceeding 300% of the peripheral dose—a feat unattainable with standard SBRT hotspots, which typically cannot exceed 150-200% of peripheral dose. This distinctive dose distribution characteristic is not merely a result of “removing limits” but rather reflects an innovative technology with sophisticated dose sculpting capabilities and enhanced biological effects (Figure 4a).

The therapeutic synergy between SCART and Lattice techniques stems from their complementary spatial dose distribution patterns. While SCART employs centralized focusing to achieve maximum dose concentration in the tumor core, Lattice radiotherapy creates multiple high-dose “islands” distributed throughout the tumor volume through spatial fractionation. This complementary approach enables comprehensive tumor coverage, with Lattice particularly effective in targeting multiple tumor regions, including potential residual peripheral areas that may receive suboptimal dose from central focusing alone. The sequential application thus provides both intensive central tumor ablation and comprehensive peripheral coverage (Figure 4c).

SCART and Lattice techniques offer distinct advantages over conventional radiotherapy approaches across multiple dimensions:

1. Precision Targeting and Dose Optimization: Highly conformal dose distribution ensures accurate tumor coverage while maximizing normal tissue sparing. Advanced dose optimization eliminates hotspots and coldspots, ensuring uniform tumor coverage. Particularly advantageous for pelvic lesions where critical structure protection is essential.

2. Treatment Efficiency and Safet.

Shorter treatment courses without delaying systemic therapy initiation. Demonstrated efficacy in less sensitive large-volume tumors when surrounding tissue safety is maintained.

3. Technological Superiority: Compared with existing radiotherapy modalities (Figure 4b):

This combined spatial fractionation approach provides new treatment options for bulky tumors that have historically presented therapeutic challenges. The sequential SCART and Lattice application strategy establishes a new paradigm for managing large-volume malignancies, offering a balance between aggressive tumor control and treatment tolerability. This approach warrants further investigation in prospective clinical trials to establish standardized protocols and validate efficacy across broader patient populations.

Sequential SCART followed by Lattice radiotherapy appears safe, feasible, and effective for managing bulky bone metastases in ovarian cancer. SCART incorporates tumor heterogeneity and regional variations in radiosensitivity into its treatment planning, with sequential optimization strategies employed to maximize therapeutic efficacy. Significant tumor regression, symptomatic improvement, and tolerable toxicity were observed. Larger prospective studies are needed to determine long-term outcomes and identify ideal treatment candidates.