“Incomplete H Sign” in Sacral Insufficiency- A Rare Scintigraphic ‘H’ Sign Variant on Technetium-99m Methylene Diphosphonate (Tc-99m MDP) Bone Scan

A 52-year-old woman, follow-up case of treated carcinoma right breast, status post right modified radical mastectomy, adjuvant chemotherapy and radiotherapy to the right chest wall 8 years back, presented with pain in the lower back and gluteal region following a fall in the bathroom. She was referred to the department of Nuclear Medicine for a Tc-99m methylene diphosphonate (MDP) bone scan to rule out skeletal metastases [Fig 1]. Tc-99m MDP bone scintigraphy revealed intense horizontal linear uptake in the S3-S4 sacral segments, consistent with a sacral insufficiency fracture (SIF). SIFs are a type of stress fracture that can occur spontaneously or following minimal-impact trauma in osteoporotic bone. These injuries are associated with substantial pain, diminished functional autonomy, and significant economic implications for both patients and the health-care system. Potential risk factors for SIFs include osteoporosis, rheumatoid arthritis, corticosteroid therapy, and prior pelvic irradiation.^[1] Because of their complex anatomy and deep location, these fractures are challenging to identify with conventional radiography; consequently, computed tomography (CT) is useful for delineating cortical fracture lines, while magnetic resonance imaging (MRI) is regarded as the most sensitive modality for early detection due to its ability to demonstrate bone marrow enema.^[2] In the early phases of injury, bone scintigraphy may serve as a more sensitive imaging modality, particularly in patients presenting with nonspecific clinical symptoms.^[3]The ‘H’ or Honda sign is a widely reported characteristic sign of SIF, on numerous imaging modalities such as bone scan, fluorodeoxyglucose (FDG)^[4], prostate-specific membrane antigen (PSMA)^[5], fibroblast activation protein inhibitor (FAPI)^[6] and DOTATATE.^[7] Characterised by horizontal tracer uptake in the sacral promontory and vertical tracer uptake in the sacral ala, resembling the letter H. Although the Honda sign is widely recognised as a characteristic indicator of sacral insufficiency fractures, the broader spectrum of scintigraphy patterns associated with these fractures has not been thoroughly investigated. To our knowledge, most reported variants of the Honda sign involve partial or asymmetric uptake in the sacral ala or upper sacral segments (S1–S2). Isolated horizontal uptake without associated sacral ala activity, particularly involving the lower sacral segments, is rarely described in the literature. The case is unusual due to isolated involvement of the horizontal component of the Honda sign localised to the S3-S4 segments, without associated sacral ala uptake, a pattern that may closely mimic metastatic disease in oncology patients.

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Fig 1: Tc-99m MDP delayed static (a) Anterior and (b) Posterior planar images showing intense linear horizontal radiotracer uptake in the sacral region, just superior to bladder activity (red arrow); Single photon emission computed tomography (SPECT-CT) of the pelvic region was acquired, which showed no definite fracture line on (c) Trans axial and (d) Sagittal CT cross-sections; Fused (e) Trans axial and (f) Sagittal SPECT CT cross sections revealed intense horizontal linear radiotracer uptake in S3-S4 sacral segments (white and yellow arrows in e and f), suggestive of sacral insufficiency fracture (SIF). SPECT-CT: Single photon emission computed tomography-computed tomography; MDP: Methylene diphosphonate

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This case also highlights the complementary role of bone scintigraphy with Single photon emission computed tomography/computed tomography (SPECT/CT) in detecting early or occult sacral insufficiency fractures, particularly in cases where CT does not demonstrate a definite fracture line. The patient was managed conservatively with analgesics and activity modification. She was also advised to undergo further evaluation for osteoporosis and to follow up with the oncology and orthopaedic teams. Her symptoms improved on subsequent clinical follow-up.

Fujii et al reported a complete Honda sign in only 63% of patients with SIFs, whereas 35% demonstrated Honda Sign variants.^[8] Consequently, recognising variant scintigraphy patterns, such as the horizontal bar, half-butterfly, and tramline configurations, is essential to avoid misinterpretation as metastatic disease and prevent unnecessary investigations.