1. Imaging Findings

Based on the ultrasound and MRI images provided by the patient, there is a nodular lesion of approximately 4 cm in size located in the left perineal region near the ischial tuberosity. The specific imaging findings are as follows:

• Ultrasound (US): In the left perineal region near the ischial tuberosity, there is a relatively well-defined hypoechoic (or anechoic) lesion with no obvious blood flow signal within it.
• MRI: On T1-weighted images (T1WI), the lesion appears as low or isointense signal, while on T2-weighted images (T2WI), it shows high signal. There is no obvious enhancement after contrast administration. It is located adjacent to the ischial tuberosity, and there are no signs of extensive infiltration of the surrounding soft tissue or bone destruction.

Overall, the imaging suggests a cystic or pseudocystic lesion along with possible fibrotic and collagenous changes in the surrounding tissue.

2. Potential Diagnoses

Combining the patient’s one-year history of prolonged cycling and the imaging characteristics, the following differential diagnoses are considered:

• Ischial bursitis / ischial pseudobursa-type lesion (Ischial hygroma or perineal nodular induration): A common condition in cyclists caused by chronic pressure and friction, leading to soft tissue injury, collagen degeneration, and pseudocyst formation. On imaging, these lesions typically present as low/isointense T1 and high T2 cystic lesions without blood flow signals. In conjunction with the patient’s typical cycling history, this is the primary consideration.
• Benign soft tissue tumors (e.g., lipoma, fibroma): Although the patient’s history and symptoms might not be typical, common soft tissue tumors should be considered for differential diagnosis. However, lipomas usually show high signal intensity on T1 and certain characteristic fat suppression features on MRI, which conflicts with this case, making them less probable.
• Infectious or inflammatory lesions (e.g., abscess): Usually accompanied by local pain, redness, elevated inflammatory markers, and enhancement on MRI, which is not evident in this case. Hence, it is less likely.

3. Final Diagnosis

Taking into account the patient’s prolonged cycling history, painless cystic lesion, and typical imaging appearance, the most likely diagnosis is: Perineal nodular induration (ischial fluid-filled cyst, also known as the “third testicle” or “buttock nodule”).

If uncertainty remains or to rule out other rare soft tissue tumors, further pathological biopsy or continued follow-up (monitoring lesion size and changes via imaging) may be necessary.

4. Treatment Plan and Rehabilitation Program

Treatment Plan:

• Conservative Management: First, adjust cycling posture and replace or modify the saddle to relieve pressure. Temporarily reduce or stop cycling or any activities that cause repeated friction and compression. Local physical therapy and warm compresses can be considered to promote local circulation.
• Medication Injection: For patients experiencing recurrent pain or significant symptoms, local injection of corticosteroids or hyaluronidase may be considered to alleviate inflammatory responses and reduce nodule size.
• Surgical Treatment: If the lesion is large and markedly affects daily life, or symptoms are severe and unresponsive to conservative treatment, surgical excision of the lesion may be considered.

Rehabilitation / Exercise Prescription Suggestions (FITT-VP Principle):

• Type of Exercise (Type): Initially, choose activities that place minimal pressure on the perineal region (e.g., swimming or upper-limb strength training). Gradually transition to cycling, ensuring proper saddle height and angle, and using a cushion that matches individual anatomy.
• Frequency (Frequency): Begin with training sessions 2-3 times per week. Ensure rest days for adequate recovery of the local tissues.
• Intensity (Intensity): Start with a lower intensity. Avoid abruptly resuming long-distance or high-load cycling. Reduce intensity promptly if discomfort arises.
• Duration (Time): Each session can begin with 20-30 minutes and be gradually increased depending on pain levels or local response.
• Progression (Progression): Once perineal pain subsides, the nodule does not enlarge, and there is no significant discomfort after activity, gradually increase cycling duration and intensity. If repeated friction occurs, stop cycling and seek medical evaluation.
• Precautions (Precautions): Maintain local hygiene and avoid intense friction. If the patient has osteoporosis or other chronic conditions, any increase in load intensity should be gradual and guided by a professional physician or physical therapist.

5. Disclaimer

This report is based on the current medical history and imaging data for reference purposes only and cannot replace an in-person consultation or professional medical advice. If you have any concerns or if symptoms worsen, please seek medical attention promptly and consider further clinical examinations and pathological diagnosis.