SOLIS is a prospective, single-arm, single-institution pilot trial evaluating single-fraction stereotactic radiotherapy (24 Gy to the planning target volume with a simultaneous integrated boost of 30 Gy to the clinical target volume) delivered with a cleared linear accelerator as definitive non-operative local treatment for early-stage, hormone receptor-positive, HER2-negative breast cancer with planned omission of lumpectomy. Adjuvant endocrine therapy is prescribed on-label. The primary endpoint is the incidence of CTCAE v5.0 Grade ≥2 skin adverse events within 90 days of treatment, monitored with a pre-specified Bayesian safety stopping rule. Secondary endpoints include biopsy-confirmed absence of residual invasive carcinoma at 12 months, imaging response on multiparametric MRI and FDG PET/CT, late toxicity, cosmesis, patient-reported outcomes, salvage surgery, and disease control through 36 months.
Invasive ductal carcinoma or favorable special histology confirmed by core biopsy.
Unifocal tumor ≤2 cm; estrogen receptor-positive and HER2-negative; grade 1-2; clinically node-negative (cN0).
Age ≥65, or age 50-64 when surgery is unsuitable or declined (capped at 10 of 30 participants).
Tumor ≥5 mm from skin and pectoralis; willing to receive adjuvant endocrine therapy.
Multifocal or multicentric disease; pure DCIS; invasive lobular carcinoma >10%; extensive intraductal component or diffuse calcifications.
Grade 3, triple-negative, or HER2-positive disease; nodal or distant metastases.
Prior ipsilateral breast radiation or excisional biopsy/lumpectomy; neoadjuvant chemotherapy.
Tumor <5 mm from skin or pectoralis; inability to meet dose constraints or PTV >60 cm³.
Single group
Stereotactic Radiation Therapy
RadiationSingle fraction 24 Gy to PTV with SIB 30 Gy to CTV on Varian Edge with image guidance.
Endocrine Therapy
DrugAdjuvant aromatase inhibitor or tamoxifen initiated within 30 days of SDRT; on-label; recommended 5 years.
Incidence of CTCAE v5.0 Grade ≥2 skin adverse events
Proportion of treated patients with at least one Grade ≥2 skin AE within 90 days, graded per protocol by study research nurse practitioner.
Absence of residual invasive carcinoma on protocol-mandated tissue sampling
Proportion with no residual invasive carcinoma on mandated sampling.
Modified RECIST 1.1 response on multiparametric breast MRI
MRI response by modified RECIST 1.1.
PERCIST 1.0 metabolic response on FDG PET/CT
Metabolic response assessed by PERCIST 1.0.
Late Grade ≥2 skin adverse events
Incidence of late Grade ≥2 skin adverse events after treatment.
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