Segment-Targeted Precision Ablative Radiation Therapy With Exact Delivery (SPARE): A Novel Anatomically Guided Stereotactic Body Radiation Therapy Technique.

Stereotactic body radiation therapy (SBRT) for early-stage non-small cell lung cancer achieves excellent local control but may cause radiation pneumonitis in uninvolved lung segments. Conventional SBRT planning using ring-shaped optimization structures enhances conformity but disregards segmental anatomy, allowing low-dose spillage into adjacent intact lung. We developed Segment-targeted Precision Ablative Radiotherapy with Exact delivery (SPARE), an anatomically-guided SBRT technique that confines radiation exposure to the tumor-bearing bronchopulmonary segment, essentially performing a "radiation segmentectomy." SPARE integrates three complementary strategies: segment-specific beam orientation that avoids adjacent uninvolved segments, optimization constraints that penalize dose to neighboring segments, and high-gradient dose shaping with Monte Carlo or Acuros calculation for precision near tissue interfaces. The tumor-bearing segment is identified using high-resolution CT to trace bronchial branches and vascular landmarks, defining natural intersegmental boundaries. A representative case of left upper lobe adenocarcinoma located in the S1+2b subsegment treated with 56 Gy in 4 fractions demonstrated successful dose confinement to the target segment while sparing adjacent S3. Grade 1 radiation pneumonitis at 4 months was localized exclusively to the treated subsegment, with compensatory expansion observed in uninvolved areas, confirming predictable toxicity localization. SPARE offers functional preservation through anatomically-guided dose containment, transforms diffuse lung injury into localized injury, and enables safer retreatment situations. This technique is executable with standard SBRT equipment and compatible with conventional fractionation schemes. This concept extends beyond lung to other segmented organs like liver and kidney, where anatomical units can be targeted while preserving overall organ function. SPARE represents a paradigm shift in SBRT planning that aligns radiation therapy with surgical principles of anatomical preservation while maintaining oncologic efficacy.