KEY POINTS
- This single-institution retrospective series included 31 consecutive patients treated with pencil-beam scanning IMPT-based hemithoracic pleural irradiation between 2019 and 2025. Histologies were malignant pleural mesothelioma in 15 patients (48%), thymic malignancies with pleural metastases in 12 (39%), and thoracic soft-tissue sarcoma in 4 (13%); 97% had ECOG 0–1.
- The median whole-pleural prescription was 50.4 GyRBE in 28 fractions. Gross disease was boosted to a median 59.4 GyRBE, using a simultaneous integrated boost in 10 patients and sequential boost in three; median CTV was 2,128 cm³, with median CTV V95 of 98%.
- Proton therapy produced very low contralateral lung exposure despite treating an entire hemithorax. Median contralateral lung mean dose was 0.3 Gy, V5 1.2%, and V20 approximately 0%; total-lung mean dose was 15.3 Gy and total-lung V20 34.7%. Historical photon IMPRINT series cited by the authors reported contralateral lung mean doses around 4–6 Gy and V5 values ranging roughly 14–52%.
- Other organs also showed substantial sparing. Median mean heart dose was 14.2 Gy, including 11.6 Gy for right-sided and 15.6 Gy for left-sided disease; corresponding historical photon values cited in the paper were approximately 22–32 Gy. Mean esophageal dose was 20.6 Gy, liver dose 16.9 Gy, and spinal-cord Dmax 39.3 Gy.
- Seven patients (23%) had previously received thoracic radiotherapy. Prior plans were registered to the current CT and cumulative EQD2 was considered when available; 2 of these 7 patients developed pneumonitis, but previous thoracic irradiation was not associated with significantly greater toxicity in this small cohort.
- At median follow-up of 13.8 months, 8/31 patients (26%) developed grade ≥2 pneumonitis, including 2/31 (6%) with grade 3 pneumonitis. Median onset was 2.2 months, there were no grade 4–5 pneumonitis events, and no other acute or late grade ≥3 toxicities were observed.
- No evaluated dosimetric or treatment parameter significantly predicted pneumonitis. The apparent advantage over historical photon series therefore remains primarily dosimetric, because the study was small, histologically heterogeneous, nonrandomized, and not designed to establish whether lower organ doses translate into lower pulmonary or late cardiac morbidity.
CLINICAL TAKEAWAY
IMPT can make one of thoracic radiotherapy's most difficult geometries remarkably clean dosimetrically, particularly by almost eliminating low-dose spill into the opposite lung while preserving pleural coverage. The clinical toxicity profile is encouraging, but whether those dosimetric gains translate into meaningfully safer treatment than modern photon IMPRINT requires prospective comparison.
SOURCE
International Journal of Radiation Oncology, Biology, Physics