Patterns of Local Recurrence for Mesothelioma Patients Treated With Extrapleural Pneumonectomy and Radiation Therapy Using the Electron-Photon Technique: The Dosimetric Challenges of Electron-Photon Match Lines and Blocks

Abstract

Hemi-thoracic radiation therapy (RT) after extra-pleural pneumonectomy (EPP) appears to reduce local recurrence (LR). The large target volume and proximity of critical organs make RT planning challenging. The electron-photon technique (EPT) developed at Memorial Sloan-Kettering Cancer Center involves photon and electron fields matched in an attempt to deliver uniform dose to areas at risk. Dose inhomogeneities may occur at these electron-photon match lines and under blocks. The purpose of this retrospective study was to describe patterns of LR relative to electron-photon match lines and block placement in patients treated with the EPT. From 2003-2009, 34 patients with non-metastatic mesothelioma underwent EPP and adjuvant RT via the EPT. RT consisted of AP-PA photon fields to the hemithorax and mediastinum. Matched electron fields treated the chest wall under blocked abdominal and cardiac regions. Abdominal blocks were added at the start of RT; heart blocks were added for left-sided cases at 19.8 Gy; spinal cord blocks were added at 39.6 Gy. Prescribed dose was 54 Gy. CT scans with LRs were registered with planning CT scans to determine LR locations relative to match lines, blocks, and dosimetry. Twenty-eight patients were male (82%). Median age was 56.5 years. Histology was epithelial for 19 patients (56%) and mixed/biphasic for 15 (44%). Median dose was 54 Gy (range: 45-54). Median follow-up was 24.6 months; median time to LR was 14.8 months. Fifteen patients (44%) had LRs in 38 local sites; ten of these patients (67%) had additional distant failures. The most common LR sites were lateral costophrenic sulcus (53%) and mediastinum (40%). Twenty-one LRs (55%) occurred under a block, six (16%) in the full-dose photon field abutting a block, six (16%) out-of-field, and five (13%) in the full-dose photon field (Table). Twelve LRs (32%) were associated with a match line in areas where the actual dose (median 42.5 Gy) was significantly less than the nominal prescribed dose of 54 Gy (p = 0.003). LRs under heart or abdominal blocks also occurred in under-dosed locations (median actual dose 45 Gy vs prescribed dose 54 Gy, p = 0.05). The majority of LRs after hemithoracic RT using EPT occurred at match lines and/or under blocks in under-dosed areas. These limitations of EPT should be considered when selecting the adjuvant RT technique most appropriate for individual patients following EPP.Scientific Abstract 3010; TableNominal Prescribed Dose vs Median Actual Dose, by Location of LRs Relative to EPTLocation of LRNumber of LRs (%)Nominal Prescribed Dose (Gy)Median Actual Dose (Range) from Graphic Plan (Gy)P valueIn full-dose photon field5 (13)5454 (54)1.0In full-dose photon field abutting block (at match line)6 (16)5447.5 (30-54).08Abutting spinal cord block15445 (45)-Abutting heart or abdominal block55452 (30-54)-Under block21 (55)39.6-5439.6 (15-54)-Under spinal cord block1039.639.6 (30-50).45Under heart or abdominal block55445 (36-54).05Under abdominal block at match line65436 (15-50).02Out of field inferiorly, superior to L2 (inferior field borders for these patients: T11 and T12)6 (16)0<5 (<5-10)1.0 Open table in a new tab