Evaluating the Hounsfield unit assignment and dose differences between CT-based standard and deep learning-based synthetic CT images for MRI-only radiation therapy of the head and neck.

BACKGROUND: Magnetic resonance image only (MRI-only) simulation for head and neck (H&N) radiotherapy (RT) could allow for single-image modality planning with excellent soft tissue contrast. In the MRI-only simulation workflow, synthetic computed tomography (sCT) is generated from MRI to provide electron density information for dose calculation. Bone/air regions produce little MRI signal which could lead to electron density misclassification in sCT. Establishing the dosimetric impact of this error could inform quality assurance (QA) procedures using MRI-only RT planning or compensatory methods for accurate dosimetric calculation. PURPOSE: The aim of this study was to investigate if Hounsfield unit (HU) voxel misassignments from sCT images result in dosimetric errors in clinical treatment plans. METHODS: Fourteen H&N cancer patients undergoing same-day CT and 3T MRI simulation were retrospectively identified. MRI was deformed to the CT using multimodal deformable image registration. sCTs were generated from T1w DIXON MRIs using a commercially available deep learning-based generator (MRIplanner, Spectronic Medical AB, Helsingborg, Sweden). Tissue voxel assignment was quantified by creating a CT-derived HU threshold contour. CT/sCT HU differences for anatomical/target contours and tissue classification regions including air (<250 HU), adipose tissue (-250 HU to -51 HU), soft tissue (-50 HU to 199 HU), spongy (200 HU to 499 HU) and cortical bone (>500 HU) were quantified. t-test was used to determine if sCT/CT HU differences were significant. The frequency of structures that had a HU difference > 80 HU (the CT window-width setting for intra-cranial structures) was computed to establish structure classification accuracy. Clinical intensity modulated radiation therapy (IMRT) treatment plans created on CT were retrospectively recalculated on sCT images and compared using the gamma metric. RESULTS: The mean ratio of sCT HUs relative to CT for air, adipose tissue, soft tissue, spongy and cortical bone were 1.7 ± 0.3, 1.1 ± 0.1, 1.0 ± 0.1, 0.9 ± 0.1 and 0.8 ± 0.1 (value of 1 indicates perfect agreement). T-tests (significance set at t = 0.05) identified differences in HU values for air, spongy and cortical bone in sCT images compared to CT. The structures with sCT/CT HU differences > 80 HU of note were the left and right (L/R) cochlea and mandible (>79% of the tested cohort), the oral cavity (for 57% of the tested cohort), the epiglottis (for 43% of the tested cohort) and the L/R TM joints (occurring > 29% of the cohort). In the case of the cochlea and TM joints, these structures contain dense bone/air interfaces. In the case of the oral cavity and mandible, these structures suffer the additional challenge of being positionally altered in CT versus MRI simulation (due to a non-MR safe immobilizing bite block requiring absence of bite block in MR). Finally, the epiglottis HU assignment suffers from its small size and unstable positionality. Plans recalculated on sCT yielded global/local gamma pass rates of 95.5% ± 2% (3 mm, 3%) and 92.7% ± 2.1% (2 mm, 2%). The largest mean differences in D95, Dmean , D50 dose volume histogram (DVH) metrics for organ-at-risk (OAR) and planning tumor volumes (PTVs) were 2.3% ± 3.0% and 0.7% ± 1.9% respectively. CONCLUSIONS: In this cohort, HU differences of CT and sCT were observed but did not translate into a reduction in gamma pass rates or differences in average PTV/OAR dose metrics greater than 3%. For sites such as the H&N where there are many tissue interfaces we did not observe large scale dose deviations but further studies using larger retrospective cohorts are merited to establish the variation in sCT dosimetric accuracy which could help to inform QA limits on clinical sCT usage.

Dose-volume thresholds and smoking status for the risk of treatment-related pneumonitis in inoperable non-small cell lung cancer treated with definitive radiotherapy.

PURPOSE: To identify clinical risk factors and dose-volume thresholds for treatment-related pneumonitis (TRP) in patients with non-small cell lung cancer (NSCLC). METHODS AND MATERIALS: Data were retrospectively collected from patients with inoperable NSCLC treated with radiotherapy with or without chemotherapy. TRP was graded according to Common Terminology Criteria for Adverse Events, version 3.0, with time to grade > or = 3 TRP calculated from start of radiotherapy. Clinical factors and dose-volume parameters were analyzed for their association with risk of TRP. RESULTS: Data from 576 patients (75% with stage III NSCLC) were included in this study. The Kaplan-Meier estimate of the incidence of grade > or = 3 TRP at 12 months was 22%. An analysis of dose-volume parameters identified a threshold dose-volume histogram (DVH) curve defined by V(20) < or = 25%, V(25) < or = 20%, V(35) < or = 15%, and V(50) < or = 10%. Patients with lung DVHs satisfying these constraints had only 2% incidence of grade > or = 3 TRP. Smoking status was the only clinical factor that affected the risk of TRP independent of dosimetric factors. CONCLUSIONS: The risk of TRP varied significantly, depending on radiation dose-volume parameters and patient smoking status. Further studies are needed to identify biological basis of smoking effect and methods to reduce the incidence of TRP.

Bladder-conserving surgery and interstitial brachytherapy for lymph node negative transitional cell carcinoma of the urinary bladder: results of a 28-year single institution experience.

BACKGROUND AND PURPOSE: We retrospectively analyzed results for lymph node negative transitional cell carcinoma of the bladder treated with brachytherapy. PATIENTS AND METHODS: From 1975-2002, 58 patients received preoperative external irradiation, partial cystectomy (in 69%), iliac node dissection, and iridium-192. Pathologic stage was: 10 pT1, 41 pT2, and 7 pT3. A median total brachytherapy dose of 60 Gy was delivered to the tumor bed. RESULTS: Mean follow-up was 76 months (range, 0.5-296). Tumor stage significantly impacted cause-specific and disease-free survival (P=0.02). Eight pT1 patients were free of disease and 2 died of other cause. For pT2 patients, 5-year cause-specific and overall survival rates were, respectively, 70% (CI 95%: 53-87) and 60% (CI 95%: 43-77). Three pT3 patients died of cancer. For the pT2 patients, the probability of 5-year local control was 65% (CI 95%: 47-83) and being alive without disease with a functional bladder, 50% (CI 95%: 33-67). Previous transurethral resection (TUR) increased the bladder relapse risk among pT2 patients (P=0.03). Twelve patients had severe acute complications and 5 had severe late effects. A high dose of external irradiation increased risk of late complications (P=0.01). Most complications occurred in patients treated before 1985. CONCLUSIONS: Highly select patients presenting with pT2 tumors less than 5 cm with no history of previous TUR may be successfully treated with low-dose external irradiation, limited partial cystectomy, and interstitial brachytherapy. High-risk pT1 patients may also benefit. Postoperative complications and late side effects are minimized with modern management. We recommend lifelong cystoscopic surveillance, with prompt surgical salvage for recurrence.