A Model for Secondary Monitor Unit Calculations of PBS Proton Therapy Treatment Plans.

PURPOSE: This article summarizes a volume-based method by which secondary monitor unit (MU) calculations may be performed for pencil beam scanning, single field uniform dose (SFUD) proton therapy treatment plans. MATERIALS AND METHODS: Treatment planning system (TPS) simulations were performed by using the local beam model to define relationships between planning target volume (PTV) characteristics and the MUs required to deliver a uniform dose for a given beam orientation. Relevant target attributes included volume, depth (ie, beam range), range-shifter air gap, and the projected area of the target volume in the beam's eye view (BEV). The proposed model approximates the PTV as a simplified cuboid region of interest as defined by its volume and BEV projected area. Output factors (cGy/MU) were then tabulated for the idealized geometry through TPS simulations using region of interests with a range of dimensions expected to be seen clinically. Correction factors were applied that account for differences between the PTV and the idealized conditions, and MUs for each beam were then scaled according to the measured spread out Bragg peak (SOBP) dose in water. RESULTS: Our model was applied to various treatment sites, including pelvis, brain, lung, and head and neck. Monitor units prescribed by the TPS were compared to those predicted by using the model for 78 treatment beams. The total mean percentage difference for all beams was -0.2% ± 3.8%. CONCLUSION: This work demonstrates the potential for reasonably accurate secondary verification of MUs in pencil beam scanning proton therapy for SFUD treatment plans with the proposed method. Required inputs are few, and are readily accessible, facilitating automation and clinical application. Further investigation will expand the current model to accommodate a broader range of potential optimization problems, and intensity-modulated proton therapy treatment plans.

Quantifying the effect of air gap, depth, and range shifter thickness on TPS dosimetric accuracy in superficial PBS proton therapy.

This study quantifies the dosimetric accuracy of a commercial treatment planning system as functions of treatment depth, air gap, and range shifter thickness for superficial pencil beam scanning proton therapy treatments. The RayStation 6 pencil beam and Monte Carlo dose engines were each used to calculate the dose distributions for a single treatment plan with varying range shifter air gaps. Central axis dose values extracted from each of the calculated plans were compared to dose values measured with a calibrated PTW Markus chamber at various depths in RW3 solid water. Dose was measured at 12 depths, ranging from the surface to 5 cm, for each of the 18 different air gaps, which ranged from 0.5 to 28 cm. TPS dosimetric accuracy, defined as the ratio of calculated dose relative to the measured dose, was plotted as functions of depth and air gap for the pencil beam and Monte Carlo dose algorithms. The accuracy of the TPS pencil beam dose algorithm was found to be clinically unacceptable at depths shallower than 3 cm with air gaps wider than 10 cm, and increased range shifter thickness only added to the dosimetric inaccuracy of the pencil beam algorithm. Each configuration calculated with Monte Carlo was determined to be clinically acceptable. Further comparisons of the Monte Carlo dose algorithm to the measured spread-out Bragg Peaks of multiple fields used during machine commissioning verified the dosimetric accuracy of Monte Carlo in a variety of beam energies and field sizes. Discrepancies between measured and TPS calculated dose values can mainly be attributed to the ability (or lack thereof) of the TPS pencil beam dose algorithm to properly model secondary proton scatter generated in the range shifter.

Commissioning of the world's first compact pencil-beam scanning proton therapy system.

This paper summarizes clinical commissioning of the world's first commercial, clinically utilized installation of a compact, image-guided, pencil-beam scanning, intensity-modulated proton therapy system, the IBA Proteus® ONE, at the Willis-Knighton Cancer Center (WKCC) in Shreveport, LA. The Proteus® ONE is a single-room, compact-gantry system employing a cyclotron-generated proton beam with image guidance via cone-beam CT as well as stereoscopic orthogonal and oblique planar kV imaging. Coupling 220° of gantry rotation with a 6D robotic couch capable of in plane patient rotations of over 180° degrees allows for 360° of treatment access. Along with general machine characterization, system commissioning required: (a) characterization and calibration of the proton beam, (b) treatment planning system commissioning including CT-to-density curve determination, (c) image guidance system commissioning, and (d) safety verification (interlocks and radiation survey). System readiness for patient treatment was validated by irradiating calibration TLDs as well as prostate, head, and lung phantoms from the Imaging and Radiation Oncology Core (IROC), Houston. These results confirmed safe and accurate machine functionality suitable for patient treatment. WKCC also successfully completed an on-site dosimetry review by an independent team of IROC physicists that corroborated accurate Proteus® ONE dosimetry.

Helical image-guided stereotactic body radiotherapy (SBRT) for the treatment of early-stage lung cancer: a single-institution experience at the Willis-Knighton Cancer Center.

AIMS AND BACKGROUND: Our aim is to report on the clinical methods and outcomes of helical intensity-modulated stereotactic body radiotherapy (SBRT) for the treatment of early-stage non-small cell lung cancer (NSCLC). METHODS AND STUDY DESIGN: Seventy-nine patients with stage I NSCLC underwent helical SBRT with 48 Gy in 4 fractions or 60 Gy in 5 fractions. All patients underwent 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) or FDG-PET/computed tomography (CT) scanning in the immobilized treatment position for planned fusion with a separate kilovoltage (KV) CT simulation prior to treatment. Megavoltage CT images were obtained on the treatment unit prior to therapy and repeated at mid-fraction with comparison and fusion to the KV CT simulation planning images to assure setup accuracy. Serial follow-up with FDG-PET or FDG-PET/CT was performed at 3-4 months and every 6 months thereafter. RESULTS: Median follow-up was 27 months (range, 4-82 months). Overall local control rate (LCR) was 93.6% (95% confidence interval [CI], 86.0-97.3%) and 3-year overall survival (OS) was 58.4% (95% CI, 47.2-69.5%). For patients with T1N0M0 disease (n = 59) the LCR was 94.9% (95% CI, 86.1-98.3%) and the 3-year OS was 62.8% (95% CI, 49.9-73.9%). Patients treated with 60 Gy had longer 3-year OS than patients treated with 48 Gy (65.2% vs 37.5%; P = 0.044). SBRT-related toxicity was modest, with 10 patients developing grade 1/2 chest wall toxicity based on the Common Terminology Criteria for Adverse Events (CTCAE). CONCLUSION: Image-guided SBRT with helical IMRT delivered in 4 or 5 fractions of 12 Gy with rigid immobilization, FDG-PET-assisted targeting, and repeat mid-fraction CT scan is an effective treatment for early NSCLC.

Influence of patient characteristics on survival following treatment with helical stereotactic body radiotherapy (SBRT) in stage I non-small-cell lung cancer.

BACKGROUND: To determine the influence of patient and tumor characteristics on clinical outcomes in patients with early-stage non-small cell lung cancer (NSCLC) treated with helical intensity modulated stereotactic body radiotherapy (SBRT). METHODS: From March 2005 to August 2010 a total of 62 patients with biopsy proven Stage I NSCLC underwent helical SBRT with 48 Gy in 4 fractions or 60 Gy in 5 fractions. Patient and tumor characteristics including tumor stage, age, sex, tumor histology, maximal tumor diameter, and smoking history, were evaluated in regard to local control and overall survival using Kaplan-Meier survival curves and the Cox proportional hazard method. Treatment related toxicity in the patient subgroups was evaluated. RESULTS: The median follow-up was 28 months. Total cohort local control was 93.55% and 3-year overall survival (OS) was 53.4%. Those patients with Stage IA disease had a 3-year OS of 64.4% versus 32.1% for Stage IB disease (P = 0.042). Tumors classified as T1a (≤20 mm) and T1b (20-30 mm) had significantly increased overall survival compared to T2 (>30 mm) tumors (P = 0.046). There was a slight survival advantage in those patients with adenocarcinoma. No correlation between age, gender or smoking history, and overall survival was found. Nine patients had radiation related toxicity, which was increasingly more common with advancing age. CONCLUSION: Helical SBRT is an effective method to treat NSCLC and the most significant prognostic factors were tumor stage and size. There was no correlation between age, gender, and smoking history.