Relative stopping powers (RSP) for proton therapy are estimated using single-energy CT (SECT), calibrated with standardised tissues of the adult male. It is assumed that those tissues are representative of tissues of all age and sex. Female, male and paediatric tissues differ from one another in density and composition. In this study, we use tabulated paediatric tissues and computational phantoms to investigate the impact of this assumption on paediatric proton therapy.The potential of dual-energy CT (DECT) to improve the accuracy of these calculations is explored. Methods:We study 51 human body tissues, categorised into male/female for the age groups newborn, 1-, 5-, 10-, 15-year old and adult, with given compositions and densities. CT numbers are simulated and RSPs are estimated using SECT and DECT methods. Estimated tissue RSPs from each method are compared to theoretical RSP. The dose and range errors of each approach is evaluated on 3 computational phantoms (Ewing's sarcoma, salivary sarcoma, glioma) derived from paediatric proton therapy patients.Results: With SECT, soft tissues have mean estimation errors and standard deviation up to (1.96 ± 4.18)% observed in newborns, compared to (0.20 ± 1.15)% in adult males. Mean estimation errors for bones are up to (-3.35 ± 4.76)% in paediatrics as opposed to (0.10 ± 0.66)% in adult males. With DECT, mean errors reduce to (0.17 ± 0.13)% and (0.23 ± 0.22)% in newborns (soft tissues/bones). With SECT, dose errors in a Ewing's sarcoma phantom are exceeding 5 Gy (10% of prescribed dose) at the distal end of the treatment field, with volumes of dose errors >5 Gy of V diff>5 = 4630.7 mm 3 . Similar observations are made in the head and neck phantoms, with overdoses Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

The aim of this work is to establish if the new CT‐based total body irradiation (TBI) planning techniques used at University College London Hospital (UCLH) and Royal Free Hospital (RFH) are comparable to the previous technique at the Middlesex Hospital (MXH) by analyzing predicted and measured diode results. TBI aims to deliver a homogeneous dose to the entire body, typically using extended SSD fields with beam modulation to limit doses to organs at risk. In vivo dosimetry is used to verify the accuracy of delivered doses. In 2005, when the Middlesex Hospital was decommissioned and merged with UCLH, both UCLH and the RFH introduced updated CT‐planned TBI techniques, based on the old MXH technique. More CT slices and in vivo measurement points were used by both; UCLH introduced a beam modulation technique using MLC segments, while RFH updated to a combination of lead compensators and bolus. Semiconductor diodes were used to measure entrance and exit doses in several anatomical locations along the entire body. Diode results from both centers for over five years of treatments were analyzed and compared to the previous MXH technique for accuracy and precision of delivered doses. The most stable location was the field center with standard deviations of 4.1% (MXH), 3.7% (UCLH), and 1.7% (RFH). The least stable position was the ankles. Mean variation with fraction number was within 1.5% for all three techniques. In vivo dosimetry can be used to verify complex modulated CT‐planned TBI, and demonstrate improvements and limitations in techniques. The results show that the new UCLH technique is no worse than the previous MXH one and comparable to the current RFH technique.PACS numbers: 87.55.Qr, 87.56.N‐

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accuracy with realistic patient motion. Sample RMTC plans for 10 patients were created with patients assessed for breathing patterns and targets assessed for location, size, motion and contrast to make patient selection criteria. These plans were delivered under RMTC to various phantoms placed on motion platforms. LED placement was also tested. The measured 3D doses were compared with planned doses using gamma analysis with the AAPM TG218 recommended criteria. In addition, 3D dose distributions were reconstructed on treatment MVCT images using an adaptive RT tool. Results: The performance of the RMTC system was within specifications. The gamma passing rates of 3D dose measurements were all within tolerance. Mean doses to PTV and organs-at-risk for reconstructed doses based on MVCT were within 1% of planned doses. The typical RMTC imaging dose was <1 cGy. Optimal target location and size were determined. Target motions up to 3 cm were tracked accurately. Implanted fiducials separated by >5mm with HU>1500 were easily identifiable. Lung tumors with HU>600 relative to those of surrounding lung were easily detectable with kV radiographs. LEDs must be placed on stable locations with representative motion. While some tracking parameters were found to be patient independent, target searching window and target offset need to be determined based on patient 4DCT. Based on these tests, we defined roles of each team member and developed workflows and checklists for the clinical use of the RMTC system. Conclusion: Performance of the first clinical motion tracking and correction system for helical tomotherapy delivery is within specifications. Practical workflows and checklists were developed. Ideal patient selection parameters for lung tumor treatment (without fiducials) and ideal fiducial geometry for other cases were defined. The system has now been successfully implemented in our clinic for patient treatments.