Background: Different computational methods have been used for the prediction of X-ray spectra and beam quality in diagnostic radiology. The purpose of this study was to compare X-ray beam qualities based on half-value layers (HVLs) determined through measurements and computational model estimations. Methods: The HVL estimations calculated by IPEM78 (Spectrum Processor of the Institute of Physics and Engineering in Medicine’s Report 78) and SpekCalc software were compared with those determined through measurements. In this study, the HVLs of both Philips (Phil) (Philips Healthcare, Best, NL) and General Electric Company (GE) (GE Global Research, Niskayuna, US) diagnostic range X-ray machines (50 kVp to 125 kVp) were evaluated. Results: In the HVL estimations, SpekCalc and IPEM78 showed maximum differences of 10% and 9%, respectively, compared with direct measurements. Both models provided means and SDs of HVLs that were within 5% of the HVL measurements of GE and Phil machines. Conclusion: Both computational models provide an alternative method for estimating the HVL of diagnostic range X-ray. These models are user-friendly in predicting HVLs, which are used to characterise the quality of the X-ray beam, and these models provide predictions almost instantly compared with experimental measurements.

Background: Set-up errors are errors which are inevitable in radiotherapy. However, they should be kept to a minimum to achieve the maximum radiation dose to a tumour as to maximise treatment efficacy. This study aims to quantify those errors and assess if they remain within the tolerance limit of 5 mm in all directions. This study will also determine the adequacy of the margins for set up error for 3DCRT of rectal cancers at University of Malaya Medical Centre (UMMC). Methods: A total of 20 rectal cancer patients (July 2018 to May 2019) who were treated with radiotherapy amounting to a total of 119 CBCT images were included in the study. Population systematic errors and random setup errors were calculated. Results: Population systematic errors and random setup errors in the vertical, longitudinal and lateral direction were tabulated in Table 1. There is a large deviation (>5 mm) noted in some patients’ setup between the first 3 days and the next successive day of imaging. Clinical target volume (CTV) to planning target volume (PTV) margin were calculated using Van Herk’s margin recipe (M=2.5Σ+0.7σ). The margins were 5.0 mm, 6.2 mm, and 4.0 mm for vertical, longitudinal and lateral directions, respectively. The systematic error for the population was 1.1 mm, 0.9 mm, 0.9 mm in the vertical, longitudinal and lateral directions respectively, while the random error is 3.2 mm, 5.7 mm and 2.5 mm in the vertical, longitudinal and lateral directions respectively. Conclusion All of the patients involved in the study were within tolerance limits at some point in their treatment. The results demonstrated that a larger margin is needed in the longitudinal direction. Weekly CBCT is also necessary after the initial 3-day imaging to ensure that patients are kept within the tolerance limits.
Radiotherapy