Our understanding of radiation-induced cellular damage has greatly improved over the past few decades. Despite this progress, there are still many obstacles to fully understand how radiation interacts with biologically relevant cellular components, such as DNA, to cause observable end points such as cell killing. Damage in DNA is identified as a major route of cell killing. One hurdle when modeling biological effects is the difficulty in directly comparing results generated by members of different research groups. Multiple Monte Carlo codes have been developed to simulate damage induction at the DNA scale, while at the same time various groups have developed models that describe DNA repair processes with varying levels of detail. These repair models are intrinsically linked to the damage model employed in their development, making it difficult to disentangle systematic effects in either part of the modeling chain. These modeling chains typically consist of track-structure Monte Carlo simulations of the physical interactions creating direct damages to DNA, followed by simulations of the production and initial reactions of chemical species causing so-called “indirect” damages. After the induction of DNA damage, DNA repair models combine the simulated damage patterns with biological models to determine the biological consequences of the damage. To date, the effect of the environment, such as molecular oxygen (normoxic vs. hypoxic), has been poorly considered. We propose a new standard DNA damage (SDD) data format to unify the interface between the simulation of damage induction in DNA and the biological modeling of DNA repair processes, and introduce the effect of the environment (molecular oxygen or other compounds) as a flexible parameter. Such a standard greatly facilitates inter-model comparisons, providing an ideal environment to tease out model assumptions and identify persistent, underlying mechanisms. Through inter-model comparisons, this unified standard has the potential to greatly advance our under-standing of the underlying mechanisms of radiation-induced DNA damage and the resulting observable biological effects when radiation parameters and/or environmental conditions change.

No abstract

A new calibration protocol, developed by the AAPM Task Group 51 (TG‐51) to replace the TG‐21 protocol, is based on an absorbed‐dose to water standard and calibration factor (ND,w), while the TG‐21 protocol is based on an exposure (or air‐kerma) standard and calibration factor (Nx). Because of differences between these standards and the two protocols, the results of clinical reference dosimetry based on TG‐51 may be somewhat different from those based on TG‐21. The Radiological Physics Center has conducted a systematic comparison between the two protocols, in which photon and electron beam outputs following both protocols were compared under identical conditions. Cylindrical chambers used in this study were selected from the list given in the TG‐51 report, covering the majority of current manufacturers. Measured ratios between absorbed‐dose and air‐kerma calibration factors, derived from the standards traceable to the NIST, were compared with calculated values using the TG‐21 protocol. The comparison suggests that there is roughly a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed‐dose to water determined by TG‐51 and that determined by TG‐21 for photon beam calibrations. The typical change in a 6 MV photon beam calibration following the implementation of the TG‐51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV photon beam. On the other hand, the results for 9 and 16 MeV electron beams show larger changes up to 2%, perhaps because of the updated electron stopping power data used for the TG‐51 protocol, in addition to the inherent 1% discrepancy presented in the calibration factors. The results also indicate that the changes may be dependent on the electron energy.PACS number(s): 87.66.–a, 87.53.–j

The dosimetric characteristics for modern computer‐controlled linear accelerators with the same make, model, and nominal energy are known to be very similar, as long as the machines are unaltered from the manufacturer's original specifications. In this preliminary study, a quantitative investigation of the similarity in the basic photon dosimetry data from the Siemens Primus linear accelerators at eight different institutions is reported. The output factor, percentage depth dose (PDD), and in‐air off‐axis factor (OAF) for the 6 and 18 MV photon beams measured or verified by the Radiological Physics Center (RPC) were analyzed. The RPC‐measured output factors varied by less than about 2% for each field size. The difference between the maximum and minimum RPC‐verified PDD values at each depth was less than about 3%. The difference between the maximum and minimum RPC‐measured in‐air OAF was no more than 4% at all off‐axis distances considered in this study. These results strongly suggest that it is feasible to establish a reference photon dosimetry data set for each make, model, and nominal energy, universally applicable to those machines unaltered from the manufacturers' original specifications, within a clinically acceptable tolerance (e.g., ∼±2%).PACS number(s): 87.53.–j, 87.66.–a

The dosimetric characteristics for modern computer‐controlled linear accelerators with the same make, model, and nominal energy are known to be very similar, as long as the machines are unaltered from the manufacturer's original specifications. In this preliminary study, a quantitative investigation of the similarity in the basic photon dosimetry data from the Siemens Primus linear accelerators at eight different institutions is reported. The output factor, percentage depth dose (PDD), and in‐air off‐axis factor (OAF) for the 6 and 18 MV photon beams measured or verified by the Radiological Physics Center (RPC) were analyzed. The RPC‐measured output factors varied by less than about 2% for each field size. The difference between the maximum and minimum RPC‐verified PDD values at each depth was less than about 3%. The difference between the maximum and minimum RPC‐measured in‐air OAF was no more than 4% at all off‐axis distances considered in this study. These results strongly suggest that it is feasible to establish a reference photon dosimetry data set for each make, model, and nominal energy, universally applicable to those machines unaltered from the manufacturers' original specifications, within a clinically acceptable tolerance (e.g., ∼±2%).PACS number(s): 87.53.–j, 87.66.–a