Commissioning of a commercially available system for intensity-modulated radiotherapy dose delivery with dynamic multileaf collimation.

PURPOSE: To commission commercially available equipment for intensity-modulated radiotherapy (IMRT) using dynamic multileaf collimation (DMLC). MATERIALS AND METHODS: First, the stability in leaf positioning and in realized IMRT profiles on a Varian 2300 C/D machine were determined as a function of time and gantry angle, and as a result of treatment interrupts. Second, dose distributions calculated with the CadPlan (Varian) treatment planning system, using leaf trajectories calculated with the leaf motion calculator (LMC) algorithm, were compared with distributions realized at the 2300 C/D unit. RESULTS: Day-to-day and gantry angle variations in leaf positioning and dose delivery were very small (less than 0.1-0.2 mm and 2%). The effect of treatment interrupts on measured dose distributions was less than 2%. The agreement between the final dose distribution calculated by CadPlan and the measured dose was generally within 2%, or 2 mm at steep dose gradients, using a leaf transmission value of 1.8% and a leaf separation value of 2 mm in LMC. For narrow peaks, deviations of up to 6% were observed. LMC does not synchronize adjacent leaf trajectories resulting in tongue-and-groove underdosages of up to 29% for extreme cases. CONCLUSIONS: The 2300 C/D machine is suitable for accurate and reproducible DMLC treatments. The agreement between dose predictions with LMC and CadPlan, and realized doses at this unit is clinically acceptable for most cases. However, differences between calculated and actual dose values may exist in peaked fluences or due to tongue-and-groove effects. Therefore, pretreatment dosimetric verification for each patient is recommended.

Differentiating the biosynthesis of pseudomonic acids A and B.

Pseudomonic acid A (1) has been the dominant commercial pseudomonate antibiotic produced by Pseudomonas fluorescens. In specific shaken flask conditions initial fermentation accumulation of 1 is followed by preferential accumulation of the 8-hydroxy derivative, pseudomonic acid B (2). Biosynthetic probing with a pulse of [1-14C] acetate or L-[methyl-14C] methionine at early, mid and late stages of the fermentation gave relative patterns of radioactivity in 1 and 2 that are inconsistent with an assumption that 2 arises by oxidation of 1, or that 1 is formed by reduction of 2. Since [methyl-14C] methionine only labels carbons in the 12-carbon part of the pseudomonate molecule that is thought to be an early biosynthetic moiety, the evidence from radiolabelling experiments implies that preferential early oxidation of this biosynthetic intermediate causes the pathway diversion to accumulate 2 instead of 1.