SU-E-T-490: Comparison of XVMC Monte Carlo Dose Calculations with Eclipse AAA Calculations for RapidArc Plans

PURPOSE

The consistency between the AAA and XVMC algorithm in the treatment planning for RapidArc is investigated. While the majority of the radiation field is blocked by the MLC system, multiple small dose islands with MLC opened only slightly can be observed in one control point. This raises questions on how accurate the clinically used AAA algorithm in Eclipse is able to calculate RapidArc dose distributions. The fast Monte Carlo Code XVMC was used as a benchmark to test the AAA algorithm.

METHODS

RadpidArc plans of 25 patients were calculated with AAA and XVMC. The patient cohort consisted of 4 different cancer sites (H&N, upper abdominal, lung, prostate). Dose distributions, PTV and OAR coverage were compared looking at the PTV mean dose Dmean, the volume V95% of the PTV receiving 95% of the prescribed dose, the dose D95% delivered to 95% of the PTV Volume, the percentage PTV mean dose with respect to the prescribed dose Dmean/prescr and OAR mean dose.

RESULTS

The recalculation of RapidArc plans yielded good agreement of both calculation algorithms for treatment plans of all four cancer sites. PTV mean dose differences of AAA and XVMC were found to be in between -0.11% and 4.89% of the prescribed dose. The mean dose difference found was 0.48±0.77 Gy. Local dose differences were found when comparing dose distributions in regions of big mass density differences and in high dose regions. One head and neck plan and one prostate plan revealed significant differences in PTV coverage (ΔDmean=3.25 Gy) and OAR mean dose (prostate mean dose -13.71 Gy) respectively.

CONCLUSIONS

The vast majority of treatment plans calculated with the AAA algorithm were found to agree within the expected and acceptable tolerances compared to XVMC results. Nevertheless in some cases dose differences were observed that could be of clinical significance. This work was funded by a Varian grant. Wolfram Laub is working in the physics group of CMS.

[Neonatal intracerebral bleeding as initial symptom of gliomatosis cerebri WHO III--neurological outcome after partial hemispherectomy]

INTRODUCTION

Anaplastic astrocytomas presenting as gliomatosis cerebri in neonates are extremely rare. Tumours in newborns are mostly of neuroectodermal origin.

CASE REPORT

We report about a female newborn at term [birth weight 3 600 g (P 90), head circumference 35 cm (P 95) APGAR 9/10/10] with an intracerebral partially clotted bleeding in the left parieto-occipital region. The bleeding was expansive leading to axial and lateral cerebral herniation. The intracerebral bleeding in the left occipital region was surgically removed. Macroscopically no solid tumour was seen, but small fragments of an anaplastic astrocytic tumour (WHO grade III) were diagnosed histologically. After surgery, no remaining tumour was visible in the MRI. 6 weeks later, a recurrent tumour (4×4 cm) was found in the area of the initial bleeding. Further treatment was initially refused by the parents. The child was readmitted to our hospital at the age of 11 months in good clinical condition and presented with left-sided hemiparesis, right-sided hemianopsia and intermittent strabismus convergens alternans. Because of the good clinical condition further therapeutic treatment was initiated. Due to the final extension of the tumour into the temporal, parietal and occipital lobes, a gliomatosis cerebri WHO III was diagnosed. An extended partial hemispherectomy was done. After neurosurgery, no further neurological failures occurred. In the follow-up examination, MRI showed no relapse of the tumour. Chemotherapy according to the HIT SKK protocol was initiated. A relapse did not occur over a follow-up of 2 years.

CONCLUSION

This is a rare case report of a congenital gliomatosis cerebri WHO grade III, treated with partial hemispherectomy, leading to a good clinical and neurological long-term outcome.

Estimation of protein and domain interactions in the switching motility system of Myxococcus xanthus

The gram-negative myxobacterium Myxococcus xanthus is equipped with an interesting motility system that allows it to reverse direction on average every 8 minutes by switching the construction of two motility engines at the ends of this rod-shaped bacterium. While the mechanisms responsible for timing and engine construction/deconstruction are relatively well understood, there are several competing hypotheses as to how they are coupled together. In this paper we examine the evidence for protein interactions underlying these possible couplings using a novel framework consisting of a probabilistic model describing protein and domain interactions and a belief propagation inference algorithm. When provided with large amount of indirect pieces of information, such as high-throughput experiment results, and protein structures, we can reliably determine the relative likelihoods of these hypotheses, even though each individual piece of evidence by itself has very limited reliability. The same framework can be used to map large protein and domain interaction networks in myxobacteria and other organisms.

A virtual source model of electron contamination of a therapeutic photon beam

The most efficient way of generating particles for Monte Carlo (MC) dose calculation is through a virtual source model (VSM) of the linear accelerator head. We have previously developed a VSM based on three sources: a primary photon source, a secondary photon source and an electron contamination source (Sikora et al 2007). In this work, we present an improvement of the electron contamination source. The VSM of contamination electrons (eVSM) is derived from a full MC simulation of the accelerator head with the BEAMnrc MC system. It comprises a Gaussian source located at the base of the flattening filter. The eVSM models two effects: an energy-dependent source diameter and an angular dependence of the particle fluence. The air scatter of the contamination electrons is approximated by energetic properties of the eVSM so that explicit in-air transport is not required during MC simulation of the dose distributions in the patient. The calculations of electron dose distributions were compared between the eVSM and the full MC simulation. Good agreement was achieved for various rectangular field sizes as well as for complex conformal segment shapes for the contamination electrons of 6 and 15 MV beams. The 3D dose evaluation of the surface dose in a CT-based patient geometry shows high accuracy (2%/2 mm) of the eVSM for both energies. The model has one tunable parameter, the mean energy of the spectrum at the patient surface. High accuracy and efficiency of particle generation make the eVSM a valuable virtual source of contamination electrons for MC treatment planning systems.