Clinical evaluation of X-ray voxel Monte Carlo versus pencil beam-based dose calculation in stereotactic body radiotherapy of lung cancer under normal and deep inspiration breath hold.

The purpose of this study is to evaluate the differences between dose distributions calculated with the pencil beam (PB) and X-ray voxel Monte Carlo (MC) algorithms for patients with lung cancer using intensity-modulated radiotherapy (IMRT) or HybridArc techniques. The 2 algorithms were compared in terms of dose-volume histograms, under normal and deep inspiration breath hold, and in terms of the tumor control probability (TCP). The dependence of the differences in tumor volume and location was investigated. Dosimetric validation was performed using Gafchromic EBT3 (International Specialty Products, ISP, Wayne, NJ). Forty-five Computed Tomography (CT) data sets were used for this study; 40 Gy at 8 Gy/fraction was prescribed with 5 noncoplanar 6-MV IMRT beams or 3 to 4 dynamic conformal arcs with 3 to 5 IMRT beams distributed per arc. The plans were first calculated with PB and then recalculated with MC. The difference between the mean tumor doses was approximately 10% ± 4%; these differences were even larger under deep inspiration breath hold. Differences between the mean tumor dose correlated with tumor volume and path length of the beams. The TCP values changed from 99.87% ± 0.24% to 96.78% ± 4.81% for both PB- and MC-calculated plans (P = .009). When a fraction of hypoxic cells was considered, the mean TCP values changed from 76.01% ± 5.83% to 34.78% ± 18.06% for the differently calculated plans (P < .0001). When the plans were renormalized to the same mean dose at the tumor, the mean TCP for oxic cells was 99.05% ± 1.59% and for hypoxic cells was 60.20% ± 9.53%. This study confirms that the MC algorithm adequately accounts for inhomogeneities. The inclusion of the MC in the process of IMRT optimization could represent a further step in the complex problem of determining the optimal treatment plan.

Anatomical and dose changes of gross tumour volume and parotid glands for head and neck cancer patients during intensity-modulated radiotherapy: effect on the probability of xerostomia incidence.

AIMS: To quantify the changes in dose as well as in the prediction of parotid gland toxicity due to anatomical changes during therapy of head and neck cancer patients. MATERIALS AND METHODS: Fifteen patients with advanced locoregional head and neck cancer, with no evidence of distant metastasis, were enrolled in a prospective study. All patients were treated with intensity-modulated radiotherapy. Multiple computed tomography scans were repeated at the end of each treatment week. The original treatment plans were copied to the per-treatment scans to create hybrid plans. The normal tissue complication probability (NTCP) was calculated assuming the end point to be grade ≥3 xerostomia according to the Radiation Therapy Oncology Group late toxicity scale. RESULTS: The gross tumour volume dose coverage was slightly affected by the anatomical changes, whereas the mean dose (D(mean)) to the parotids changed from 26.1 ± 6.0 to 27.4 ± 7.4 Gy, with a mean increase of 0.22 Gy/treatment week. Consequently, the mean NTCP increased from 0.15 ± 0.06 to 0.18 ± 0.10, primarily due to a few patients exhibiting a marked increase. The absolute gross tumour volume shrinkage and the percentage parotids shrinkage were the best independent predictors for the NTCP variations. CONCLUSIONS: On average, the increase in the parotids D(mean) as well as in NTCP during treatment is limited, and the observed variations were strongly patient-dependent.

Radiation exposure of personnel during intraoperative radiotherapy (IORT): radiation protection aspects.

Intraoperative radiotherapy (IORT) is a multidisciplinary procedure which combines two conventional methods of cancer treatment surgery and radiation therapy. The purpose is to deliver a large single dose to the surgically exposed tumor bed while minimizing doses to normal tissues. Intraoperative radiation therapy (IORT) is a technique which allows irradiating the patient directly after the surgical operation using a linear accelerator that can be situated in the operating room. For medical accelerators with energy over 10MeV the need to characterize the neutron spectra for this particular situation arises from the fact that, when neutron spectra is not fully known, it becomes necessary to be more cautious introducing a weight factor wR of 20 (maximum value). This leads to overesteem the equivalent dose due to neutrons and it indicates to introduce additional (mobile) shields for photon and neutrons radiation not easily achievable in an operating room.