Evaluation of Acuros® XB accuracy for static small fields dose calculations based on the IAEA/AAPM TRS-483 recommendation.

PURPOSE: Evaluate Acuros® XB dose calculation accuracy following TRS-483 recommendations in small static fields for flattened and un-flattened 6 MV X-ray beams. METHODS: Field output factors were measured following TRS-483 recommendations using four radiation detectors. Two sets of field output factors were measured. One set was used to configure the beam model into Acuros® XB down to a jaw-defined field size of 1.0 cm × 1.0 cm. The second set was used to evaluate the differences between calculated and measured field output factors for MLC-fields down to a field size of 0.5 cm × 0.5 cm. RESULTS: Acuros® XB showed an accuracy within 1.5% down to an MLC-field of 1.0 cm × 1.0 cm, for a focal spot size of 1.0 and 0.0 mm in the cross and in-plane directions. For an MLC-field of 0.5 cm × 0.5 cm, an agreement was found within 3% between calculated and measured field output factors. These results were addressed by optimizing the focal spot size to minimize the differences between calculated and measured dose profiles. CONCLUSIONS: By optimizing the focal spot size, Acuros® XB showed an acceptable agreement within 3% down to an MLC-field of 0.5 cm × 0.5 cm. The results of this work suggest that if static and modulated delivery of very small targets is planned, then a field output factor table down to a field size of 1.0 cm is required in the beam configuration model.

Bilateral Simultaneous Temporal Relapses of Medulloblastoma.

Supratentorial relapses are a common component of medulloblastoma after failure of treatment. Craniospinal irradiation (CSI) to cerebrospinal fluid-bearing areas is an essential part of the management of these tumors both in adults and children. Failure of treatment in specific anatomical regions can be attributable to technical inaccuracies in CSI technique leading to radiation underdosing in such areas. We present two cases of patients with bilateral simultaneous metastasis of a primary medulloblastoma treated, in both cases, four years before the recurrence. In both patients the tumors were mirror images, at the right and left temporal pole. Radiotherapeutic plans were analyzed in both cases, and a possible mechanism determining the pattern of relapse is discussed. We consider, in agreement with the literature, that a prone position during treatment, shielding blocks at the cribiform/subfrontal region, and anatomic inadequacies in the CSI fields could have contributed to the presented pattern of relapse.

Evaluation of the Gafchromic(®) EBT2 film for the dosimetry of radiosurgical beams.

PURPOSE: Radiosurgery uses small fields and high-radiation doses to treat intra- and extracranial lesions in a single session. The lack of a lateral electronic equilibrium and the presence of high-dose gradients in these fields are challenges for adequate measurements. The availability of radiation detectors with the high spatial resolution required is restricted to only a few. Stereotactic diodes and EBT radiochromic films have been demonstrated to be good detectors for small-beam dosimetry. Because the stereotactic diode is the standard measurement for the dosimetry of radiosurgical beams, the goal of this work was to perform measurements with the radiochromic film Gafchromic(®) EBT2 and compare its results with a stereotactic diode. METHODS: Total scatter factors, tissue maximum, and off-axis ratios from a 6 MV small photon beams were measured using EBT2 radiochromic film in a water phantom. The film-measured data were evaluated by comparing it with the data measured with a stereotactic field diode (IBA-Dosimetry). RESULTS: The film and diode measurements had excellent agreement. The differences between the detectors were less than or equal to 2.0% for the tissue maximum and the off-axis ratios. However, for the total scatter factors, there were significant differences, up to 4.9% (relative to the reference field), for field sizes less than 1.0 cm. CONCLUSIONS: This work found that the Gafchromic(®) EBT2 film is adequate for small photon beam measurements, particularly for tissue maximum and off-axis ratios. However, careful attention must be taken when measuring output factors of small beams below 1.0 cm due to the film's energy dependence. The measurement differences may be attributable to the film's active layer composition because EBT2 incorporates higher Z elements (i.e., bromide and potassium), hence revealing a potential energy dependence for the dosimetry of small photon beams.