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Purpose: Electron beam radiotherapy is frequently administered based on clinical setups without formal treatment planning. We felt, therefore, that it was important to monitor electron beam treatments by in vivo dosimetry to prevent errors in treatment delivery. In this study, we present our clinical experience with patient dose verification using electron diodes and quantitatively assess the dose perturbations caused by the diodes during electron beam radiotherapy. Methods and Materials: A commercial diode dosimeter was used for the in vivo dose measurements. During patient dosimetry, the patients were set up as usual by the therapists. Before treatment, a diode was placed on the patient's skin surface and secured with hypoallergenic tape. The patient was then treated and the diode response registered and stored in the patient radiotherapy system database via our in-house software. A customized patient in vivo dosimetry report showing patient details, expected and measured dose, and percent difference was then generated and printed for analysis and record keeping. We studied the perturbation of electron beams by diodes using film dosimetry. Beam profiles at the 90% prescription isodose depths were obtained with and without the diode on the beam central axis, for 6–20 MeV electron beams and applicator/insert sizes ranging from a 3-cm diameter circular field to a 25 × 25 cm open field. Results: In vivo dose measurements on 360 patients resulted in the following ranges of deviations from the expected dose at the various anatomic sites: Breast (222 patients) −20.3 to +23.5% (median deviation 0%); Head and Neck (63 patients) −21.5 to +14.8% (median −0.7%); Other sites (75 patients) −17.6 to +18.8% (median +0.5%). Routine diode dosimetry during the first treatment on 360 patients (460 treatment sites) resulted in 11.5% of the measurements outside our acceptable ±6% dose deviation window. Only 3.7% of the total measurements were outside ±10% dose deviation. Detailed investigations revealed that the dose discrepancies, overwhelmingly, were due to inaccurate diode orientation and positioning, especially in the regions with rapidly changing contours and/or sloping surfaces. The presence of a diode in the treatment field was found, in some cases, to cause significant dose reduction, most noticeable with smaller fields and lower energy beams. The reduction in dose ranged from 16% (for a 6 MeV beam and a 3 cm diameter circular field) to 4% (for a 12 MeV beam and a 10× 10 cm field). Conclusions: Diode dosimetry was found to be convenient and valuable for verifying in real time the dose delivery accuracy of electron beam treatments, but with some caveats. When treating a small field by low energy electrons, frequent use of diodes is undesirable, because it might result in appreciable reduction of dose to the target volume.

To compare different treatment techniques for unilateral treatment of parotid gland tumors.The CT-scans of a representative parotid patient were used. The field size was 9 x 11 cm, the separation was 15.5 cm, and the prescription depth was 4.5 cm. Using 3D dose distributions, tissue inhomogeneity corrections, scatter integration (for photons) and pencil beam (for electrons) algorithms and dose-volume histogram (DVH), nine treatment techniques were compared. [1] unilateral 6 MV photons [2] unilateral 12 MeV electrons [3] unilateral 16 MeV electrons [4] an ipsilateral wedge pair technique using 6 MV photons [5] a 3-field AP (wedged), PA (wedged) and lateral portal technique using 6 MV photons [6] a mixed beam technique using 6 MV photons and 12 MeV electrons (1:4 weighting) [7] a mixed beam technique using 6 MV photons and 16 MeV electrons (1:4 weighting) [8] a mixed beam technique using 18 MV photons and 20 MeV electrons (2:3 weighting) [9] a mixed beam technique using 18 MV photons and 20 MeV electrons (1:1 weighting).Using dose-volume histograms to evaluate the dose to the contralateral parotid gland, the percentage of contralateral parotid volume receivingor = 30% of the prescribed dose was 100% for techniques [1], [8] and [9], and5% for techniques [2] through [7]. Evaluating the "hottest" 5 cc of the ipsilateral mandible and temporal lobes, the hot spots were: 152% and 150% for technique [2], 132% and 130% for technique [6]. Comparing the exit doses, techniques [1], [8] and [9] contributed toor = 50% of the prescribed dose to the contralateral mandible and the temporal lobes. Only techniques [2] and [6] kept the highest point doses to both the brain stem and the spinal cord below 50% of the prescribed dose.The single photon lateral field [1] and the mixed electron-photon beams [8] and [9] are not recommended treatment techniques for unilateral parotid irradiation because of high doses delivered to the contralateral parotid gland and high exit doses which are associated with Xerostomia. The en face electron beam technique [2] and the mixed electron-photon beam technique [6] are unacceptable due to the excessive dose heterogeneity to the contiguous normal structures. In spite of optimal dose fall-off achieved using the en face technique [3], most patients cannot tolerate the resulting high skin doses. We conclude that the ipsilateral wedge pair [4], the 3-field [5], and the mixed electron-photon beam [7] techniques are optimal techniques in providing relatively homogeneous dose distributions within the target area and for minimizing dose to the relevant normal structures.

Purpose : To study the attenuation and scattering effects of ocular implants, made from different materials, on the dose distributions of a 6 MV photon beam, and 6, 9, and 12 MeV electron beams used in orbital radiotherapy. Methods and Materials : Central axis depth-dose measurements were performed in a polystyrene phantom with embedded spherical ocular implants using film dosimetry of a 6 MV photon beam and electron beams of 6, 9, and 12 MeV energy. The isodose distributions were also calculated by a computerized treatment planning system using computerized tomography (CT) scans of a polystyrene phantom that had silicone, acrylic, and hydroxyapatite ocular implants placed into it. Results : Electron beam dose distributions display distortions both on the measured and calculated data. This effect is most accentuated for the hydroxyapatite implants, for which the transmissions through ocular implants are on the order of 93% for the 6 MV photon beam, and range from 60% for 6 MeV electrons to 90% for 12 MeV electrons. Conclusion : We studied the effect of ocular implants of various materials, embedded in a polystyrene phantom, on the dose distributions of a 6 MV photon beam, and 6, 9, and 12 MeV electron beams. Our investigations show that while 6 MV photons experience only a few percent attenuation, lower energy electron beam with 60% transmission is not a suitable choice of treating tumors behind the ocular implants.