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High-density metallic implants can introduce considerable uncertainties in proton therapy treatment planning. These uncertainties eventually translate into proton range errors, which may cause significant underdosing to the target volume or overdosing to normal tissue beyond the target. This study investigated the dosimetric impact of a 0.6 mm titanium (Ti) mesh implant in passive-scattering proton beam therapy through the study of the depth dose and output in water, and the dose profiles in solid water at various depths. The measurements were performed for a beam with a range of 8.5 cm and a modulation of 7.5 cm. The titanium mesh was placed at a depth of 1 cm below the surface of the phantom for all measurements. A range reduction of 0.5 ± 0.1 mm was observed for a beam perpendicular to the mesh, with no further reductions when the incident angle increased to 60°. We conclude that the dosimetric effect of a 0.6 mm titanium mesh implant is small for a passive scattering proton beam. With proper correction applied to metal artifacts, consistent results were observed in the phantom study in the treatment planning system.

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PURPOSE: Radiotherapy planning for iliac pelvic nodes can be challenging due to the close proximity of sensitive healthy tissues such as the bowel and rectum. Modern treatment techniques like photon intensity-modulated radiotherapy (IMRT) and intensity-modulated proton therapy (IMPT) offer improved healthy tissue sparing for similar target coverage. In this study we compare IMRT and IMPT plans for six post-cystectomy patients. METHODS: A dose of 50.4 Gy was prescribed to the planning target volume (PTV), which for IMRT is the clinical target volume (CTV) plus a 5 mm expansion for geometric uncertainties due to CTV and patient positioning errors, and for proton beams is the CTV plus the lateral 5 mm margin plus an additional longitudinal margin to allow for the proton range uncertainty. The optimization objectives are: 98% of the PTV receive at least 95% of the prescription, target maximum dose = 107% of prescription, rectum V[40Gy] < 30% and max = 105%, and bowel V[45Gy] < 125 cc and max = 107%. All IMRT and IMPT plans are made to achieve the target coverage objective. RESULTS: Using IMPT, the rectum would receive a mean dose of 9.0 Gy with an average (over the six patients) maximum dose of 38.1 Gy. Using IMRT, the rectum would receive a mean dose of 13.0 Gy and an average maximum dose of 37.6 Gy. The IMPT plans give a mean dose of 17.9 Gy and a maximum dose of 53.4 Gy for the bowel, whereas the IMRT plans give a mean dose of 23.8 Gy and a maximum dose of 53.2 Gy. Both the rectum and bowel show slightly lower mean doses for IMPT. CONCLUSIONS: Our results indicate that IMPT plans improve normal tissue sparing as compared to IMRT plans and provide adequate dose coverage of the target volume.

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AIMS: To find the cure rate of late (second year of age) and very late (3-5 years of age) initial probing for congenital nasolacrimal duct obstruction (CNLDO) and to identify the factors contributing to the failure rate of the probing in older children. METHODS: In a prospective interventional case series study, 169 eyes of 125 consecutive patients (1-5 years old) with CNLDO underwent probing under general anaesthesia. Cure was defined as absence of tearing and discharge in the affected eye. RESULTS: 138 eyes of 101 patients aged 13-60 months (mean 23.4 (SD 10.2)) were included. Of 15 eyes (10.8%) with complex CNLDO, 80% presented after 24 months of age (p<0.0001). The cure rate was 89% in patients 13-24 months of age and 72% after the age of 24 months (p = 0.01). It was 90.2% in the membranous and 33.3% in the complex CNLDO in both late and very late probing (p<0.0001). There was a high correlation (r = 0.97) and no significant difference between the cure rate at 1 week and final follow up. CONCLUSION: Accumulation of the complex CNLDO is the main risk factor for failure of probing in the older children. The outcome of the nasolacrimal duct probing at 1 week follow up is highly indicative of the final outcome.

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An en face, medium energy electron boost of approximately 10 Gy is often given to stomal and peristomal tissues. Because the boost is considered a simple treatment, CT-based treatment planning is rarely used. Further, the results of such a plan might be inaccurate, as the complex dose distribution surrounding the stoma air cavity is poorly modeled by many treatment planning systems. We constructed three phantoms-two with a central vertical cavity to mimic the size and shape of the stoma and proximal trachea and one with a cavity inclined at 45 degrees to the horizontal to better simulate anatomy. These were used to investigate the dose distribution surrounding the stoma. In all cases, the entrance to the stoma opening was centered in a field defined by a 7-cm circular cutout and the phantom was irradiated at a source-surface distance (SSD) of 100 cm with either vertically incident 9- or 12-MeV electrons. Film measurements were made at a range of depths below and lateral to the cavity. For the vertical cavity phantoms, diode measurements were performed and isodose plans using CT scans of the phantoms were generated on a modern treatment planning system. For these two phantoms, the combined effects of lateral scatter from surrounding material and reduced equivalent thickness for electrons which pass directly through the cavity increases the dose within a centimeter of the bottom of cavity by as much as 50% for 9 MeV and 70% for 12 MeV. In material at the shallower ("superior") end of the inclined cavity, a 40-50% overdose was noted. The dose increase is geometry dependent and is not predicted by the available treatment planning system. The potential of such a dose increase to affect normal tissues such as the neopharynx should be considered.

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An extended source-surface distance (SSD) is often employed in total-body irradiation (TBI) to obtain fields sufficiently large to encompass the total body. It is clinically desirable to administer a uniform dose to all tissues, including the surface, especially to counteract the skin-sparing of high-energy X-rays. The electrons generated from the air contribute more to the dose near the surface at extended distances than at regular SSDs. However, if further increase of the dose at shallow depths is desired a spoiler can be employed. Monte Carlo simulations were used to study these effects for the electron component of the beam and the parameters needed for the pencil-beam dose calculation of this component. Measurements were performed to confirm these studies. The energy spectrum of electrons generated in air is independent of SSD. All the electrons in the beam, however, originate from the spoiler when one is used. The spectrum of these electrons is nearly independent of the SSD. The pencil-beam dose kernels were calculated using the derived electron spectra. To calculate doses in the buildup region with and without a spoiler, we use one pencil-beam kernel for the electrons generated in air and one for the electrons generated from the spoiler. Measured depth-dose data agreed with the calculation results.

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When parallel opposed 6-MV x-ray beams are used for treatment of head and neck tumors, superficial tissues and lymphatics at shallow depths of < or =4 to 6 mm may be at cancer risk but receive less than full radiation dose. In these cases, the use of either a beam spoiler or bolus material can increase dose to superficial tissues. The potential benefit of a beam spoiler relative to bolus is preservation of skin-sparing characteristics for cases in which the skin surface does not require full dose. In this study, we evaluate the application of a beam spoiler and compare it to bolus for standard treatments of head and neck tumors. Measurements of both depth dose in-water and in-air profiles were made with a beam spoiler for a 6-MV photon beam. The measurements were combined with Monte Carlo calculations to obtain the energy spectrum of the spoiler-generated electrons. An in-house pencil beam treatment-planning algorithm was used to calculate the dose distribution with spoiler. The dose distribution in the buildup region was then studied with and without the spoiler for a typical head and neck treatment with parallel-opposed beams. Dose distributions and partial-volume dose histograms (PVDH) demonstrate the benefits provided by spoilers for the head and neck treatments and the limitations of their use. The beam spoiler is useful in treating the superficial lymphatics in the buildup region near head and neck tumors. Guidelines for use of beam spoiler versus bolus are discussed.

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A method to determine the spectrum of a clinical photon beam from measured depth-dose data is described. At shallow depths, where the range of Compton-generated electrons increases rapidly with photon energy, the depth dose provides the information to discriminate the spectral contributions. To minimize the influence of contaminating electrons, small (6 x 6 cm2) fields were used. The measured depth dose is represented as a linear combination of basis functions, namely the depth doses of monoenergetic photon beams derived by Monte Carlo simulations. The weights of the basis functions were obtained with the Cimmino feasibility algorithm, which examines in each iteration the discrepancy between predicted and measured depth dose. For 6 and 15 MV photon beams of a clinical accelerator, the depth dose obtained from the derived spectral weights was within about 1% of the measured depth dose at all depths. Because the problem is ill conditioned, solutions for the spectrum can fluctuate with energy. Physically realistic smooth spectra for these photon beams appeared when a small margin (about +/- 1%) was attributed to the measured depth dose. The maximum energy of both derived spectra agreed with the measured energy of the electrons striking the target to within 1 MeV. The use of a feasibility method on minimally relaxed constraints provides realistic spectra quickly and interactively.

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PURPOSE: To determine the change in volume of the prostate as a result of neoadjuvant androgen deprivation prior to prostate implant and in the early postimplant period following transperineal ultrasound guided palladium-103 brachytherapy for early-stage prostate cancer. METHODS AND MATERIALS: Sixty-nine men received 3 to 6 months of androgen deprivation therapy followed by treatment planning ultrasound followed 4 to 8 weeks later by palladium-103 implant of the prostate. All patients had clinical and radiographic stage T1c-T2b adenocarcinoma of the prostate. A second ultrasound study was carried out 11 to 13 days following the implant to determine the change in volume of the prostate as a result of the implant. The prehormonal and preimplant volumes were compared to the postimplant volume to determine the effect of hormones and brachytherapy on prostate volume. RESULTS: The median decrease in prostate volume as a result of androgen deprivation was 33% among the 54 patients with prostate volume determinations prior to hormonal therapy. The reduction in volume was greatest in the quartile of men with the largest initial gland volume (59%) and least in the quartile of men with smallest glands (10%). The median reduction in prostate volume between the treatment planning ultrasound and the follow-up study after implant was 3%, but 23 (33%) patients had an increase in prostate volume, including 16 (23%) who had an increase in volume >20%; 11 of these patients (16%) had an increase in volume >30%. The time course of development and resolution of this edema is not known. The severity of the edema was not related to initial or preimplant prostate volume or duration of hormonal therapy. CONCLUSIONS: Prostate edema may significantly affect the dose delivered to the prostate following transperineal ultrasound guided brachytherapy. The effect on the actual delivered dose will be greater when shorter lived isotopes are used. It remains to be observed whether this edema will affect outcome.

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Approximately rectangular low or medium energy electron fields at extended SSD are often used to boost over the spinal cord in the treatment of head and neck cancer after cord tolerance is reached. A separate abutting photon field is used to continue treatment anterior to the spine. Typically, the electron and photon fields have different central axes and the electron cutout is symmetrically centered in the cone. However, a good match between the photon and electron fields is achieved more readily if the central axis of the electron field is located at the center of the area treated by the photon field. This displaces the electron cutout toward the edge of the cone. We measured and compared the percent depth dose (PDD), output factors and profiles for matched pairs of centered and off-centered rectangular and square cutouts for the 6, 9 and 12 MeV beams of three Varian linacs. The 10 x 10 and 15 x 15 cones were used at SSDs of 100 cm and 110 cm. Differences between centered and off-centered cutouts of the same dimensions were less than 1% for PDD and profiles and less than 5% for output factors. Therefore, the same central axis can be used for abutting photon and electron fields, without requiring extra dosimetric data to account for the off-center location of the electron cutout.

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BACKGROUND AND PURPOSE: Independent collimation conveniently allows for the junctioning of abutting fields with non-diverging beam edges. When this technique is used at the junction of multiple fields, e.g. lateral and low anterior fields in three-field head and neck set-ups, there should be a dosimetric match with no overdose or underdose at the matchline. We set out to evaluate the actual dosimetry at the central match plane. MATERIALS AND METHODS: Independent jaws were used to mimic two half-beam blocked fields abutting at the central axis. X-Ray verification film was exposed in a water-equivalent phantom and the dose at the matchline was evaluated with laser densitometry. Collimators were then programmed to force a gap or overlap of the radiation fields to evaluate the effect of jaw misalignment within the tolerance of the manufacturer's specification. Diode measurements of the field edges were also performed. Four beam energies from four different linear accelerators were evaluated. RESULTS: Small systematic inhomogeneities were found along the matchline in all linear accelerators tested. The maximum dose on the central axis varied linearly with small programmed jaw misalignments. For a gap or overlap of 2 mm between the jaws, the matchline dose increased or decreased by 30-40%. The region of overdose or underdose around the matchline is 3-4 mm wide. The discrepancy between the width of jaw separation and the width of the region of altered dose is explained by a penumbra effect. CONCLUSION: We recommend that independent jaw alignment be evaluated routinely and provide a simple method to estimate dose inhomogeneity at the match plane. If there is a field gap or overlap resulting in a clinically significant change in dosimetry, jaw misalignment should be corrected. If it cannot be corrected, part of the benefit of asymmetric collimation is lost and other methods of field junctioning may have to be considered. We routinely use a small block over the spinal cord at the mono-isocenter set-up plane for three-field head and neck treatments to prevent an overdose.

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PURPOSE: The ideal lens-sparing radiotherapy technique for retinoblastoma calls for 100% dose to the entire retina including the ora serrata and zero dose to the lens. Published techniques, most of which use photons, have not accomplished this ideal treatment. We describe here a technique that approaches this ideal configuration using electron beam therapy. METHODS AND MATERIALS: Dose-modeling calculations were made using a computer program built around a proprietary algorithm. This program calculates 3D dose distribution for electrons and photons and uses the Cimmino feasibility method for the inverse problem of beam weighting to achieve the prescribed dose. The algorithm has been verified in the ocular region by measurements in a RANDO phantom. To search for an ideal lens-sparing beam setup, a stylized phantom of an 8-month-old infant was generated with built-in inhomogeneities, and a phantom of a 5-year-old child was generated from a patient CT series. RESULTS: Of more than 100 different beam setups tested, two 9 MeV electron beams at gantry angles plus and minus 26 degrees from the optic nerve axis achieved the best distribution. Both fields have a lens block and an isocenter between the globe and origin of the optic nerve. When equal doses are given to both fields, the entire extent of the retina (including ora serrata) received 100%, while the lens received 10% or less. CONCLUSION: The two-oblique-electron-beam technique here described appears to meet most of the stringent dosimetry needed to treat retinoblastoma. It is suitable for a range of ages, from infancy to early childhood years.

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To match corresponding seed images in different radiographs so that the 3D seed locations can be triangulated automatically and without ambiguity requires (at least) three radiographs taken from different perspectives, and an algorithm that finds the proper permutations of the seed-image indices. Matching corresponding images in only two radiographs introduces inherent ambiguities which can be resolved only with the use of non-positional information obtained with intensive human effort. Matching images in three or more radiographs is an 'NP (Non-determinant in Polynomial time)-complete' problem. Although the matching problem is fundamental, current methods for three-radiograph seed-image matching use 'local' (seed-by-seed) methods that may lead to incorrect matchings. We describe a permutation-sampling method which not only gives good 'global' (full permutation) matches for the NP-complete three-radiograph seed-matching problem, but also determines the reliability of the radiographic data themselves, namely, whether the patient moved in the interval between radiographic perspectives.

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Electron beam characteristics were investigated for different applicator cone designs. Low-energy scattered electrons from the cone housing can contribute to the skin dose. The depth dose was measured as a function of electron energies and field size for different cones. The measurements were compared with calculated 3-D dose distributions obtained from Monte-Carlo-derived energy deposition kernels [Phys. Med. Biol. 37, 391 (1992)]. This comparison shows that beam-defining cones can be fabricated to eliminate low-energy scattered electrons from the cone housing. Eliminating low-energy electrons from the cones provides depth dose (DD) characteristics for clinical electron beams that are comparable to those (1) from nearly monoenergetic electrons (as determined from the practical range of the electrons), and (2) for which the DD is nearly independent of source-to-skin distance. Measurements of DD for small circular fields were also performed. For very small field sizes, corresponding to the width of the lateral spread of a pencil beam, the theoretical and measured DD were found to depend on field size. Three-dimensional dose calculations of small and irregularly shaped electron fields are necessary to appreciate these effects.

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A difficult, error-prone, and time consuming problem in brachytherapy is the determination of the three-dimensional (3-D) positions of implanted seeds. To triangulate 3-D seed positions, the corresponding seed images in radiographs taken at different parallax (gantry) angles must be identified (matched). With only two radiographs, there is an inherent problem of ambiguity in matching individual seed images. The use of the assignment algorithm cannot bypass this ambiguity. However, when seeds are implanted within ribbons, their positions in 3-D space can be determined accurately and (almost always) without ambiguity. This follows because the matching of ribbon images is much less ambiguous than the matching of individual seed images. We describe a semi-automated procedure to match the ribbon images on two radiographs, determine the 3-D loci of the ribbons, and locate the 3-D position of each seed on each ribbon. The ribbon matching problem involves a double application of the assignment algorithm. After image matching, the 3-D locus of each ribbon is found from the intersection of the ray sets from its radiographic images and the analysis of a 3-D spanning tree through the derived ray set intersection points. The seed positions on a ribbon locus are determined by interpolation of equally spaced seed intervals along the 3-D ribbon locus. Experiments with a phantom show that for two radiographs separated by any parallax between 20 degrees and 90 degrees, our technique provides a systematic error in 3-D seed location of 3.0 mm, and a random error of less than 2 mm.

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Previous Monte Carlo calculations for the energy spectra of electrons produced in water irradiated by photons are extended to 1 GeV. All of the physical processes believed to be important in the transport of electrons and positrons above 100 keV and photons starting with the ejection of L photoelectrons are considered. The results are presented in tabular form and can be conveniently used to compute kerma in water. The contributions of several physical processes, such as Compton scattering and pair-production to electron spectra, are separately tabulated. The results are compared with those of Todo et al. (1982) for the single interactions of monoenergetic photons. It is found that the inclusion of processes such as multiple Compton scattering, bremsstrahlung production, positron annihilation in flight, Møhiller and Bhabha scattering from electrons and Molière multiple scattering from atomic nuclei make a considerable difference in the inferred electron spectrum in water.

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A rigorous formalism using the dyadic Green's function to determine the relative heating pattern in a two-layer fat-muscle medium of a dipole-corner reflector applicator in direct contact with the finite fat layer is described. The results are compared with the earlier approximate calculations of Guy and Lehmann. Though the two theories give identical results for the relative heat produced in the muscle layer, the relative heating patterns predicted by the present theory in the fat layer are different.

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