Gamma knife treatment of subfoveal, classic neovascularization in age-related macular degeneration: a pilot study

Kilovoltage stereotactic radiosurgery for age-related macular degeneration: assessment of optic nerve dose and patient effective dose

Age-related macular degeneration (AMD) is a leading cause for vision loss for people over the age of 65 in the United States and a major health problem worldwide. Research for new treatments of the wet form of the disease using kilovoltage stereotactic radiosurgery is currently underway at Oraya Therapeutics, Inc. In the present study, the authors extend their previous computational stylized model of a single treated eye [Med. Phys. 35, 5151-5160 (2008)] to include full NURBS-based reference head phantoms of the adult male and female using anatomical data from ICRP Publication 89. The treatment was subsequently modeled in MCNPX 2.5 using a 1 x 1 x 1 mm3 voxelized version of the NURBS models. These models incorporated several organs of interest including the brain, thyroid, salivary glands, cranium, mandible, and cervical vertebrae. A higher resolution eye section at 0.5 x 0.5 x 0.5 mm3 voxel resolution was extracted from the head phantoms to model smaller eye structures including the macula target, cornea, lens, vitreous humor, sclera/retina layer, and optic nerve. Due to lack of literature data on optic nerve pathways, a CT imaging study was undertaken to quantify the anatomical position of the optic nerve. The average absorbed doses to the organs of interest were below generally accepted thresholds for radiation safety. The estimated effective dose was 0.28 mSv which is comparable to diagnostic procedures such as a head radiograph and a factor of 10 lower than a head CT scan.

Dosimetry characterization of a multibeam radiotherapy treatment for age-related macular degeneration

Age-related macular degeneration (ARMD) is a major health problem worldwide. Advanced ARMD, which ultimately leads to profound vision loss, has dry and wet forms, which account for 20% and 80% of cases involving severe vision loss, respectively. A new device and approach for radiation treatment of ARMD has been recently developed by Oraya Therapeutics, Inc. (Newark, CA). The goal of the present study is to provide a initial dosimetry characterization of the proposed radiotherapy treatment via Monte Carlo radiation transport simulation. A 3D eye model including cornea, anterior chamber, lens, orbit, fat, sclera, choroid, retina, vitreous, macula, and optic nerve was carefully designed. The eye model was imported into the MCNPX2.5 Monte Carlo code and radiation transport simulations were undertaken to obtain absorbed doses and dose volume histograms (DVH) to targeted and nontargeted structures within the eye. Three different studies were undertaken to investigate (1) available beam angles that maximized the dose to the macula target tissue, simultaneously minimizing dose to normal tissues, (2) the energy dependency of the DVH for different x-ray energies (80, 100, and 120 kVp), and (3) the optimal focal spot size among options of 0.0, 0.4, 1.0, and 5.5 mm. All results were scaled to give 8 Gy to the macula volume, which is the current treatment requirement. Eight beam treatment angles are currently under investigation. In all eight beam angles, the source-to-target distance is 13 cm, and the polar angle of entry is 300 from the geometric axis of the eye. The azimuthal angle changes in eight increments of 45 degrees in a clockwise fashion, such that an azimuthal angle of 0 degreee corresponds to the 12 o'clock position when viewing the treated eye. Based on considerations of nontarget tissue avoidance, as well as facial-anatomical restrictions on beam delivery, treatment azimuthal angles between 135 degrees and 225 degrees would be available for this treatment system (i.e., directly upward and entering the eye from below). At beam directions approaching 225 degrees and higher, some dose contribution to the optic nerve would result under the assumption that the optic nerve is tilted cranially above the geometric axis in a given patient, a feature not typically seen in past studies. A total treatment dose of 24 Gy would be delivered in three 8 Gy treatments at these selected azimuthal angles. Dose coefficients, defined as the macula radiation absorbed dose per unit air kerma in units of Gy/Gy, were 16% higher for 120 kVp x-ray beams in comparison to those at 80 kVp, thus requiring only 86% of the integrated tube current (mAs) for equivalent dose delivery. When 0.0, 0.4, and 1.0 mm focal spot sizes were used, the dose profiles in the macula are very similar and relatively uniform, whereas a 5.5 mm focal spot size produced a more nonuniform dose profile. The results of this study dem onstrate the therapeutic promise of this device and provide important information for further design and clinical implementation for radiotherapy treatments for ARMD.

Gamma Knife Radiosurgery in the Treatment of Choroidal Neovascularization (Wet-Type Macular Degeneration)

We evaluated retrospectively our institutional experience in the treatment of macular degeneration with gamma knife radiosurgery (GKR). Treatment was delivered in a single shot of 12 Gy. Seven patients were treated between March of 1999 and May of 2000. The median duration of follow-up was 2.2 years. The majority of patients maintained stable visual acuity after treatment. Our series indicates that GKR may be useful as a salvage treatment for patients who have failed or are ineligible for other treatments for their macular degeneration. Further studies are needed to better define the role of GKR in the treatment of macular degeneration.

Gamma knife surgery for choroidal neovascularization in age-related macular degeneration

The authors conducted a study to determine a way of overcoming the poor-quality demonstration of choroidal neovascularization (CNV) associated with age-related macular degeneration (AMD) on conventional magnetic resonance (MR) imaging studies.

The poor MR imaging demonstration of CNV in patients with AMD makes the use of gamma knife surgery more difficult. This difficulty, however, can be overcome by use of a modified time-of-flight MR imaging sequence with Gd enhancement and coronal reconstruction.

Gamma knife surgery for choroidal neovascularization in age-related macular degeneration

The authors conducted a study to determine a way of overcoming the poor-quality demonstration of choroidal neovascularization (CNV) associated with age-related macular degeneration (AMD) on conventional magnetic resonance (MR) imaging studies.

The poor MR imaging demonstration of CNV in patients with AMD makes the use of gamma knife surgery more difficult. This difficulty, however, can be overcome by use of a modified time-of-flight MR imaging sequence with Gd enhancement and coronal reconstruction.

Radiotherapy for subfoveal choroidal neovascularization in age-related macular degeneration: a randomized clinical trial

PURPOSE

To report results of 18-month follow up of external beam radiation therapy with photons for subfoveal classic or occult choroidal neovascularization (CNV) in age-related macular degeneration (ARMD).

DESIGN

Randomized clinical trial.

METHODS

A total of 161 patients with subfoveal CNV in ARMD were recruited in a prospective double-masked study. The posterior pole of the afflicted eye was given 1 Gy (4 x 0.25 Gy) in the control group and 8 Gy (4 x 2 Gy) or 16 Gy (4 x 4 Gy) in the treatment groups. At the time of treatment, and 6, 12, and 18 months post treatment, best-corrected visual acuity (BCVA), reading ability, and CNV size were measured.

RESULTS

At the completion of the study 150 (93.2%), 139 (86.3%), and 137 (85.1%) patients were followed for 6, 12, and 18 months, respectively. The mean number of lines lost in the BCVA was -1.69, -2.2, and -3.23 in the 1 Gy group; -0.94, -1.25, and -1.73 in the 8 Gy group; -0.51, -0.67, and -1.93 in the 16 Gy group. The difference was significant after 12 months (P =.016 for 8 Gy vs. 1 Gy; P =.006 for 16 Gy vs. 1 Gy), and 18 months (P =.011 for 8 Gy vs. 1 Gy; P =.05 for 16 Gy vs. 1 Gy). The patients with classic CNV, or with an initial distance visual acuity >or=20/100, benefited more from treatment. A significant difference was not found between control group and treatment groups in the reading ability and in the CNV size. No radiation-associated side effects were reported thus far.

CONCLUSION

The number of lines lost in the BCVA was less in the 8 Gy and 16 Gy treatment groups than in the control group during the complete follow up examination. Radiation therapy with 8 Gy and 16 Gy, without showing any difference in efficacy, resulted in a near stabilization of the BCVA in patients with subfoveal classic or occult CNV in ARMD. Further studies are necessary to determine the significance of repeated radiotherapy series with a dose of 8 Gy to improve the effect on the CNV size and thereby to prolong stabilization of distance visual acuity.