106Ru/106Rh plaque and proton radiotherapy for ocular melanoma: a comparative dosimetric study

A Systematic Comparison of Dose Distributions Delivered in 125I Plaque Brachytherapy and Proton Radiation Therapy for Ocular Melanoma

PURPOSE

To characterize dose distributions with ¹²⁵I plaque brachytherapy compared with proton radiation therapy for ocular melanoma for relevant clinical scenarios, based on tumor base diameter (d), apical height (h), and location.

METHODS AND MATERIALS

Plaque and proton treatment plans were created for 4 groups of cases: (1) REF: 39 instances of reference midsize circular-base tumor (d = 12 mm, h = 5 mm), in locations varying by retinal clock hours and distance to fovea, optic disc, and corneal limbus; (2) SUP: 25 superiorly located; (3) TEMP: 25 temporal; and (4) NAS: 25 nasally located tumors that were a fixed distance from the fovea but varying in d (6-18 mm) and h (3-11 mm). For both modalities, 111 unique scenarios were characterized in terms of the distance to points of interest, doses delivered to fovea, optic disc, optic nerve at 3 mm posterior to the disc (ON@3mm), lens, and retina. Comparative statistical evaluation was performed with the Mann-Whitney U test.

RESULTS

Superior dose distributions favored plaque for sparing of (1) fovea in large (d + h ≥ 21 mm) NAS tumors; (2) ON@3mm in REF cases located ≤4 disc diameters from disc, and in NAS overall. Protons achieved superior dose sparing of (1) fovea and optic disc in REF, SUP, and TEMP; (2) ON@3mm in REF >4 disc diameters from disc, and in SUP and TEMP; and (3) the lens center overall and lens periphery in REF ≤6 mm from the corneal limbus, and in TEMP with h = 3 mm. Although protons could completely spare sections of the retina, plaque dose was more target conformal in the high-dose range (50% and 90% of prescription dose).

CONCLUSIONS

Although comparison between plaque and proton therapy is not straightforward because of the disparity in dose rate, prescriptions, applicators, and delivery techniques, it is possible to identify distinctions between dose distributions, which could help inform decisions by providers and patients.

Design and optimizing a novel ocular plaque brachytherapy with dual-core of 103Pd and 106Ru

In recent decades, eye plaques of brachytherapy have been extensively used as primary treatment as well as a complementary treatment for ocular cancer. The purpose of this study is the development of the eye plaque brachytherapy throughout a new design of eye plaque by combining the COMS plaque and the CCB BEBIG plaque loaded by IRA1-¹⁰³Pd and ¹⁰⁶Ru, respectively. A new dual-core plaque with a diameter of 20 mm was designed in the way that the BEBIG plaque with a diameter of 20 mm loaded by ¹⁰⁶Ru plate is attached to the COMS plaque with a diameter of 20 mm loaded by 24 of IRA1-¹⁰³Pd seeds. Dose calculations for the new plaque were performed by using the MCNP5 code. Dose calculations of dual-core plaque including ¹⁰³Pd seeds (gamma) and ¹⁰⁶Ru plate (beta) were separately done for the sake of MCNP constraints in gamma and beta particle transfer simultaneously. The new dual-core plaque delivers a much higher dose rate to the tumor compared with every single plaque, while the dose rate reached to healthy tissues is slightly higher than each plaque separately. Of course, this is acceptable because the treatment time reduces and subsequently the error in radiation therapy reduces.

3D image-guided treatment planning for Ruthenium-106 brachytherapy of choroidal melanomas

BACKGROUND

Current standard treatment procedures for Ruthenium-106 (Ru-106) brachytherapy for choroidal melanomas do not use 3D image-guided treatment planning. We evaluated the potential impact of introducing 3D treatment planning and quantified the theoretical clinical benefits in terms of tumour control probability (TCP) and normal tissue complication probability (NTCP).

MATERIALS AND METHODS

Treatment plans for thirty-two patients were optimized using 3D image-guided treatment planning and compared to the original 2D clinical plans. Optimization of plans was done in an image-based treatment planning system by optimizing the plaque position and treatment time such that the entire tumour received the prescribed dose of 100 Gy. TCP and NTCP for 2D clinical plans and optimized 3D image-guided plans were estimated from published outcome prediction models and compared within patients using Wilcoxon signed-rank test.

RESULTS

The median minimum tumour dose (D_99% ) for 2D clinical plans was 93 Gy (range: 23-158 Gy), corresponding to 5-year TCP of 75% (IQR 61-86%), while median tumour D_99% for optimized 3D image-guided plans was 115 Gy (range 103-141 Gy), corresponding to TCP of 82% (IQR 80-84%). This was a statistically significant increase in estimated TCP (median increase in TCP 8% (IQR: -5-23, p = 0.006). While the dose to normal tissue increased somewhat, there was no significant change in NTCP.

CONCLUSION

3D treatment planning theoretically allows for improved tumour dose delivery for Ru-106 brachytherapy of choroidal melanomas, resulting in a significant increase in expected tumour control compared to traditional approaches using 2D calculations. The deliverability of optimized plans, and potential increased risk of late complications, will have to be confirmed in future clinical studies.

Monte Carlo Computation of Dose-Volume Histograms in Structures at Risk of an Eye Irradiated with Heterogeneous Ruthenium-106 Plaques

Background/Aims

The aim of this work is to compare Monte Carlo simulated absorbed dose distributions obtained from ¹⁰⁶Ru eye plaques, whose heterogeneous emitter distribution is known, with the common homogeneous approximation. The effect of these heterogeneities on segmented structures at risk is analyzed using an anthropomorphic phantom.

Methods

The generic CCA and CCB, with a homogeneous emitter map, and the specific CCA1364 and CCB1256 ¹⁰⁶Ru eye plaques are modeled with the Monte Carlo code PENELOPE. To compare the effect of the heterogeneities in the segmented volumes, cumulative dose-volume histograms are calculated for different rotations of the aforementioned plaques.

Results

For the cornea, the CCA with the equatorial placement yields the lowest absorbed dose rate while for the CCA1364 in the same placement the absorbed dose rate is 33% higher. The CCB1256 with the hot spot oriented towards the cornea yields the maximum dose rate per unit of activity while it is 44% lower for the CCB.

Conclusions

Dose calculations based on a homogeneous distribution of the emitter substance yield the lowest absorbed dose in the analyzed structures for all plaque placements. Treatment planning based on such calculations may result in an overdose of the structures at risk.

AAPM recommendations on medical physics practices for ocular plaque brachytherapy: Report of task group 221

The American Association of Physicists in Medicine (AAPM) formed Task Group 221 (TG-221) to discuss a generalized commissioning process, quality management considerations, and clinical physics practice standards for ocular plaque brachytherapy. The purpose of this report is also, in part, to aid the clinician to implement recommendations of the AAPM TG-129 report, which placed emphasis on dosimetric considerations for ocular brachytherapy applicators used in the Collaborative Ocular Melanoma Study (COMS). This report is intended to assist medical physicists in establishing a new ocular brachytherapy program and, for existing programs, in reviewing and updating clinical practices. The report scope includes photon- and beta-emitting sources and source:applicator combinations. Dosimetric studies for photon and beta sources are reviewed to summarize the salient issues and provide references for additional study. The components of an ocular plaque brachytherapy quality management program are discussed, including radiation safety considerations, source calibration methodology, applicator commissioning, imaging quality assurance tests for treatment planning, treatment planning strategies, and treatment planning system commissioning. Finally, specific guidelines for commissioning an ocular plaque brachytherapy program, clinical physics practice standards in ocular plaque brachytherapy, and other areas reflecting the need for specialized treatment planning systems, measurement phantoms, and detectors (among other topics) to support the clinical practice of ocular brachytherapy are presented. Expected future advances and developments for ocular brachytherapy are discussed.

[CyberKnife®: new treatment option for uveal melanoma]

BACKGROUND

CyberKnife® stereotactic radiosurgery is a new treatment option for uveal melanoma.

OBJECTIVE

This review outlines the technique of robot-assisted CyberKnife® therapy, as well as the pros and cons in the treatment of uveal melanoma.

METHODS

The study provides a PubMed literature review and own preliminary clinical experiences.

RESULTS

CyberKnife® therapy for choroidal and ciliary body melanomas shows comparable results concerning local tumor control and overall survival matching those of conventional therapies. With only low complication rates, a high level of quality of life can be conserved by possible preservation of visual acuity as well as the ocular globe.

CONCLUSION

Stereotactic radiosurgery using CyberKnife® seems to be an efficient and safe therapeutic option for malignant melanomas affecting the choroid and ciliary body. Comparative studies with conventional radiation strategies are now a high priority.

Monte Carlo Estimation of Absorbed Dose Distributions Obtained from Heterogeneous 106Ru Eye Plaques

BACKGROUND

The distribution of the emitter substance in ¹⁰⁶Ru eye plaques is usually assumed to be homogeneous for treatment planning purposes. However, this distribution is never homogeneous, and it widely differs from plaque to plaque due to manufacturing factors.

METHODS

By Monte Carlo simulation of radiation transport, we study the absorbed dose distribution obtained from the specific CCA1364 and CCB1256 ¹⁰⁶Ru plaques, whose actual emitter distributions were measured. The idealized, homogeneous CCA and CCB plaques are also simulated.

RESULTS

The largest discrepancy in depth dose distribution observed between the heterogeneous and the homogeneous plaques was 7.9 and 23.7% for the CCA and CCB plaques, respectively. In terms of isodose lines, the line referring to 100% of the reference dose penetrates 0.2 and 1.8 mm deeper in the case of heterogeneous CCA and CCB plaques, respectively, with respect to the homogeneous counterpart.

CONCLUSIONS

The observed differences in absorbed dose distributions obtained from heterogeneous and homogeneous plaques are clinically irrelevant if the plaques are used with a lateral safety margin of at least 2 mm. However, these differences may be relevant if the plaques are used in eccentric positioning.

Gold nanoparticle-based brachytherapy enhancement in choroidal melanoma using a full Monte Carlo model of the human eye

The effects of gold nanoparticles (GNPs) in 125I brachytherapy dose enhancement on choroidal melanoma are examined using the Monte Carlo simulation technique. Usually, Monte Carlo ophthalmic brachytherapy dosimetry is performed in a water phantom. However, here, the compositions of human eye have been considered instead of water. Both human eye and water phantoms have been simulated with MCNP5 code. These simulations were performed for a fully loaded 16 mm COMS eye plaque containing 13 125I seeds. The dose delivered to the tumor and normal tissues have been calculated in both phantoms with and without GNPs. Normally, the radiation therapy of cancer patients is designed to deliver a required dose to the tumor while sparing the surrounding normal tissues. However, as the normal and cancerous cells absorbed dose in an almost identical fashion, the normal tissue absorbed radiation dose during the treatment time. The use of GNPs in combination with radiotherapy in the treatment of tumor decreases the absorbed dose by normal tissues. The results indicate that the dose to the tumor in an eyeball implanted with COMS plaque increases with increasing GNPs concentration inside the target. Therefore, the required irradiation time for the tumors in the eye is decreased by adding the GNPs prior to treatment. As a result, the dose to normal tissues decreases when the irradiation time is reduced. Furthermore, a comparison between the simulated data in an eye phantom made of water and eye phantom made of human eye composition, in the presence of GNPs, shows the significance of utilizing the composition of eye in ophthalmic brachytherapy dosimetry Also, defining the eye composition instead of water leads to more accurate calculations of GNPs radiation effects in ophthalmic brachytherapy dosimetry.

Monte Carlo dosimetry for 103Pd, 125I, and 131Cs ocular brachytherapy with various plaque models using an eye phantom

PURPOSE

To investigate dosimetry for ocular brachytherapy for a range of eye plaque models containing(103)Pd, (125)I, or (131)Cs seeds with model-based dose calculations.

METHODS

Five representative plaque models are developed based on a literature review and are compared to the standardized COMS plaque, including plaques consisting of a stainless steel backing and acrylic insert, and gold alloy backings with: short collimating lips and acrylic insert, no lips and silicone polymer insert, no lips and a thin acrylic layer, and individual collimating slots for each seed within the backing and no insert. Monte Carlo simulations are performed using the EGSnrc user-code BrachyDose for single and multiple seed configurations for the plaques in water and within an eye model (including nonwater media). Simulations under TG-43 assumptions are also performed, i.e., with the same seed configurations in water, neglecting interseed and plaque effects. Maximum and average doses to ocular structures as well as isodose contours are compared for simulations of each radionuclide within the plaque models.

RESULTS

The presence of the plaque affects the dose distribution substantially along the plaque axis for both single seed and multiseed simulations of each plaque design in water. Of all the plaque models, the COMS plaque generally has the largest effect on the dose distribution in water along the plaque axis. Differences between doses for single and multiple seed configurations vary between plaque models and radionuclides. Collimation is most substantial for the plaque with individual collimating slots. For plaques in the full eye model, average dose in the tumor region differs from those for the TG-43 simulations by up to 10% for(125)I and (131)Cs, and up to 17% for (103)Pd, and in the lens region by up to 29% for (125)I, 34% for (103)Pd, and 28% for (131)Cs. For the same prescription dose to the tumor apex, the lowest doses to critical ocular structures are generally delivered with plaques containing (103)Pd seeds.

CONCLUSIONS

The combined effects of ocular and plaque media on dose are significant and vary with plaque model and radionuclide, suggesting the importance of model-based dose calculations employing accurate ocular and plaque media and geometries for eye plaque brachytherapy.

Monte Carlo Simulation of the Treatment of Eye Tumors with (106)Ru Plaques: A Study on Maximum Tumor Height and Eccentric Placement

BACKGROUND/AIMS

Ruthenium plaques are used for the treatment of ocular tumors. There is, however, a controversy regarding the maximum treatable tumor height. Some advocate eccentric plaque placement, without a posterior safety margin, to avoid collateral damage to the fovea and optic disc, but this has raised concerns about marginal tumor recurrence. There is a need for quantitative information on the spatial absorbed dose distribution in the tumor and adjacent tissues. We have overcome this obstacle using an approach based on Monte Carlo simulation of radiation transport.

METHODS

CCA and CCB (106)Ru plaques were modeled and their geometry embedded in a computerized tomography scan of the eye of a patient. Different tumor sizes and locations were simulated with the general-purpose Monte Carlo code PENELOPE.

RESULTS

Cumulative dose-volume histograms were obtained for the tumors and the tissues at risk considered. Plots of isodose lines for both plaques were obtained in a computerized tomography study.

CONCLUSIONS

Ruthenium eye plaques are an adequate treatment option for tumors up to around 5 mm in height. According to our results, assuming a correct placement of the plaque, a tumor of 6.5 mm apical height is about the maximum size that can be treated safely with the large CCB plaque.

Preliminary results of proton radiotherapy for choroidal melanoma - the Kraków experience

AIM OF THE STUDY

The objective of the study was to present the preliminary results of proton radiotherapy as a method for treating 15 patients with choroidal melanoma.

MATERIAL AND METHODS

The proton radiotherapy was administered using beams providing energy levels of 60 MeV, which ensures a clinical range of 28.4 mm. In addition, the beam has a very narrow penumbra of 1.3 mm and a sharp distal dose fall-off. All patients received the dose of 60 CGE (cobalt gray equivalent) given to the PTV (planning target volume). This dose was administered in 4 fractions over 4 successive days of treatment.

RESULTS

The tumour had regressed in 8 patients (53.3%) and remained stable in 3 patients (20%). The large tumours in another 3 patients (20%) were removed during vitrectomy (endoresection), which increased the number of patients with tumour regression up to 11 (73.3%). In the case of 1 patient, despite intraocular tumour regression occurring the choroidal melanoma had spread multifocally into the orbit, which necessitated orbit exenteration. The results ensured that the eyeballs of 14 patients (93.3%) could be saved. The follow-up period for the 15 patients ranged between 8 and 26 months (average: 17.4 months, median: 19 months). In this period some side effects were noted: an increase in intraocular pressure, retinal detachment, cataract, maculopathy, neuropathy and vitreous haemorrhaging.

CONCLUSIONS

The preliminary results confirm that proton radiotherapy is an effective method for treating patients with choroidal melanoma. This method ensures an eyeball preservation rate of 93%, with the vision function of 80% of the patients being saved.

Monte Carlo study for designing a dedicated "D"-shaped collimator used in the external beam radiotherapy of retinoblastoma patients

PURPOSE

Retinoblastoma is the most common intraocular malignancy in the early childhood. Patients treated with external beam radiotherapy respond very well to the treatment. However, owing to the genotype of children suffering hereditary retinoblastoma, the risk of secondary radio-induced malignancies is high. The University Hospital of Essen has successfully treated these patients on a daily basis during nearly 30 years using a dedicated "D"-shaped collimator. The use of this collimator that delivers a highly conformed small radiation field, gives very good results in the control of the primary tumor as well as in preserving visual function, while it avoids the devastating side effects of deformation of midface bones. The purpose of the present paper is to propose a modified version of the "D"-shaped collimator that reduces even further the irradiation field with the scope to reduce as well the risk of radio-induced secondary malignancies. Concurrently, the new dedicated "D"-shaped collimator must be easier to build and at the same time produces dose distributions that only differ on the field size with respect to the dose distributions obtained by the current collimator in use. The scope of the former requirement is to facilitate the employment of the authors' irradiation technique both at the authors' and at other hospitals. The fulfillment of the latter allows the authors to continue using the clinical experience gained in more than 30 years.

METHODS

The Monte Carlo code PENELOPE was used to study the effect that the different structural elements of the dedicated "D"-shaped collimator have on the absorbed dose distribution. To perform this study, the radiation transport through a Varian Clinac 2100 C/D operating at 6 MV was simulated in order to tally phase-space files which were then used as radiation sources to simulate the considered collimators and the subsequent dose distributions. With the knowledge gained in that study, a new, simpler, "D"-shaped collimator is proposed.

RESULTS

The proposed collimator delivers a dose distribution which is 2.4 cm wide along the inferior-superior direction of the eyeball. This width is 0.3 cm narrower than that of the dose distribution obtained with the collimator currently in clinical use. The other relevant characteristics of the dose distribution obtained with the new collimator, namely, depth doses at clinically relevant positions, penumbrae width, and shape of the lateral profiles, are statistically compatible with the results obtained for the collimator currently in use.

CONCLUSIONS

The smaller field size delivered by the proposed collimator still fully covers the planning target volume with at least 95% of the maximum dose at a depth of 2 cm and provides a safety margin of 0.2 cm, so ensuring an adequate treatment while reducing the irradiated volume.

Radiation dose to the surgeon during plaque brachytherapy

PURPOSE

To evaluate the radiation dose to a surgeon's hands during I eye plaque procedures.

METHODS

Sixteen consecutive patients with uveal melanomas were scheduled for eye plaque brachytherapy. The same surgeon wore thermoluminescent dosimeters on the dominant index finger and thumb while placing and removing the eye plaque to measure radiation dose. Additional laboratory experiments were performed to measure unobstructed (by surgical gloves or other parts of the hand) radiation exposure from a plaque.

RESULTS

Hand radiation doses during eye plaque brachytherapy are very low, but measurable, with plaques containing an average of 1.3 GBq of 125I.

CONCLUSION

Using these data, a surgeon would need to perform more than 1,000 cases each year to approach or exceed the annual regulatory radiation dose limits for the extremities.

Dosimetry of (125)I and (103)Pd COMS eye plaques for intraocular tumors: report of Task Group 129 by the AAPM and ABS

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy.

Ruthenium plaque radiation therapy for iris and iridociliary melanomas

PURPOSE

To determine the long-term effects of ruthenium-106 plaque radiation therapy for iris and iridociliary melanomas in terms of tumour regression and complications.

METHODS

Between 1 November 1997 and 31 December 2007, 36 patients with an iris or iridociliary melanoma were treated with Ruthenium-106 (Ru-106) ophthalmic plaque radiation therapy. The median follow-up was 6.5 years with a range of 2-11 years. The mean tumour apex dose was 151.5 Gy. Main outcome measures were local tumour control and radiation-related ocular complications.

RESULTS

The mean age of the patients at the time of treatment was 54 years (range 14-82). The tumours had a median largest basal dimension of 4.8 mm (range 3-11) and a prominence of 2.3 mm (range 0.8-5), with a median involvement of three clock hours (range 1-6). The tumours were confined to the iris in 14 patients (39%), extended into the anterior ciliary body in 12 (33%), while the anterior ciliary body tumour extended into the iris in ten patients (28%). Tumours regressed in all patients (100%) with a mean regression of 80% of the original tumour size at 7 years of follow-up. Radiation-related complications included corneal erosions on the first postoperative day in nine patients (25%), cataract in four of 11 patients free of cataract before treatment (36%) and postradiation glaucoma in one patient (3%). Visual acuity of 20/200 or worse was present in one of 20 patients (5%) at 6 years of follow-up. Local recurrences occurred in two patients (5%), 2 and 5 years after the radiation therapy respectively, and both underwent enucleation. Distant metastases occurred in one of these enucleated patients (3%) 2 years after treatment.

CONCLUSION

Ru-106 plaque radiation therapy for iris and iridociliary melanoma resulted in good local tumour control and preservation of visual acuity with few and treatable side-effects.

Assessment of targeting accuracy of a low-energy stereotactic radiosurgery treatment for age-related macular degeneration

Age-related macular degeneration (AMD), a leading cause of blindness in the United States, is a neovascular disease that may be controlled with radiation therapy. Early patient outcomes of external beam radiotherapy, however, have been mixed. Recently, a novel multimodality treatment was developed, comprising external beam radiotherapy and concomitant treatment with a vascular endothelial growth factor inhibitor. The radiotherapy arm is performed by stereotactic radiosurgery, delivering a 16 Gy dose in the macula (clinical target volume, CTV) using three external low-energy x-ray fields while adequately sparing normal tissues. The purpose of our study was to test the sensitivity of the delivery of the prescribed dose in the CTV using this technique and of the adequate sparing of normal tissues to all plausible variations in the position and gaze angle of the eye. Using Monte Carlo simulations of a 16 Gy treatment, we varied the gaze angle by ±5° in the polar and azimuthal directions, the linear displacement of the eye ±1 mm in all orthogonal directions, and observed the union of the three fields on the posterior wall of spheres concentric with the eye that had diameters between 20 and 28 mm. In all cases, the dose in the CTV fluctuated <6%, the maximum dose in the sclera was <20 Gy, the dose in the optic disc, optic nerve, lens and cornea were <0.7 Gy and the three-field junction was adequately preserved. The results of this study provide strong evidence that for plausible variations in the position of the eye during treatment, either by the setup error or intrafraction motion, the prescribed dose will be delivered to the CTV and the dose in structures at risk will be kept far below tolerance doses.

Assessment of the accuracy of an MCNPX-based Monte Carlo simulation model for predicting three-dimensional absorbed dose distributions

In recent years, the Monte Carlo method has been used in a large number of research studies in radiation therapy. For applications such as treatment planning, it is essential to validate the dosimetric accuracy of the Monte Carlo simulations in heterogeneous media. The AAPM Report no 105 addresses issues concerning clinical implementation of Monte Carlo based treatment planning for photon and electron beams, however for proton-therapy planning, such guidance is not yet available. Here we present the results of our validation of the Monte Carlo model of the double scattering system used at our Proton Therapy Center in Houston. In this study, we compared Monte Carlo simulated depth doses and lateral profiles to measured data for a magnitude of beam parameters. We varied simulated proton energies and widths of the spread-out Bragg peaks, and compared them to measurements obtained during the commissioning phase of the Proton Therapy Center in Houston. Of 191 simulated data sets, 189 agreed with measured data sets to within 3% of the maximum dose difference and within 3 mm of the maximum range or penumbra size difference. The two simulated data sets that did not agree with the measured data sets were in the distal falloff of the measured dose distribution, where large dose gradients potentially produce large differences on the basis of minute changes in the beam steering. Hence, the Monte Carlo models of medium- and large-size double scattering proton-therapy nozzles were valid for proton beams in the 100 MeV-250 MeV interval.

Monte Carlo calculations and measurements of absorbed dose per monitor unit for the treatment of uveal melanoma with proton therapy

The treatment of uveal melanoma with proton radiotherapy has provided excellent clinical outcomes. However, contemporary treatment planning systems use simplistic dose algorithms that limit the accuracy of relative dose distributions. Further, absolute predictions of absorbed dose per monitor unit are not yet available in these systems. The purpose of this study was to determine if Monte Carlo methods could predict dose per monitor unit (D/MU) value at the center of a proton spread-out Bragg peak (SOBP) to within 1% on measured values for a variety of treatment fields relevant to ocular proton therapy. The MCNPX Monte Carlo transport code, in combination with realistic models for the ocular beam delivery apparatus and a water phantom, was used to calculate dose distributions and D/MU values, which were verified by the measurements. Measured proton beam data included central-axis depth dose profiles, relative cross-field profiles and absolute D/MU measurements under several combinations of beam penetration ranges and range-modulation widths. The Monte Carlo method predicted D/MU values that agreed with measurement to within 1% and dose profiles that agreed with measurement to within 3% of peak dose or within 0.5 mm distance-to-agreement. Lastly, a demonstration of the clinical utility of this technique included calculations of dose distributions and D/MU values in a realistic model of the human eye. It is possible to predict D/MU values accurately for clinical relevant range-modulated proton beams for ocular therapy using the Monte Carlo method. It is thus feasible to use the Monte Carlo method as a routine absolute dose algorithm for ocular proton therapy.