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Novel low-kVp beamlet system for choroidal melanoma.

Esquivel C, Fuller CD, Waggener RG, Wong A, Meltz M, Blough M, Eng TY, Thomas CR - Radiat Oncol (2006)

Bottom Line: The BLOKX technique showed a significant reduction of dose, 89.8%, to the macula compared to the episcleral plaque.A minimum 71.0 % decrease in dose to the optic nerve occurred when the BLOKX was used.The BLOKX technique allows more favorable dose distribution in comparison to standard COMS brachytherapy, as simulated using a Monte Carlo iterative mathematical modeling.

View Article: PubMed Central - HTML - PubMed

Affiliation: Cancer Therapy and Research Center, San Antonio, TX, USA. cesquive@ctrc.net

ABSTRACT

Background: Treatment of choroidal melanoma with radiation often involves placement of customized brachytherapy eye-plaques. However, the dosimetric properties inherent in source-based radiotherapy preclude facile dose optimization to critical ocular structures. Consequently, we have constructed a novel system for utilizing small beam low-energy radiation delivery, the Beamlet Low-kVp X-ray, or "BLOKX" system. This technique relies on an isocentric rotational approach to deliver dose to target volumes within the eye, while potentially sparing normal structures.

Methods: Monte Carlo N-Particle (MCNP) transport code version 5.0(14) was used to simulate photon interaction with normal and tumor tissues within modeled right eye phantoms. Five modeled dome-shaped tumors with a diameter and apical height of 8 mm and 6 mm, respectively, were simulated distinct positions with respect to the macula iteratively. A single fixed 9 x 9 mm2 beamlet, and a comparison COMS protocol plaque containing eight I-125 seeds (apparent activity of 8 mCi) placed on the scleral surface of the eye adjacent to the tumor, were utilized to determine dosimetric parameters at tumor and adjacent tissues. After MCNP simulation, comparison of dose distribution at each of the 5 tumor positions for each modality (BLOKX vs. eye-plaque) was performed.

Results: Tumor-base doses ranged from 87.1-102.8 Gy for the BLOKX procedure, and from 335.3-338.6 Gy for the eye-plaque procedure. A reduction of dose of at least 69% to tumor base was noted when using the BLOKX. The BLOKX technique showed a significant reduction of dose, 89.8%, to the macula compared to the episcleral plaque. A minimum 71.0 % decrease in dose to the optic nerve occurred when the BLOKX was used.

Conclusion: The BLOKX technique allows more favorable dose distribution in comparison to standard COMS brachytherapy, as simulated using a Monte Carlo iterative mathematical modeling. Future series to determine clinical utility of such an approach are warranted.

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MCNP output simulation of BLOKX procedure.
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Figure 5: MCNP output simulation of BLOKX procedure.

Mentions: Doses to the tumor base, apex of the tumor, macula, optic nerve and center of the lens can be seen in Table 1. The doses to the base of the tumor were 87.1, 101.8 and 102.8 Gy respectively, for tumors located at 60, 90 and 270° with respect to the macula. For tumors located at 45 and 30° with respect to the macula, minimum dose to the base of the tumor and all doses to other structures were normalized to the base of the tumor which received 85 Gy. The apex of the tumor received between 93.1 to 112.0 Gy for these locations. The macula received doses between 1.5 to 3.2 Gy for tumors located at 60, 90, and 270° with respect to the macula. For the 45 and 30° tumor locations, the macula received 51.0 and 70.4 Gy respectively. Another simulation was run for the tumor located at 45° from the macula. This time the BLOKX was repositioned and the beam was aimed at the tumor from another direction (Figure 4). A drop in the dose to the macula, from 51.0 to 4.0 Gy was observed. The optic nerve received doses between 1.2 to 2.9 Gy and was well below the recommended dose limit of 10 Gy for all tumor locations. The MCNP-derived doses to the lens range from 1.8 to 16.54 Gy. The uncertainties affecting the measurements were estimated to be approximately 0.32 to 3.1%. Figure 5 illustrates the MCNP simulations of the BLOKX irradiating the tumor in the eye. The primary beam is directed at the tumor. Only the scattered radiation will reach critical structures in the eye.


Novel low-kVp beamlet system for choroidal melanoma.

Esquivel C, Fuller CD, Waggener RG, Wong A, Meltz M, Blough M, Eng TY, Thomas CR - Radiat Oncol (2006)

MCNP output simulation of BLOKX procedure.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC1586203&req=5

Figure 5: MCNP output simulation of BLOKX procedure.
Mentions: Doses to the tumor base, apex of the tumor, macula, optic nerve and center of the lens can be seen in Table 1. The doses to the base of the tumor were 87.1, 101.8 and 102.8 Gy respectively, for tumors located at 60, 90 and 270° with respect to the macula. For tumors located at 45 and 30° with respect to the macula, minimum dose to the base of the tumor and all doses to other structures were normalized to the base of the tumor which received 85 Gy. The apex of the tumor received between 93.1 to 112.0 Gy for these locations. The macula received doses between 1.5 to 3.2 Gy for tumors located at 60, 90, and 270° with respect to the macula. For the 45 and 30° tumor locations, the macula received 51.0 and 70.4 Gy respectively. Another simulation was run for the tumor located at 45° from the macula. This time the BLOKX was repositioned and the beam was aimed at the tumor from another direction (Figure 4). A drop in the dose to the macula, from 51.0 to 4.0 Gy was observed. The optic nerve received doses between 1.2 to 2.9 Gy and was well below the recommended dose limit of 10 Gy for all tumor locations. The MCNP-derived doses to the lens range from 1.8 to 16.54 Gy. The uncertainties affecting the measurements were estimated to be approximately 0.32 to 3.1%. Figure 5 illustrates the MCNP simulations of the BLOKX irradiating the tumor in the eye. The primary beam is directed at the tumor. Only the scattered radiation will reach critical structures in the eye.

Bottom Line: The BLOKX technique showed a significant reduction of dose, 89.8%, to the macula compared to the episcleral plaque.A minimum 71.0 % decrease in dose to the optic nerve occurred when the BLOKX was used.The BLOKX technique allows more favorable dose distribution in comparison to standard COMS brachytherapy, as simulated using a Monte Carlo iterative mathematical modeling.

View Article: PubMed Central - HTML - PubMed

Affiliation: Cancer Therapy and Research Center, San Antonio, TX, USA. cesquive@ctrc.net

ABSTRACT

Background: Treatment of choroidal melanoma with radiation often involves placement of customized brachytherapy eye-plaques. However, the dosimetric properties inherent in source-based radiotherapy preclude facile dose optimization to critical ocular structures. Consequently, we have constructed a novel system for utilizing small beam low-energy radiation delivery, the Beamlet Low-kVp X-ray, or "BLOKX" system. This technique relies on an isocentric rotational approach to deliver dose to target volumes within the eye, while potentially sparing normal structures.

Methods: Monte Carlo N-Particle (MCNP) transport code version 5.0(14) was used to simulate photon interaction with normal and tumor tissues within modeled right eye phantoms. Five modeled dome-shaped tumors with a diameter and apical height of 8 mm and 6 mm, respectively, were simulated distinct positions with respect to the macula iteratively. A single fixed 9 x 9 mm2 beamlet, and a comparison COMS protocol plaque containing eight I-125 seeds (apparent activity of 8 mCi) placed on the scleral surface of the eye adjacent to the tumor, were utilized to determine dosimetric parameters at tumor and adjacent tissues. After MCNP simulation, comparison of dose distribution at each of the 5 tumor positions for each modality (BLOKX vs. eye-plaque) was performed.

Results: Tumor-base doses ranged from 87.1-102.8 Gy for the BLOKX procedure, and from 335.3-338.6 Gy for the eye-plaque procedure. A reduction of dose of at least 69% to tumor base was noted when using the BLOKX. The BLOKX technique showed a significant reduction of dose, 89.8%, to the macula compared to the episcleral plaque. A minimum 71.0 % decrease in dose to the optic nerve occurred when the BLOKX was used.

Conclusion: The BLOKX technique allows more favorable dose distribution in comparison to standard COMS brachytherapy, as simulated using a Monte Carlo iterative mathematical modeling. Future series to determine clinical utility of such an approach are warranted.

Show MeSH
Related in: MedlinePlus