Optic disc dose reduction for a prescription depth of 5 mm
Case #1: one seed removal
Fig. 1a-f presents optic disc dose (dashed lines) as a function of DT for various BDs for six notched COMS plaques with one seed removed in comparison to that (solid lines) for six standard COMS plaques. A prescription depth was 5 mm. As expected, when one seed was removed, optic disc dose was reduced.
(Gy) by notched COMS plaques with one seed removed is shown in Fig. 2a-f. A prescription depth was 5 mm. Following unique trends for optic disc dose reduction are observed.
1) (Gy) increases with DT, reaches the maximum value () and then decreases with DT. For the largest 2-3 BDs, however, dose reduction continuously decreases with DT. Examples include those for BDs of 7 mm and 9 mm in 12 mm notched plaque (Fig. 2a) and those for BDs of 15 mm, 17 mm and 19 mm in 22 mm notched plaque (Fig. 2f).
2) Maximum absolute optic disc dose reduction () is usually larger with smaller plaques but does not continuously decrease with plaque size.
3) The magnitude of does not vary with BD in each plaque except for those for the largest 2-3 BDs (Table 2).
4) DT at which occurs (DTmaxD) decreases by about 1 mm with increasing BD by 2 mm. For example, for 12 mm notched plaque, DTmaxD decreases from 2.7 mm to 1.7 mm as BD increases from 1 mm to 3 mm (Fig. 2a). On the other hand, for the largest 2-3 BDs, DTmaxD is always 0 mm or close to 0 mm. For instance, in 12 mm notched plaque, DTmaxD becomes 0 mm when BD is 7 mm or 9 mm (Fig. 2a).
5) DTmaxD increases with plaque size for the same BD. For BD of 5 mm (blue line in Fig. 2), DTmaxD are 0.7 mm, 1.9 mm, 2.2 mm, 3.5 mm, 4.4 mm and 5.0 mm for 12 mm, 14 mm, 16 mm, 18 mm, 20 mm and 22 mm plaques, respectively (Table 2).
Table 2 summarizes (Gy), and corresponding and DTmaxD (mm) for each BD and for each notched plaque with one seed removed (Case #1). (are 86.3 Gy (35.5%-35.6%), 57.1 Gy (27.9%-28.3%), 59.9 Gy (29.3%-29.6%), 40.9 Gy (25.3%-25.5%), 38.3 Gy (26.7%-27.1%) and 45.5 Gy (32.3%-32.7%) for 12 mm, 14 mm, 16 mm, 18 mm, 20 mm and 22 mm plaques, respectively, excluding those for the largest 2-3 BDs (Fig. 2 and Table 2). For each plaque and each prescription depth, ranges within 0.4% with varying BD.
Case #2: two seeds removal
Fig. 3a-e displays optic disc dose (dashed lines) as a function of DT for various BDs for five notched COMS plaques with two seeds removed and that (solid lines) for five standard COMS plaques. A prescription depth was 5 mm. When two seeds were removed, optic disc dose was reduced more than when one seed was removed.
by notched COMS plaques with two seeds removed is shown in Fig. 4a-e. A prescription depth was 5 mm. The amount of dose reduction is larger but trends for dose reduction are similar compared with in Case #1.
Table 3 summarizes (Gy), and corresponding and DTmaxD (mm) for each BD and for each notched plaque with two seeds removed (Case #2). (are 101.4 Gy (43.5%-44.0%), 107.5 Gy (44.6%-47.1%), 75.0 Gy (41.5%-41.7%), 59.4 Gy (38.1%-38.2%) and 74.0 Gy (47.9%-48.3%) for 14 mm, 16 mm, 18 mm, 20 mm and 22 mm plaques, respectively, excluding those for the largest 2-4 BDs (Fig. 4 and Table 3).
Case #1 versus Case #2
Although trends for optic disc dose reduction are similar between Case #1 and Case #2 (Fig. 2 and 4), differences in and in DTmaxD between the two cases are fairly distinct (Tables 2-3). Differences in between the two cases are 44.3 Gy (57.1 Gy vs. 101.4 Gy), 47.6 Gy (59.9 Gy vs. 107.5 Gy), 34.1 Gy (40.9 Gy vs. 75.0 Gy), 21.1 Gy (38.3 Gy vs. 59.4 Gy) and 28.5 Gy (45.5 Gy vs. 74.0 Gy) for 14 mm - 22 mm plaques in order, respectively (Tables 2 and 3). In Case #2, DTmaxD becomes shallower by 0.7 mm - 0.8 mm for 14 mm, 16 mm, 18 mm and 22 mm plaques and by 0.5 mm - 0.6 mm for the 20 mm plaque than in Case #1 for the same BD (Tables 2 and 3). For instance, in Case #1, for BD = 1 mm (red line in Fig.2), DTmaxD are 3.9 mm, 4.2 mm, 5.5 mm, 6.3 mm and 6.9 mm for 14 mm - 22 mm plaques in order, respectively, whereas in Case #2, for BD = 1 mm (red line in Fig.4), corresponding DTmaxD are 3.2 mm, 3.4 mm, 4.8 mm, 5.8 mm and 6.2 mm.
Dose conversion factors for different prescription depths
Dose conversion factors for prescription depths from 1 mm to 10 mm for standard COMS plaques were reported in our previous study [13]. In the current study, dose conversion factors are presented in Table 4 for six notched COMS plaques with one seed removed (Case #1) and in Table 5 for five notched COMS plaques with two seeds removed (Case #2). For plaques from 14 mm to 22 mm, dose conversion factors for Case #1 are similar to those for Case #2 within 2.4%. Compared to dose conversion factors for standard COMS plaques [13], those for notched COMS plaques from 14 mm to 22 mm are slightly higher (up to 2.7% for Case #1 and 3.4% for Case #2) at shallow (<5 mm) prescription depths and slightly lower (up to 1.1% for Case #1 and 1.4% for Case #2) at deeper (>5 mm) depths. However, trends for dose conversion factors between standard and notched COMS plaques are similar. The factors increase with increasing prescription depth. Also, the factors increase with increasing plaque size for depth <5 mm but decrease with increasing plaque size for depth >5 mm.
Optic disc dose reduction for different prescription depths
Optic disc dose reduction by notched COMS plaques increases with increasing prescription depth but its trends for the other prescription depths (1 mm - 4 mm & 6 mm - 10 mm) are similar to the five trends mentioned above for the depth of 5 mm (data not shown here). Tables 6 and 7 present for each prescription depth and for each notched plaque with one seed removed (Case #1) and with two seeds removed (Case #2), respectively. Those for the largest 2-4 BDs are not included in Tables 6-7. (Gy) increases with increasing prescription depth for each plaque in both cases. However, corresponding decreases slightly (up to 2.0%) with increasing prescription depth.
Estimation of optic disc dose: clinical application of this study
A clinical example (BD = 3 mm, DT = 3 mm and apical height = 3 mm) is given and practical application of the results obtained in this study for this example is demonstrated in Table 8. A prescribed dose is 85 Gy and a 14 mm COMS plaque loaded with 125I seeds (model IAI-125A) is selected. Depending on the type of COMS plaque and prescription depth, six clinical scenarios are possible and optic disc dose can be estimated for each scenario (Table 8). Of these, Scenario #1 (standard COMS plaque and prescription depth of 5 mm) and Scenario #6 (notched COMS plaque with two seeds removed and prescription depth of 3 mm) give the highest (197.6 Gy) and the lowest (65.0 Gy) optic disc doses, respectively. Optic disc dose difference between these two scenarios is 132.6 Gy. When 85 Gy is prescribed at 5 mm, dose reduction from optic disc dose (197.6 Gy) for the standard plaque is 56.9 Gy for one seed removal (Scenario #2) and 92.7 Gy for two seeds removal (Scenario #3). At 3 mm, dose reduction from optic disc dose (124.5 Gy) for the standard plaque is 37.3 Gy for one seed removal (Scenario #5) and 59.5 Gy for two seeds removal (Scenario #6). Although absolute dose reduction is larger for depth of 5 mm, relative dose reduction is similar between the two depths (i.e., approximately 29% for one seed removal (Scenarios #2 and #5) and 47% for two seeds removal (Scenarios #3 and #6)).