IS SMALLER BETTER? COMPARISON OF 3-MM AND 5-MM LEAF SIZE FOR STEREOTACTIC RADIOSURGERY: A DOSIMETRIC STUDY

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1 Int. J. Radiation Oncology Biol. Phys., Vol. 66, No. 4, Supplement, pp. S76 S81, 2006 Copyright 2006 Elsevier Inc. Printed in the USA. All rights reserved /06/$ see front matter doi: /j.ijrobp SRS/SRT SUPPLEMENT IS SMALLER BETTER? COMPARISON OF 3-MM AND 5-MM LEAF SIZE FOR STEREOTACTIC RADIOSURGERY: A DOSIMETRIC STUDY SHYH-SHI CHERN, PH.D.,* DENNIS D. LEAVITT, PH.D.,* RANDY L. JENSEN, M.D., PH.D., AND DENNIS C. SHRIEVE, M.D., PH.D.* Departments of *Radiation Oncology and Neurosurgery, University of Utah School of Medicine, Salt Lake City, Utah Purpose: To perform a dosimetric comparison of a minimal 3-mm leaf width multileaf collimator (MLC) and a minimal 5-mm MLC in dynamic conformal arc stereotactic radiosurgery for treatment of intracranial lesions. Methods and Materials: The treatment plans of 23 patients previously treated for intracranial lesions in our institution were redone using the BrainSCAN, version 5.3, stereotactic radiosurgery treatment planning system (BrainLAB). For each case, two dynamic conformal arc plans were generated: one using a minimal 3-mm micro-mlc (BrainLAB, Novalis) and one using a minimal 5-mm MLC (Varian Millennium). All arc parameters were the same in each of the two plans, except for the collimator angle settings. The collimator angle settings were optimized for each arc in each plan. A peritumoral rind structure (1 cm) was created to evaluate normal tissue sparing immediately adjacent to the target volume. Conformity indexes (CIs) were calculated for each plan. The dependence of normal tissue sparing and target conformity on target volume (TV) was determined. Results: The TV was cm 3 (median, 5.90). The CI was (median, 1.51) for the 3-mm micro-mlc and (median, 1.60) for the 5-mm MLC. Despite this small difference, it was a statistically significant increase (p < ) for the 5-mm MLC compared with the 3-mm micro-mlc. Improved normal tissue sparing was demonstrated using the 3-mm micro-mlc compared with the 5-mm MLC by examining the peritumoral rind volumes (PRVs) receiving 50% (PRV 50 ), 80% (PRV 80 ), and 90% (PRV 90 ) of the prescription dose. The reduction in the PRV 50, PRV 80, and PRV 90 for the 3-mm micro-mlc compared with the 5-mm MLC was 13.5%, 12.9%, and 11.5%, respectively. The CI decreased with a larger TV, as did the difference in the CIs between the 3-mm micro-mlc and 5-mm MLC. A reduction in the PRV increased with larger TVs. Conclusion: The 3-mm micro-mlc provided better target conformity and greater normal tissue sparing than the 5-mm MLC in stereotactic radiosurgery using dynamic conformal arcs. These differences were small but consistent in the patients examined. Future research is needed to determine whether this small improvement can yield a clinical impact on patient care Elsevier Inc. Stereotactic radiosurgery, Multileaf collimator, Dynamic conformal arc SRS. INTRODUCTION The hallmark of stereotactic radiosurgery (SRS) is the ability to conform the prescription isodose surface to the target volume, allowing delivery of a high, single radiation dose to an intracranial target, while limiting the dose to surrounding normal tissues (1). In the early development of linear accelerator-based SRS (2 4), different size circular collimators were used by way of multiple noncoplanar arcs to deliver the radiation dose to the target. With the advancement of the multileaf collimator (MLC) (5, 6), techniques were developed that allowed the use of noncircular apertures, leading to the ability to shape dose distributions to better conform to the target volume (7 9). In dynamic conformal arc SRS, the micro-mlc changes shape continuously throughout the treatment arc (10). The aperture shape therefore conforms to the beam s eye view of the target Reprint requests to: Shyh-Shi Chern, Ph.D., Department of Radiation Oncology, Huntsman Cancer Hospital, University of Utah School of Medicine, 1950 Circle of Hope, Salt Lake City, UT Tel: (801) ; Fax: (801) ; Richard. volume throughout the treatment. Because MLCs of smaller leaf widths, such as the BrainLAB micro-mlc (minimal 3-mm leaf width; BrainLAB, Munich, Germany) and Varian Millennium MLC (minimal 5-mm leaf width; Varian, Palo Alto, CA), which are now commercially available, the application of the dynamic conformal arc technique is expected to yield further improvement in target conformity and normal tissue sparing. The goal of this report was to dosimetrically compare the 3-mm micro-mlc (BrainLAB Novalis) with the 5-mm MLC (Varian Millennium) in the treatment of intracranial lesions using dynamic conformal arc SRS. The evaluation of the dose volume histogram of treatment plans generated by these two MLCs provided insight into the possible advantages of one compared with the other. Possible clinical implications are discussed. Chern@hci.utah.edu Received July 27, 2005, and in revised form March 31, Accepted for publication April 4, S76

2 3-mm vs. 5-mm MLC in SRS Table 1. Patient and tumor characteristics Tumor type Age Gender TV (cm3) AVM AVM Metastatic renal Pituitary Average NA Abbreviations: AVM arteriovenous malformation; glioblastoma multiforme; NA not applicable. METHODS AND MATERIALS Patient selection A total of 23 patients previously treated for intracranial lesions in our institution by dynamic conformal arc SRS were selected for S.-S. CHERN et al. S77 analysis in this study. These patients were randomly selected from 170 patients treated with SRS during the past 4 years. Selection was based purely on finding a wide range of radiosurgical targets. These patients included 4 with meningioma, 5 with glioblastoma multiforme, 6 with acoustic neuroma, 5 with metastatic lesions, 2 with arteriovenous malformations, and 1 with a pituitary tumor. The average patient age was 61 years, with the patients with arteriovenous malformation and glioblastoma multiforme being younger and those with acoustic neuroma and meningiomas older. The volume range was cm3 (median, 5.90; Table 1). Treatment planning The team of radiation oncologists and neurosurgeons outlined the target volumes. A dynamic conformal arc plan was created for the target using multiple noncoplanar arcs. The number and length of arcs were initially set by default to five arcs and 100 for each arc. Couch positions were also initially set to five default positions. In some cases, four arcs, as well as a shorter length of arcs, could be used to avoid traversing normal structures, such as the eyes. In some cases, a longer arc length was needed because of the smaller target size and limitations on machine output per degree of arc. To compare treatment plans between the 3-mm micro-mlc and the 5-mm MLC, all arc parameters were kept the same for each target, except for the collimator angle. Optimized collimator angles were determined independently for the 3- and 5-mm MLC systems. Figure 1 shows a beam s eye view of the target for the 3- and 5-mm MLCs with their respective optimized collimator angles. As illustrated in Fig. 2, a peritumoral rind structure (1 cm) was created to evaluate the dose to the tissue immediately adjacent to the TV. The dose volume histograms of both plans were then examined. The calculation grid was set at 0.5 mm. Conformity indexes (CIs) were calculated to evaluate target conformity. Fig. 1. (a) Beam s eye view of single 100 dynamic arc for 5-mm multileaf collimator (MLC) showing collimator leaf position for each 10 interval with 90 optimized collimator angle. (b) Beam s eye view of single 100 dynamic arc for 3-mm micro-mlc showing collimator leaf position for each 10 interval, with 120 optimized collimator angle.

3 S78 I. J. Radiation Oncology Biology Physics Volume 66, Number 4, Supplement, 2006 R i n d Normal tissue sparing A typical dose volume histogram of peritumoral rind structure is illustrated in Fig. 5. A reduction in PRV for the 3-mm micro-mlc compared with the 5-mm MLC for all PISs 40% was noted. Figure 6 summarizes the compari- T a r g e t 1 c m devised to calculate the CI. In the example shown in Table 2, the 86% PIS provided coverage of 95% of the TV, but 95% of the 86% PIS (82% PIS) covered only 98% of the TV. Thus, the selection of the 86% PIS failed the second criterion, although it satisfied the first. In contrast, the 85% PIS covered 95% (96% actually) of the TV and 95% of the 85% PIS (81% PIS) covered 99% of the TV. Therefore, the selection of the 85% PIS satisfied both criteria. To quantify the difference in CI between the 3-mm micro-mlc and the 5-mm Millennium MLC, the percentage of difference of the CI (% CI) was defined as follows: % CI CI 120mlc CI mmlc CI mmlc 100% where CI 120MLC and CI mmlc are the CIs for the 5-mm and 3-mm MLCs, respectively. Fig. 2. Illustration of peritumoral rind structure and its associated target. Collimators The BrainLAB 3-mm micro-mlc has three different widths. It has a total of 26 pairs of leaves that form a maximal field size of approximately cm (5, 6). The 14 innermost pairs of leaves project to a 3-mm width at the isocenter. The intermediate 6 pairs of leaves project to a 4.5 mm width at the isocenter. The outermost 6 pairs of leaves project to a 5.5 mm width at the isocenter. The Varian 120-leaf Millenium MLC has two leaf widths, 5 mm (inner 40 pairs) and 10 mm (outer 20 pairs). They form a maximal field size of 40.0 cm 40.0 cm. The difference in dosimetric characteristics among the 3-mm micro-mlc and the 5-mm Millennium MLC is illustrated in Fig. 1. As shown, the 3-mm micro-mlc conforms to a tighter margin than the 5-mm Millennium MLC. This leads to more dose in the peripheral region of the target using the 5-mm Millennium MLC than using the 3-mm micro-mlc. Conformity index The CI, as originally described by Paddick (1) and modified by Nakamura et al. (11) was calculated as follows: (PIV PVTV) CI (PVTV TV) where TV is the target volume; PIV is the prescription isodose volume or the volume encompassed by the prescription isodose surface (PIS); and PVTV is the TV included in the PIS. Thus, PIV is equal to PVTV plus the normal tissue volume encompassed by the PIS. The CI is highly dependent on the PIS chosen. Therefore, the PIS should be chosen according to consistent criteria. Our selection of the PIS was based on the following criteria: the greatest PIS covering 95% of the TV while delivering 95% of the prescription dose to 99% of the TV. Once the PIS was determined for the 3-mm micro-mlc, the same PIS was applied to the 5-mm MLC. The CI has a minimal value of 1, indicating perfect target conformity. For this study, a Microsoft spreadsheet was PRV 50, PRV 80, PRV 90, PRV 50, PRV 80, and PRV 90 The peritumoral rind structure (1 cm) was created to evaluate normal tissue sparing adjacent to the target. The peritumoral rind volume (PRV) was calculated as the PRV 50 (PRV receiving 50% of the prescription dose), PRV 80 (PRV receiving 80% of the prescription dose), and PRV 90 (PRV receiving 90% of the prescription dose). The difference in PRV 50 ( PRV 50 ), PRV 80 ( PRV 80 ), and PRV 90 ( PRV 90 ) between the 5-mm Millennium MLC and 3-mm micro-mlc was also calculated. More specifically, 120 PRV 50 PRV mlc 50 PRV mmlc 50, 120 PRV 80 PRV mlc 80 PRV mmlc 80, 120 and PRV 90 PRV mlc mmlc 90 PRV 90 where 120-MLC and micro-mlc in superscript represent the 5-mm MLC and 3-mm micro-mlc, respectively. RESULTS Conformity index As shown in Tables 1 and 3, and the TVs was cm 3 (median, 5.90). The CI was (median, 1.505) for the 3-mm micro-mlc and (median, 1.60) for the 5-mm MLC. Despite this small difference, it was a statistically significant increase (p using the paired two-tailed Student t test) for the 5-mm MLC compared with the 3-mm micro-mlc. Figure 3 shows a scatter plot of the CI vs. TV for each MLC. In this plot, a general pattern was seen toward a decreasing CI with an increasing TV. In Fig. 4, the plot of % CI vs. TV is shown. The percentage of difference in the CI seemed to decrease with an increasing TV, although the correlation was weak.

4 3-mm vs. 5-mm MLC in SRS S.-S. CHERN et al. S79 Table 2. Microsoft spreadsheet used to select PIS and calculate CI Normal volume TV PIV PVTV Treatment isodose Percent covered 95% of PIS CI Abbreviations: PIS prescription isodose surface; CI conformity index; TV target volume; PIV prescription isodose volume; PVTV TV included in PIS. sons of PRV 50, PRV 80, and PRV 90 between the 3-mm micro-mlc and 5-mm MLC. For each PRV, a statistically significant reduction (p , paired two-tailed Student t test) was noted in the PRV for the 3-mm micro-mlc compared with the 5-mm MLC. The percentage of PRV reduction on average for PRV 50, PRV 80, and PRV 90 was 13.5%, 12.9%, and 11.5%, respectively. Figure 7 shows the plot of PRV 50, PRV 80, and PRV 90 vs. TV. They all followed a similar upward trend with an increasing TV. This result was somewhat expected because a larger TV has a larger PRV. As a result, the PRV tended to increase with an increasing TV. DISCUSSION Our comparison of the dosimetry for 3-mm compared with that for 5-mm leaf-width MLC in dynamic conformal arc SRS indicated several small, but consistent, differences. The CI for the 3-mm micro-mlc was improved compared with that for the 5-mm MLC. This result agrees with previous reports (8, 12 15) by other investigators using static conformal or dynamic conformal arc fields. For example, Monk et al. (13) reported a small, but statistically significant, inferior CI for the Varian 5-mm MLC compared with Table 3. Summary of CIs for the 3-mm micro-mlc and the 5-mm MLC Micro MLC Millennium MLC Conformity index CI mean (p ) CI Standard deviation CI range Abbreviations: MLC multileaf collimator; CI conformity index Target volume (c.c.) Fig. 3. Conformity index vs. target volume. Black square represents 5-mm multileaf collimator. Black triangle represents 3-mm micro-multileaf collimator

5 S80 I. J. Radiation Oncology Biology Physics Volume 66, Number 4, Supplement, 2006 Rind volume (c.c.) mm micro-mlc 5 mm Millennium MLC Prescription isodose surface(%) Fig. 6. Comparison of peritumoral rind volume receiving 50%, 80%, and 90% of prescription isodose surface. Volume reduction seen at various prescription isodose surfaces for 3-mm micromultileaf collimator compared with 5-mm multileaf collimator. Fig. 4. Percentage of difference in conformity index vs. target volume. the BrainLAB 3-mm micro-mlc. Jin et al. (14) showed that the average CI ratio between the 5-mm MLC and 3-mm micro-mlc was 1 for all four different tumor size groups, indicating poorer target conformity for the 5-mm MLC compared with the 3-mm micro-mlc. Note that both studies adopted the CI defined by the BrainSCAN, version 5.3, stereotactic radiosurgery treatment planning system (Brain- LAB), which differs from the one used in our report. Concerning the relationship of the difference in CI between the 5-mm MLC and 3-mm micro-mlc vs. the TV size, our results indicated a decreased pattern toward a larger TV. Jin et al. (14) reported a similar result. They found that the average CI ratio between the 5-mm MLC and 3-mm micro-mlc decreased with increasing TV. However, Monk et al. (13) found no such pattern. The creation of PRVs for different levels of dose allowed us to quantify normal tissue sparing. Unlike the study by 100 Monk et al. (13) in which they specifically measured the dose to critical structures such as the optic chiasm, optic nerves, and brainstem, we used PRV as a more general framework to evaluate the effect of leaf width on the tissues immediately adjacent to the target and included in the specified PRVs. In general, our study showed, in all cases examined, a volume reduction in PRV for all greater PISs ( 50%) for the 3-mm micro-mlc compared with the 5-mm MLC. The average volume reduction in PRV 50, PRV 80, and PRV 90 was 13.5%, 12.9%, and 11.5%, respectively. Kubo et al. (12) reported a similar result. They found a reduction in the rectum and bladder dose for three-dimensional conformal prostate plans using a 3-mm micro-mlc compared with a 10-mm MLC. Monk et al. (13) also showed an increase in brain normal tissue treated to 50% and 70% of the prescription dose for the 5-mm MLC compared with the 3-mm micro-mlc. The TV correlated fairly well (R ) with the difference in PRV 50. Thus, the PRV receiving 50% of the PIS increased for the 5-mm MLC compared with that for Peritumoal rind volume (%) Prescription isodose surface (%) Fig. 5. Dose volume histogram of peritumoral rind volume (PRV) for 1 patient s treatment plan. Note, PRV is percentages. Black square represents 5-mm multileaf collimator. Black triangle represents 3-mm micro-multileaf collimator Fig. 7. Difference in peritumoral rind volume (PRV) vs. target volume. Black square represents PRV 50 ; diamonds represent PRV 80 ; triangles represent PRV 90. Solid line represents trend line for PRV 50 vs. target volume. Its linearly fitting equation and correlation coefficient also shown.

6 3-mm vs. 5-mm MLC in SRS S.-S. CHERN et al. S81 the 3-mm micro-mlc with a larger TV. This result might have some clinical implications. For example, when a critical structure such as the optic chiasm is located immediately adjacent to the target and the target is fairly large, the 3-mm leaf MLC might allow for SRS treatment that delivers a therapeutic dose to the TV while respecting the optic nerve tolerance. For small targets ( 1 cm 3 ), the 3-mm micro-mlc provides a 10% improvement, on average, in CI compared with the 5-mm MLC (Fig. 4). CONCLUSION The 3-mm leaf width MLC offers some dosimetric advantages compared with the 5-mm leaf width MLC in terms of target conformity and dose to the surrounding normal tissues. The differences were small but consistently seen for all targets analyzed. Whether this small improvement translates into a clinically significant advantage is not yet clear. REFERENCES 1. Paddick I. A simple scoring ratio to index the conformity of radiosurgical treatment plans. J Neurosurg 2000;93: Lutz W, Winston K, ki N. A system for stereotactic radiosurgery with a linear accelerator. Int J Radiat Oncol Biol Phys 1988;14: Friedman WA, Bova FJ. The University of Florida radiosurgery system. Surg Neurol 1989;32: Shrieve DC, Larson DA, Loeffler JS. Radiosurgery. In: Leibel SA, Phillips TL, editors. Textbook of radiation oncology. 2nd ed. Philadelphia: WB Saunders, p Xia P, Geis P, Xing L, et al. Physical characteristics of a miniature multileaf collimator. Med Phys 1999;26: Cosgrove VP, Jahn U, Pfaender M, et al. Commissioning of a micro multi-leaf collimator and planning system for stereotactic radiosurgery, Radiother Oncol 1999;50: Hacker FL, Kooy HM, Bellerive MR, et al. Beam shaping for conformal fractionated stereotactic radiotherapy: A modeling study. Int J Radiat Oncol Biol Phys 1997;38: Leavitt DD, Gibbs FA Jr, Heilbrun MP, et al. Dynamic field shaping to optimize stereotactic radiosurgery. Int J Radiat Oncol Biol Phys 1991;21: Leavitt DD. Beam shaping for stereotactic radiosurgery/stereotactic radiotherapy. Med Dosim 1998;23: Grebe G, Pfaender M, Roll M, Luedemann L. Dynamic arc radiosurgery and radiotherapy: Commissioning and verification of dose distributions. Int J Radiat Oncol Biol Phys 2001; 49: Nakamura JL, Verhey LJ, Smith V, et al. Dose conformity of gamma knife radiosurgery and risk factors for complications. Int J Radiat Oncol Biol Phys 2001;51: Kubo HD, Wilder RB, Pappas CT. Impact of collimator leaf width on stereotactic radiosurgery and 3D conformal radiotherapy treatment plans. Int J Radiat Oncol Biol Phys 1999; 44: Monk JE, Perks JR, Doughty D, et al. Comparison of a micro-multileaf collimator with a 5-mm-leaf-width collimator for intracranial stereotactic radiotherapy. Int J Radiat Oncol Biol Phys 2003;57: Jin JY, Yin FF, Ryu S, Ajlouni M, Kim JH. Dosimetric study using different leaf width MLCs for treatment planning of dynamic conformal arcs and intensity-modulated radiosurgery. Med Phys 2005;32: Fiveash JB, Murshed H, Duan J, et al. Effect of multileaf collimator leaf width on physical dose distributions in the treatment of CNS and head and neck neoplasms with intensity modulated radiation therapy. Med Phys 2002;29:

Feasibility of using the Vero SBRT system for intracranial SRS

JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 15, NUMBER 1, 2014 Feasibility of using the Vero SBRT system for intracranial SRS Manuela Burghelea, 1,2a Dirk Verellen, 1 Thierry Gevaert, 1 Tom Depuydt,