|Year : 2016 | Volume
| Issue : 2 | Page : 852-857
Dosimetric comparison of intensity modulated radiosurgery with dynamic conformal arc radiosurgery for small cranial lesions
Juan F Calvo-Ortega, David Delgado, Sandra Moragues, Miquel Pozo, Joan Casals
Department of Radiation Oncology, Hospital Quirón Barcelona, Barcelona, Spain
|Date of Web Publication||25-Jul-2016|
Juan F Calvo-Ortega
Department of Radiation Oncology, Hospital Quiron Barcelona, Plaza Alfonso Comin, 5, 08023 Barcelona
Source of Support: None, Conflict of Interest: None
Aims: To dosimetrically compare the fixed gantry intensity modulated radiosurgery (IMRS) with dynamic conformal arc radiosurgery (DCARS) for cranial lesions. This study investigates whether IMRS can be an adequate dosimetric alternative to DCARS for cranial stereotactic radiosurgery (SRS).
Subjects and Methods: Forty-five SRS procedures for solitary brain metastasis (range: 0.44–29.18 cm 3) performed at our institution were selected for this study. Two plans were generated per patient: One IMRS plan using a multileaf collimation (MLC) of 5 mm, and one DCARS plan designed with a 3 mm micro-MLC. Dosimetric comparison metrics include the target coverage (Cov), conformity index (CI), homogeneity index (HI), gradient index (GI), and volume of the normal brain tissue receiving ≥12 Gy (V12). In addition, maximum doses to organs at risk (OAR) (brainstem, optic apparatus and cochlea) were compared for both techniques.
Results: Compared to DCARS, IMRS improved mean CI (IMRS: 0.81 vs. DCARS: 0.63, P < 0.001), with no significant difference in target Cov (IMRS: 0.99 vs. DCARS: 0.99, P > 0.05), HI (IMRS: 1.22 vs. DCARS: 1.24, P > 0.05), GI (IMRS: 5.44 vs. DACRS: 5.44, P > 0.05). A weak significant difference in V12 (IMRS: 4.6 cm 3 vs. 5.2 cm 3, P = 0.033) was obtained. Subgroup analysis per target volume (small: <1 cm 3, intermediate: ≤1 cm 3 and <5 cm 3 and large: ≥5 cm 3) only revealed the statistically difference for CI metric (P < 0.001). No significant differences were found for maximum dose to the OAR.
Conclusions: We have shown that IMRS provides the dosimetric advantages compared with DCARS. Based on the dosimetric findings in this study, fixed gantry IMRS technique can be adopted as a standard procedure for cranial SRS when micro-MLC technology is not available on the linear accelerator.
Keywords: Dynamic conformal arc radiosurgery, eclipse, intensity modulated radiosurgery, iPlan, stereotactic radiosurgery
|How to cite this article:|
Calvo-Ortega JF, Delgado D, Moragues S, Pozo M, Casals J. Dosimetric comparison of intensity modulated radiosurgery with dynamic conformal arc radiosurgery for small cranial lesions. J Can Res Ther 2016;12:852-7
|How to cite this URL:|
Calvo-Ortega JF, Delgado D, Moragues S, Pozo M, Casals J. Dosimetric comparison of intensity modulated radiosurgery with dynamic conformal arc radiosurgery for small cranial lesions. J Can Res Ther [serial online] 2016 [cited 2019 Dec 11];12:852-7. Available from: http://www.cancerjournal.net/text.asp?2016/12/2/852/163680
| > Introduction|| |
Stereotactic radiosurgery (SRS) is a well-established irradiation technique to deliver the focal ablative doses to small intracranial lesions (typically <35 cm 3) both benign and malignant. SRS was initially developed by Lars Leksell by using the Gamma Knife machine, but linear accelerator (linac) technology has been increasingly adopted for SRS treatments.
SRS linac-based radiosurgery started using cone shaped collimators, but the introduction of the multileaf collimator (MLC) device allowed new delivery techniques as the dynamic leaf motions during arcing (dynamic conformal arcs [DCAs]) and fixed beam intensity modulated radiotherapy (IMRT; typically referred as IMRS in case of SRS). The dosimetric advantage of the use of DCA technique as compared to traditional cone shaped collimators has been demonstrated by several authors.,
IMRS has been shown to provide the improved tumor coverage (Cov) and normal tissue sparing for small cranial tumors relative to conventional cone shaped collimator arc-based stereotactic techniques. Several studies have demonstrated that IMRS improve the conformity index (CI) at the cost of inferior dose falloff compared with the conventional static field or dynamic arc therapy.,,,,
Our institution introduced DCA-based SRS DCA radiosurgery (DCARS) using a micro-MLC in 2004, and fixed gantry IMRS using a conventional MLC was potentially available from 2009. The objective of this work is to compare our IMRS and DCARS approaches in terms of target Cov, dose conformity, dose homogeneity, dose gradient and normal brain dose for a variety of irregularly shaped intracranial lesions. The results of the study will indicate whether the IMRS technique is dosimetrically able to replace the well-established DCARS therapy routinely used in our department for cranial SRS.
| > Subjects and Methods|| |
Patient dataset selection and simulation
Forty-five radiosurgical treatments for single cranial lesion were randomly selected from our SRS database. Lesions included 8 vestibular schwannomas, 27 brain metastases, 4 arteriovenous malformations, 3 meningiomas, 1 metastasis surgical resection cavity, 1 pineocytoma and 1 neuroma of the cavernous sinus. The patients were immobilized using the BrainLAB mask system (BrainLAB AG, Germany). For each patient, the planning computed Tomography (1 mm slice thickness) and contrast-enhanced T1 and T2 magnetic resonance imaging scans (slice thickness 1 mm) were co-registered using the BrainLAB iPlan RT version 4.1 treatment planning system (TPS), for contouring of the lesion and organs at risk (OARs: Brainstem, chiasm and optic nerves in all patients; in addition, ipsilateral cochleas for the vestibular schwannoma cases).
The planning target volume (PTV) was generated from each lesion by adding a 2 mm isotropic margin according to our margin policy. From now on, the terms “target” and “PTV” are used indistinctively in this work. PTVs ranged in volume from 0.44 to 29.18 cm 3 (equivalent diameter from 9.4 to 38.2 mm), and they were categorized in three subgroups: (1) “Small targets” (PTV <1 cm 3; n = 10), (2) “intermediate lesions” (1 ≤ PTV <5 cm 3; n = 21), and (3) “large targets” (PTV ≥ 5 cm 3; n = 14). The median prescription dose (PD) was 16 Gy (range: 12–24 Gy). We followed the volume classification described by Tanyi et al.
For each patient, two plans (IMRS and DCARS) were designed as it is described in next two paragraphs.
Fixed gantry intensity modulated radiosurgery treatment planning
For each patient, a noncoplanar beam arrangement (range: 9–15 fields, mean: 12 fields) was generated in the Eclipse version 10.0.28 TPS (Varian Medical Systems, Palo Alto, CA). All treatments were planned with 6 MV photons produced by a Varian Clinac 2100 CD equipped with the Varian Millennium 120 MLC (central leaves of 5 mm width).
IMRS plans were inversely optimized (sliding-window technique) using the Dose Volume Optimizer (DVO, version 10.0.28) algorithm of the Eclipse, that produces modulated field fluences on the basis of user-defines dose volume constraints. In addition, a rind volume, defined as 30 mm wall from the surface of the PTV, was used during optimization in order to reduce the dose to the normal brain and produce high dose conformity. The final dose calculations were done using the Anisotropic Analytical Algorithm version 10.0.28 of the Eclipse, with a 1.0 mm calculation grid and tissue heterogeneity correction. The dosimetric accuracy of IMRS designed using the Millennium 120 and the Eclipse TPS was investigated by our group.
Dynamic conformal arc radiosurgery treatment planning
The DCARS modality is an arc rotational delivery technique in which the leaves move during the rotation of the gantry to dynamically adapt the shape of the treatment beam to the beam's eye view of the target. The beam collimation for DCARS plans was made using the BrainLAB M3 MLC attached to the linac. This collimator has 26 pairs of leaves, of which the central ones have a width of 3.0 mm and the external ones a width of 5.5 mm relative to the isocenter. The DCARS plans were calculated with the BrainLAB iPlan RT Dose version 4.1 TPS using the pencil beam algorithm commissioned for 6 MV beams from a Varian Clinac 2100 CD linac. Dose calculations were made with the radiological path length for tissue heterogeneity correction. An adaptive calculation of grid size <1.0 mm was applied to smaller lesions. The dynamic arc plans normally consisted of 5 noncoplanar arcs. When required, the arc position and lengths were modified to avoid the eyes. A MLC margin of 1–2 mm to the PTV was used in each DCARS plan.
Quality assessment of clinical plans
Target dose homogeneity and conformation criteria according to the radiation therapy oncology group (RTOG) guidelines were required for all plans. In general, both the IMRS and DCARS plans were normalized with the same criterion so that 99% of the PTV volume received the PD. OAR dose tolerance values given by the American Association of Physicists in Medicine Task Group 101 were followed for treatment planning.
The IMRS and DCARS techniques were quantitatively compared using a series of metrics: Target Cov, CI, target dose homogeneity index (HI), gradient index (GI) and volume of the normal brain tissue receiving 12 Gy (V12).
The Cov parameter was defined as the fraction of PTV receiving the PD. The CI measures how the prescribed isodose volume conforms to the target volume. The CI described by Paddick was the index used in this work. CI is equal to 1.00 for a perfect plan and decreases as the conformity is worsening.
Another metric we analyzed is the HI. This is a tool to analyze the uniformity of dose distribution in the target volume. In this study, we had used the RTOG HI defined as the ratio of the maximum point dose received by the planning target to the PD. The ideal value is 1 and it increases as the plan becomes less homogeneous.
The low doses outside the PD volume may cover the significant amounts of normal tissues and can be responsible for complications, especially when the target is in proximity to critical structures. Therefore, the dose falloff (or gradient) beyond the target volume extending into normal tissue structures must be rapid in all directions. We compared IMRS versus DCARS using the GI defined as the ratio of the volume of tissue receiving 50% of the PD to the PTV. A lower value indicates a lower dose spillage outside the target and a sharper dose falloff.
The unified dosimetry index (UDI) is a figure of merit for ranking rival plans (IMRS vs. DCARS, in our study) that combines into one single score the above mentioned metrics: Cov, CI, HI, and GI. The composite UDI score is equals to 1.0 for an ideal plan. For an actual physically realizable dosimetry plan, the composite UDI score is always >1.0 and its value increases as any dosimetric component is worsening. Therefore, a low UDI score indicates a “better” plan, whereas a high score corresponds to a “worse” plan.
The V12 has been shown to correlate with both the incidence of radiation necrosis and asymptomatic radiologic changes. Data for the V12 metric was collected and compared.
In both IMRS and DCARS plans, the point maximum doses to OARs were registered and compared. “Point” defined as 0.035 cc or less.
The SPSS (Statistical Package for the Social Sciences; IBM Corporation, New York, USA) was used for the statistical analysis. To analyze the differences between the IMRS and DCARS plans for each of the previously described metric, a two-tailed paired t-test was used. A Wilcoxon signed-test/two-tailed test was performed for the subgroup analysis attending to the target size. The value of α was 0.05 being used.
To graphically assess the association between some metrics and the volume of PTV, simple linear regressions were performed.
| > Results|| |
The values obtained for the analyzed metrics in this work are resumed in [Table 1].
No statistically significant difference was found for the Cov parameter. This is an expected result as same normalization criterion was used in all the plans (99% of PTV receiving at least the PD).
All IMRS plans were more conformal in comparison with DCARS plans. The CIs for the IMRS varied from 0.65 to 0.98 (mean: 0.81), while worse CIs were observed for all plans computed with the DCARS (0.40–0.80; mean: 0.63) [Figure 1]. As the CI may depend on the size of the target volume, it is interesting to evaluate this parameter according to the range of volumes of targets. IMRS plans provided significant better conformity regardless the size of the lesions: 0.86 versus 0.62 (P = 0.005), 0.77 versus 0.61 (P < 0.001), and 0.83 versus 0.67 (P = 0.001), for the small, intermediate, and large lesions, respectively.
|Figure 1: Conformity index versus planning target volume scatter plot with correlation analysis|
Click here to view
IMRS plans resulted in comparable average HI as DCARS plans (P > 0.05). HIs ranged from 1.10 to 1.30 (mean: 1.22) with IMRS, and from 1.10 to 1.60 (mean: 1.24) with DCARS. No significant differences (P > 0.05) were found for HI, when we looked at the three groups of cases ranked according to the target size.
When we compared the low dose spillage for the 45 paired plans, GIs varied from 3.07 to 8.98 (mean: 5.44) with IMRS, while ranged from 3.05 to 9.29 (mean: 5.44) with DCARS. The IMRS plans were slightly better than DCARS plans in 22 of 45 cases. Not significant differences (P > 0.05) were found for GI irrespective the volume of the lesion (small, intermediate, and large). Similar association between GI and the PTV volume was graphically observed for both techniques [Figure 2].
|Figure 2: Gradient index versus planning target volume scatter plot with correlation analysis|
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In general, IMRS plans resulted in lower UDI values (mean: 103) than DCARS plans (mean: 204), that is, IMRS plans were normally better than DCARS ones. Only 2 of 45 DCA-based plans showed better UDIs that IMRS. The lowest score was 27 with IMRS versus 76 for with DCARS. On the other hand, the highest score was 214 with IMRS versus 545 for with DCARS. In sum, a statistically significant better UID score was found for IMRS for all target size subgroup (P < 0.005).
IMRS decreased the average V12 reported for the three subgroups analyzed [Table 1]; but no statistically significant differences were found between IMRS and DCARS. A clear trend to irradiate more normal brain was observed for DCARS in case of targets with more than 5 cc [Figure 3]. Finally, the maximum doses to OARs (brainstem, chiasm, optic nerves, and cochlea) were not statistically different for IMRS and DCARS plans [Table 1].
|Figure 3: Volume of normal brain receiving ≥12 Gy versus planning target volume scatter plot with correlation analysis (V12: Volume of normal brain receiving ≥12 Gy)|
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| > Discussion|| |
The target Cov was equivalent for IMRS and DCARS in this study due to all plans were in general normalized such that the PD was covering the 99% of the PTV. In some DCARS plans, the normalization criterion was relaxed down to 95% in order to reduce dose to adjacent OARs. We consider very important to keep the Cov of target at the same or similar level for a meaningful comparison of the plans between two different delivery techniques. However, two plans with the same normalization (e.g., 99%) may not have the same values for other dosimetric parameters (HI, and CI). For instance, dose gradients may be different depending on the irradiation technique, and therefore different HI/CI values may be obtained.
The CI improved in all cases with IMRS. In eight cases, the difference in the CIs between IMRS and DCARS was quite small (<0.1) and may not represent a clinically relevant improvement of IMRS over DCARS. IMRS is able to produce plans highly conformed to the PTV with CI up to 0.98 [Figure 1], while the better CI attained with DCARS was 0.80. Similar conclusion with respect to CI was reported by Nakamura et al. for 11 small and medium sized skull base tumor cases. They described always better CI for IMRS performed with a micro-MLC versus Gamma Knife technique. In contrast, Wiggenraad et al. compared IMRS and DCARS for 25 cases. In general, they found no statistically significant difference in CI between both techniques, and IMRS plans were the best only in concave tumors. However, we do not agree with their conclusion. We think their findings could be explained due to the significant difference in the target Cov between both techniques. In our study, all plans had same target Cov and this makes more objective in the CI comparison between both techniques. Sparing the optic system and the brainstem is a challenge when the tumor is in close contact with it. According to our experience, the highest grade of conformity attained with IMRS allows to reduce the volume of an adjacent critical organ irradiated to significant dose levels, as it is illustrated in [Figure 4] for a case of PTV in contact to the brainstem. The radiation-related brainstem toxicity shall be reduced with IMRS as this organ is better spared from high doses.
|Figure 4: Example of clinical case enrolled in this study where planning target volume was near to brainstem: (a) Intensity modulated radiosurgery; (b) dynamic conformal arc radiosurgery, (c) dose volume histograms for brainstem (squares for intensity modulated radiosurgery and triangles for dynamic conformal arc radiosurgery). Only 100% (yellow) and 50% (blue) isodose levels are shown in a and b. [Figure 4] is shown a case of stereotactic radiosurgery where the planning target volume is adjacent to the brainstem. The Figure c is illustrating how the intensity modulated radiosurgery plan is able to diminish the volume of brainstem irradiated respect to the dynamic conformal arc radiosurgery modality|
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In our study, no statistical significant difference between IMRS and DCARS were found for HI. However, different results have been reported in the literature regarding to HI. For instance, Ernst-Stecken et al. compared DCARS and IMRS in very small skull base tumors, reporting lower HI with DCA compared with IMRT. The clinical relevance of dose homogeneity within the target volume for SRS is controversial. Inhomogeneous high central doses achieved with some radiosurgical systems (e.g., gamma knife) may provide improved local control; however, this increased local control may come with an increased risk of neurologic complications. In our clinical experience, we have always emphasized the dose homogeneity, but limiting the HI to be <2.0 in order to balance the risk of local failure and neurologic injury.
Regarding to the dose gradient (GI metric), not statistical significant differences were found in our study, irrespective the volume of the lesion (small, intermediate, and large). However, Gevaert et al. reported significantly higher values of GI for IMRS compared with DCARS for 15 analyzed cases. It might argue that the ability of DCARS to reduce the low dose spillage is due to the geometry of a dynamic conformal arc consisting of many collimated sub-beams. Nevertheless, the IMRS technique described in the Gevaert's study consisted of very few beams (only 6 fields) such that a larger low dose spread is expected in comparison with the arc therapy. By increasing the number of beams to 8 and 12, Hamilton et al. were able to produce the plans equal or superior to the arc technique in terms of sparing normal brain at dose levels above 50%.
The four metrics mentioned above (Cov, CI, HI, and GI) were combined (equally weighted) using the UDI score. According this tool, IMRS resulted in better dosimetric approach than DCARS in the majority of the cases. The UDI score was only better for DCARS in 2 of 45 cases analyzed in this study.
A positive correlation between the 12 Gy-volume (V12) outside the target and radiosurgical complications is described in the literature. Globally, IMRS resulted in a statistically significant decrease of V12 in comparison with DCARS. However, no statistically significant were found in the subgroup analysis per target volume.
IMRT has been reported as a technique to perform cranial SRS from several years, but its use has not been spread so far. Scarce literature exits comparing the use of fixed gantry IMRS with the technique of multiple conformal arcs using linacs.,,, As far as we know, no studies comparing IMRS designed with the Eclipse/Millennium 120 MLC combination vs. DCARS planned with the iPlan/M3 system has been published.
The results of the present study indicate that IMRS is an attractive dosimetric alternative to DCARS. We conclude that IMRS planned with the Eclipse/Millennium 120 MLC combination may dosimetrically replace DCARS technique based on the M3 MLC and the iPlan software. Our findings may be useful for all users of the Eclipse/Millennium 120 MLC who want to implant the IMRS modality without the investment in a dedicated MLC and/or TPS specifically designed for SRS treatments.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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