Medical Dosimetry
Volume 36, Issue 1 , Pages 62-70, Spring 2011

Dosimetric Comparison of Helical Tomotherapy and Dynamic Conformal Arc Therapy in Stereotactic Radiosurgery for Vestibular Schwannomas

  • Tsair-Fwu Lee, Ph.D.

      Affiliations

    • National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan
    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • Corresponding Author InformationReprint requests to: Fu-Min Fang, M.D., Ph.D. and Tsair-Fwu Lee, Ph.D., Department of Radiation Oncology, Chang Gung Memorial Hospital-Kaohsiung Medical Center, 123 Ta-Pei Rd., Niao Sung Hsian, Kaohsiung Hsien, Taiwan
    • ,
  • Pei-Ju Chao, M.S.

      Affiliations

    • National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan
    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • ,
  • Chang-Yu Wang, M.D.

      Affiliations

    • National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan
    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • ,
  • Jen-Hong Lan, Dipl-Phy.

      Affiliations

    • National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan
    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • ,
  • Yu-Je Huang, M.D.

      Affiliations

    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • ,
  • Hsuan-Chih Hsu, M.D.

      Affiliations

    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • ,
  • Chieh-Cheng Sung, M.S.

      Affiliations

    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • ,
  • Te-Jen Su, Ph.D.

      Affiliations

    • National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan
    • ,
  • Shi-Long Lian, M.D., Ph.D.

      Affiliations

    • Chung-Ho Memorial Hospital, School of Medicine, Kaohsiung Medical University, Taiwan
    • ,
  • Fu-Min Fang, M.D., Ph.D.

      Affiliations

    • Chang Gung Memorial Hospital-Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan
    • Corresponding Author InformationReprint requests to: Fu-Min Fang, M.D., Ph.D. and Tsair-Fwu Lee, Ph.D., Department of Radiation Oncology, Chang Gung Memorial Hospital-Kaohsiung Medical Center, 123 Ta-Pei Rd., Niao Sung Hsian, Kaohsiung Hsien, Taiwan

Received 31 March 2009; accepted 24 November 2009. published online 26 February 2010.

Article Outline

Abstract 

The dosimetric results of stereotactic radiosurgery (SRS) for vestibular schwannoma (VS) performed using dynamic conformal arc therapy (DCAT) with the Novalis system and helical TomoTherapy (HT) were compared using plan quality indices. The HT plans were created for 10 consecutive patients with VS previously treated with SRS using the Novalis system. The dosimetric indices used to compare the techniques included the conformity index (CI) and homogeneity index (HI) for the planned target volume (PTV), the comprehensive quality index (CQI) for nine organs at risk (OARs), gradient score index (GSI) for the dose drop-off outside the PTV, and plan quality index (PQI), which was verified using the plan quality discerning power (PQDP) to incorporate 3 plan indices, to evaluate the rival plans. The PTV ranged from 0.27−19.99 cm3 (median 3.39 cm3), with minimum required PTV prescribed doses of 10−16 Gy (median 12 Gy). Both systems satisfied the minimum required PTV prescription doses. HT conformed better to the PTV (CI: 1.51 ± 0.23 vs. 1.94 ± 0.34; p < 0.01), but had a worse drop-off outside the PTV (GSI: 40.3 ± 10.9 vs. 64.9 ± 13.6; p < 0.01) compared with DCAT. No significant difference in PTV homogeneity was observed (HI: 1.08 ± 0.03 vs. 1.09 ± 0.02; p = 0.20). HT had a significantly lower maximum dose in 4 OARs and significant lower mean dose in 1 OAR; by contrast, DCAT had a significantly lower maximum dose in 1 OAR and significant lower mean dose in 2 OARs, with the CQI of the 9 OARs = 0.92 ± 0.45. Plan analysis using PQI (HT 0.37 ± 0.12 vs. DCAT 0.65 ± 0.08; p < 0.01), and verified using the PQDP, confirmed the dosimetric advantage of HT. However, the HT system had a longer beam-on time (33.2 ± 7.4 vs. 4.6 ± 0.9 min; p < 0.01) and consumed more monitor units (16772 ± 3803 vs. 1776 ± 356.3; p < 0.01). HT had a better dose conformity and similar dose homogeneity but worse dose gradient than DCAT. Plan analysis confirmed the dosimetric advantage of HT, although not all indices revealed a better outcome for HT. Whether this dosimetric advantage translates into a clinical benefit deserves further investigation.

Key Words: Helical TomoTherapy, Dynamic conformal arc therapy, Novalis, Vestibular schwannoma, Dosimetry

 

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Introduction 

Stereotactic radiosurgery (SRS) is an alternative to microsurgery in the treatment of vestibular schwannomas (VSs); it confers lower morbidity and comparable local control.1, 2 There have been technical developments in the delivery of radiation beams using linac-based SRS with progress from the conventional circular arc, static conformal beam to dynamic conformal arc or intensity-modulated therapy. During beam delivery, the radiation can be shaped by circular collimators of variable size or a micromultileaf collimator (mMLC). Dynamic conformal arc therapy (DCAT), which combines the concept of a circular arc and mMLC beam shaping, shapes radiation by changing the mMLC patterns along the arc pathway of a single isocenter.3, 4 In our department, we use the DCAT technique to treat VS using the Novalis system. The DCAT technique has been used to treat intracranial tumors, and a comparable dosimetric result was obtained in relation to intensity-modulated radiotherapy (IMRT).5, 6

IMRT constitutes an advanced form of the conformal technique and uses inverse planning algorithms and iterative computer-driven optimization to generate treatment fields with varying beam intensity. IMRT has the ability to produce custom-tailored conformal dose distributions around the tumor, although most studies have examined large tumors.7 With commercial motorized mMLC systems, IMRT can be extended to smaller intracranial tumors.8 IMRT can be delivered using linac or Hi-Art Helical TomoTherapy (HT) (TomoTherapy, Madison, WI).9, 10, 11 Compared with step-and-shoot IMRT, helical IMRT is capable of calculating the MLC position every 7° of rotation; it also creates a more uniform target dose and improves critical organ sparing.12, 13

Dosimetric comparisons of HT with the gamma knife or conventional linac-based SRS have been performed for small intracranial or skull base tumors.14, 15, 16 To our knowledge, however, no dosimetric comparisons of IMRT delivered using HT with DCAT using the Novalis system in VS have been made. This study compared the 2 techniques in VS quantitatively, by using several indices for the dosimetric comparisons, including the conformity index (CI) and homogeneity index (HI) for the planned target volume (PTV), quality index (QI) and comprehensive quality index (CQI) for 9 organs at risk (OARs), gradient score index (GSI) for the dose drop-off outside the PTV, and plan quality index (PQI) to incorporate 3 plan indices to evaluate the plans quantitatively.12, 13, 17, 18, 19, 20, 21 To investigate the discerning power between the indices, the plan quality discerning power (PQDP) was used to check the discrimination between PQI and other indices, such as conformation number (CN) and conformation index (COIN).19 The beam-on time22 and monitor units (MUs) used by the 2 techniques were also measured and compared.

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Materials and Methods 

Study population 

Ten consecutive patients (6 females, 4 males) with VS and treated using SRS with the Novalis system between March 2007 and October 2008 were enrolled. The patient characteristics and tumor descriptions are presented in Table 1. The median age at SRS was 58 years (range 29–86). The tumor was located on the right vestibular nerve in 3 patients and on the left in seven.

Table 1. Patient and tumor characteristics (n = 10)
Patient No.Planned Target Volume (cm3)Prescription Dose (Gy)Tumor Site TreatedGenderAge (y)
11.1315RightM53
23.7712LeftF69
31.4516LeftF29
45.8310LeftM74
53.0110LeftF33
66.6410RightM67
70.2712LeftF86
84.3512LeftM59
919.9910RightF39
101.4212LeftF68
Mean ± SD (range)4.73±5.72(0.27–19.99)12(10–16) 58(29–86)

Novalis and HT 

The Novalis system, a dedicated linac-based SRS modality, is integrated with an m3-mMLC system, working on a 6-MV photon beam with stringent isocentricity standards (BrainLAB, Heimstetten, Germany, and Varian Medical Systems, Palo Alto, CA).23 There are three different leaf widths in the m3-mMLC, which has 26 pairs of tungsten alloy (95% W, 3.4% Ni and 1.6% Fe) leaves (14 × 3, 6 × 4.5, and 6 × 5.5 mm, with a maximum useful field of 9.8 × 9.8 cm2). In addition, the system is combined with ExacTrac® X-Ray 6D (BrainLAB, Heimstetten, Germany), an infrared (IR) camera, a kV stereoscopic x-ray imaging system, a relocatable stereotactic frame system, a noninvasive mask system, and ExacTrac® Robotics, for patient positioning in all 6D of freedom.24

Helical TomoTherapy combines an intensity-modulated fan beam with the helical motion of the gantry, relative to the patient. In the HT system, a 6-MV linac is mounted on a circular gantry that rotates in the transverse plane of the patient, while the patient couch moves into the gantry bore. The fan beam is modulated by a 64-leaf binary MLC (6.25-mm leaf-width at the isocenter) throughout gantry rotation.10, 11

The 10 patients were treated with the Novalis system, and a standard 5-arc noncoplanar DCAT was delivered. For the dosimetric comparison with HT, the computed tomography (CT) images and associated contours were transferred to the HT system (version 2.1) via the DICOM-RT protocol format. The same optimization parameters and prescribed doses were used in the HT as in the Novalis treatment plan system (TPS). In the HT plans, the operator must choose 3 main parameters: the field width (one of 1, 2.5, or 5 cm); pitch (range 0.01–20); and modulation factor (range 1–10); this is unique to HT. Briefly, the field width is defined as the slice thickness of the radiation field projected at the isocenter along the gantry rotation axis. The pitch is defined as the couch movement relative to the field width during one gantry rotation. The modulation factor is defined as the ratio of the maximum number of opening leaves and the average number of opening leaves in active gantry rotations.11, 18 A 1-cm field width, a pitch of 0.3, and a modulation factor of 2 were used in all of the HT plans in this study. The choices of these 3 parameter values were based on preliminary planning exercises that showed this choice was a good balance between ability at dose sculpting and efficiency of the treatment, in terms of treatment duration and feasibility for routine use. In general, small field dimensions, small pitch, and large modulation factors mean longer irradiation times and a better ability for the delivery system to sculpt complex dose distributions with steeper dose gradients.9, 11

DCAT and HT treatment plans 

After immobilization in a BrainLAB immobilization localizer frame, the patients were scanned using a CT simulator (AcQSim CT, Philips Medical Systems, Eindhoven, The Netherlands) at 1.25-mm slice thickness, containing 512 × 512 pixels in each slice.25 The field of view had a mean dimension of 34 cm (range 30–38). T1- and T2-weighted magnetic resonance imaging (MRI) was also obtained for each patient, with contrast medium, using a 3-T MRI scanner (GE Signa EXCITE MR Scanner, GE Medical Systems, Milwaukee, WI). Both image sets were transferred to the image workstation (iPlan RT image 3.0, BrainLAB AG, Heimstetten, Germany) for fusion of the anatomic structures. The target volume and 9 OARs, including the brainstem, chiasm, medulla oblongata, and bilateral optic nerves, optic tract, and cochlea, were delineated in both systems. The PTV encompassed the gross tumor volume (GTV) with an additional localization uncertainty margin of 1.0 mm in 3 dimensions. The prescribed dose was based on dose-volume histograms (DVHs), the size and location of the PTV, and the proximity to nearby critical structures. The PTV in the 10 patients ranged from 0.27−19.99 cm3 (median 3.39), and the prescribed doses from 10−16 Gy (median 12) at an isodose of at least 95%. To sculpt the dose to the target and to avoid OARs, penalties were given when the PTV received less than the prescribed dose and OARs received higher doses than the constraints. Both planning systems perform iterations during the optimization process, using the Clarkson dose algorithm for DCAT and an algebraic iteration for HT. The dose calculation grid resolution, voxel size, and dose bin size of the DVHs used in both systems were the same for the subsequent computation of various indices, with 256 × 256 pixels in each slice, approximately 0.66 × 0.66 × 1.25 mm, and 0.01 Gy, respectively.

The beam-on time taken by both systems was compared. We recorded the actual beam-on time of the 10 patients treated using Novalis, and obtained the beam-on time by simulating a test run of each plan for HT. The patient set-up time for both systems and the time in between arcs for DCAT were not included.

Nomenclature of the dosimetric indices 


1.CI: a ratio used to evaluate the goodness of fit of the PTV to the prescription isodose volume in the treatment plans: [VTV, the treatment volume of the prescribed isodose lines; VPTV, the volume of the PTV; TVPV, the volume of VPTV within the VTV]. The smaller the value of CI, the better conformal the PTV fitting is.12, 21

2.Conformity improvement ratio (ΔCI%): a ratio used to evaluate the improvement or reduction in CI in HT plans over DCAT plans. [CIht and CIdcat are the CI values in the HT and the DCAT plans, respectively].

3.HI: a ratio used to evaluate the homogeneity of the PTV. [D1% and D99% are the minimum doses delivered to 1% and 99% of the PTV]. A greater HI indicates poorer homogeneity.26, 27

4.Homogeneity improvement ratio (ΔHI%): a ratio used to evaluate the improvement in HI in the HT plans over the DCAT plans.
[HIht and HIdcat are the HI values in the HT and the DCAT plans, respectively.]

5.GSI: the GSI proposed by Wanger et al.28 for evaluating the degree of the dose drop-off outside the PTV.
 and are the effective radii of the 50% prescription isodose volume (V50%) and the treated volume (VTV), respectively, when the 2 volumes are assumed to be perfect spheres. Consequently is an estimate of the average distance it takes to drop to 50% of the prescribed dose outside the treatment volume.] GSI equal to or greater than 100 corresponds to an ideal plan, where the gradient is 3 mm or less.

6.QI: an index used to evaluate the difference in the maximum or mean absorbed dose at serial or parallel OARs, respectively, between HT and DCAT plans.26

7.CQI: the average QI for the OARs analyzed.
where i is the index of the critical structures and ‘❘’ means ‘or.’

8.PQI: an index, considering the healthy tissue conformity index (H), merit function (M), and penalty function (P), done through a prefiltering process to qualify rival plans for comparisons, to evaluate and rank the plans.19 The PQI ranges from 0 to , and a lower PQI indicates a better plan.
TVRI, the target volume covered by the reference dose; VRI, the total volume of the reference dose.
Where p is the number of cold spot checks, q is the number of hot spot checks, r is the number of targets of different prescribed doses, represents the volume of the jth target (in %) receiving a dose of at least the ith dose level, represents the minimum volume of the jth target (in %) receiving at least the ith dose level, and represents the allowable volume of the jth target (in %) receiving at least the ith dose level.
Where n is the number of critical organs to be monitored, m is the number of check points used for the jth critical organ, represents the volume of the jth critical organ (in %) receiving a dose of at least the ith dose level and represents the allowable volume of that organ (in %) receiving at least the ith dose level.

9.CN: conformation number, proposed by vant't Reit et al., which was computed for the sample plans to obtain a measure of the quality of each plan.29
[TV, target volume; TVRI, target volume covered by the reference dose; VRI, volume of the reference isodose]

10.COIN: conformation index, proposed by Baltas et al., which was computed for a reference dose.30
[CN, conformation number; NCO, number of critical organs; , critical organ volume receiving at least the reference dose; , critical organ volume]

11.PQDP: a ratio used to evaluate the discerning power for the same index of 2 rival plans. The greater the ratio, the better the PQDP is.

To check the performance of plan quality indices, we attempted to use the PQDP19 as the ratio of the absolute difference to the mean of the same index of 2 sets of plans, which is listed in the following:

Where Index1 and Index2 are the same quality indices of the rival plans, respectively. is the mean value of Index1 and Index2.

Statistical analyses 

The mean values (standard deviation) of the dosimetric data for the 10 patients were analyzed, with the exact pair t-test used to compare the difference between HT and DCAT. A 2-tailed value of p < 0.05 was deemed to indicate statistical significance. The SPSS-15.0 software was used for data processing (SPSS, Inc., Chicago, IL).

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Results 

PTV analysis-CI and HI 

The isodose distributions in the axial, coronal, and sagittal planes and the DVHs of PTV and OAR(s) in one typical case planned using both systems are shown in Fig. 1.

  • View full-size image.
  • Fig. 1. 

    DCAT and HT isodose distributions of planned target volume (PTV) in axial, coronal, and sagittal views in one VS patient planned by DCAT versus HT planning system (a)∼(f). The dose-volume histograms (DVH) of PTV and three OARs in the same VS patient by the two planning systems (g) PTV, and (h) three OARs. DVH = dose volume histograms; PTV = planning target volume; OAR = organ at risk; HT = helical tomotherapy; DCAT = dynamic conformal arc therapy.

Table 2 lists the CI and HI of the PTV for each patient planned with HT and DCAT. The mean CI was 1.51 ± 0.23 (range 1.09–1.84) with HT and 1.94 ± 0.34 (range 1.86–2.6) with DCAT, indicating significantly better conformity of the PTV with HT (p < 0.01). The average improvement in ΔCI% was 22.2% for HT vs. DCAT. The mean HI was 1.08 ± 0.03 (range 1.03–1.13) for HT and 1.09 ± 0.02 (range 1.07–1.12) for DCAT; this difference was statistically insignificant (p = 0.20).

Table 2. The dosimetric comparisons of PTV between HT and DCAT plans for each patient
CIHIGSI
PatientHTDCATΔCI (%)HTDCATΔHI (%)HTDCATΔ GSI (%)
11.842.16−14.671.071.09−1.6945.9972.5836.64
21.642.20−25.461.101.10−0.2444.0663.7730.90
31.701.77−4.151.071.08−1.1440.2369.6042.19
41.421.78−20.021.121.101.7734.9665.5746.68
51.671.84−9.551.071.070.0532.2360.7746.96
61.281.63−21.651.061.10−3.7134.7362.4544.39
71.092.60−57.931.031.07−3.8354.3082.5834.25
81.361.70−20.431.081.08−0.3349.7464.5522.94
91.401.87−25.031.131.120.9317.1231.4445.56
101.692.19−22.821.081.080.0749.1075.9435.34
Mean ± SD1.51±0.231.94±0.34−22.22±14.32†1.08±0.031.09±0.02−0.81±1.8⁎40.25±10.9064.92±13.5838.58±7.9†

Abbreviations: PTV = planning target volume; HT = helical tomotherapy; DCAT = dynamic conformal arc therapy; CI = conformity index; HI = homogeneity index; GSI = gradient score index; ΔCI (%) = the CI improvement ratio; ΔHI (%) = The HI improvement ratio; ΔGSI (%) = the GSI improvement ratio.

⁎p = 0.197.

†p < 0.01.

Outside PTV analysis—GSI and CQI of OARs 

The mean GSI was 40.3 ± 10.9 (range 17.12–54.30) for the HT plans and 64.9 ± 13.6 (range 31.44–82.58) for DCAT, indicating a significantly better dose drop-off outside the PTV in the DCAT plans (p < 0.01; Table 2). The average improvement of the GSI value was 39% in the DCAT vs. HT plans. The mean and maximum doses and QI of the 9 OARs are summarized in Table 3, Table 4. The HT had a significantly lower maximum dose in 4 OARs (chiasm, left cochlea, medulla oblongata, and left optic tract) and significantly lower mean dose in 1 OAR (left optic tract). By contrast, the DCAT had a significantly lower maximum dose in 1 OAR (left optic nerve) and significantly lower mean dose in 2 OARs (left and right optic nerve), with a CQI of 0.92 ± 0.45 for the 9 OARs between HT and DCAT.

Table 3. Analysis of mean and maximum doses of 9 OARs between HT and DCAT with 10 cases
Maximum Dose in GyMean Dose in Gy
OARsHT, mean ± SD (range)DCAT, mean ± SD (range)p-ValuesHT, mean ± SD (range)DCAT, mean ± SD (range)p-Values
BS10.84±3.17(5.50–16.80)11.09±3.34(3.50–16.8)0.5492.67±1.34(0.92–5.61)2.48±0.98(0.54–3.92)0.326
Chiasm0.61±0.56(0.24–2.11)1.02±0.68(0.48–2.80)0.0010.45±0.49(0.14–1.74)0.37±0.25(0.19–1.06)0.391
Lt cochlear2.46±1.57(0.39–10.44)4.30±3.76(0.60–10.40)0.0301.61±1.03(0.25–3.12)1.42±1.21(0.23–3.50)0.163
Rt cochlear1.73±2.26(0.19–7.56)2.81±4.14(0.60–11.85)0.1441.26±1.53(0.18–4.83)1.05±1.39(0.20–3.98)0.117
Lt ON1.10±0.44(0.59–1.67)0.68±0.38(0.30–1.60)0.0090.51±0.25(0.27–1.00)0.24±0.09(0.15–0.46)0.004
Rt ON0.62±0.54(0.20–2.02)0.67±0.68(0.16–2.30)0.3840.37±0.33(0.13–1.24)0.22±0.19(0.07–0.73)0.029
MO4.48±1.72(2.67–7.60)6.27±2.65(2.25–10.90)0.0121.28±0.62(0.53–2.80)1.55±0.82(0.44–2.80)0.620
Lt OT0.50±0.47(0.17–1.67)1.26±0.53(0.72–2.40)0.0010.30±0.21(0.13–0.83)0.47±0.25(0.24–1.05)0.004
Rt OT0.50±0.57(0.13–2.02)0.98±1.08(0.16–3.80)0.0550.34±0.34(0.09–1.24)0.42±0.42(0.15–1.52)0.273

Abbreviations: HT = helical tomotherapy; DCAT = dynamic conformal arc therapy; OARs = organs at risk; BS = brain stem; Lt = left side; Rt = right side; ON = optic nerves; MO = medulla oblongata; OT = optic tract. The two-tailed p-values were obtained from paired t-tests.

Table 4. The dosimetric comparisons of nine OARs between HT and DCAT plans
Variables of OARsQIValues,Mean±SD(range)
Brainstem1.01±0.22(1.58–0.67)
Chiasm0.58±0.23(1.03–0.23)
Lt cochlear0.79±0.39(1.46–0.46)
Rt cochlear0.79±0.89(3.23–0.11)
Lt optic nerves2.01±1.02(3.90–0.73)
Rt optic nerves1.28±1.12(4.33–0.44)
Medulla oblongata0.78±0.26(1.18–0.51)
Lt optic tract0.39±0.30(0.87–0.19)
Rt optic tract0.68±0.70(2.60–0.17)
CQI of nine OARs0.92±0.45(0.39–2.01)

Abbreviations: HT = helical tomotherapy; DCAT = dynamic conformal arc therapy; OARs = organs at risk; QI = quality index; CQI = comprehensive quality index.

PQI analysis 

Table 5 summarizes the H, M, and P values and the final PQI scores for each plan. The mean values of H, M, and P for HT vs. DCAT were 0.70 ± 0.11 vs. 0.45 ± 0.10 (p < 0.01), 1.00 ± 0.002 vs. 0.92 ± 0.10 (p = 0.05), and 0.84 ± 0.15 vs. 0.75 ± 0.19 (p = 0.03), respectively, with the final PQI scores of 0.37 ± 0.12 vs. 0.65 ± 0.08 (p < 0.01), indicating a significant dosimetric gain in HT.

Table 5. The dosimetric comparisons of PTV between HT and DCAT plans for each patient
PatientHMPPQI
HTDCATHTDCATHTDCATHTDCAT
10.560.321.001.001.001.000.440.68
20.620.411.000.931.000.970.380.62
30.600.401.001.000.930.850.400.62
40.740.510.990.800.860.530.300.73
50.630.551.001.000.630.680.520.56
60.810.561.000.980.610.510.430.66
70.910.281.001.000.980.980.090.72
80.750.511.000.940.830.590.310.64
90.740.540.990.980.610.701.422.64
100.600.381.000.890.920.770.410.69
Mean ± SD0.70±0.110.45±0.101.00±0.0020.92±0.100.84±0.150.75±0.190.37±0.120.65±0.05
p-value<0.010.050.03<0.01

Abbreviations: HT = helical tomotherapy; DCAT = dynamic conformal arc therapy; H = healthy tissue conformity index; M = target coverage; P = normal tissue sparing; PQI = quality of a treatment plan.

Discerning power of PQI 

PQDP was used to investigate the discerning power of PQI for discriminating the HT and DCAT plans. The discerning power of PQI was compared with that of the 2 other conventional plan indices: CN and COIN. The PQDP was 54.9% in PQI, which was better than 25.9% for CN or 42.8% for COIN (Table 6).

Table 6. PQDP for CN, COIN, and PQI analysis
CNCOINPQI
PatientHTDCATHTDCATHTDCAT
10.540.460.540.460.440.68
20.610.460.610.460.380.62
30.590.560.590.560.400.62
40.700.560.700.550.300.73
50.600.670.590.590.520.56
60.780.610.770.610.430.66
70.910.380.910.380.090.72
80.740.590.740.480.310.64
90.710.530.650.431.422.64
100.590.460.590.430.410.69
Mean ± SD0.68±0.110.53±0.090.67±0.110.51±0.090.37±0.120.65±0.05
PQDP25.9%42.8%54.9%

Abbreviations: HT = helical tomotherapy; DCAT = dynamic conformal arc therapy; CN = conformation number; COIN = conformation index; PQI = plan quality index; PQDP = plan quality discerning power.

Beam-on time 

The MUs for and beam-on times of the 10 patients with HT vs. DCAT are compared in Table 7. The mean MUs for HT were 16,772 ± 3803, which was significantly higher than 1776 ± 356.3 in DCAT (p < 0.01). The mean beam-on time for HT was 33.2 ± 7.4 min, which was significantly longer than 4.6 ± 0.9 min for DCAT (p < 0.01).

Table 7. Actual beam-on time in the DCAT and simulated beam-on time in the HT plans
HTDCAT
PTV (cm3)MFBOT (min)MU UsedPTV (cm3)ArcsBOT (min)MU Used
Mean ± SD (range)4.8±5.7 (0.22–20.03)1.3±0.12 (1.18–1.54)33.2±7.4 (23.3–49.0)16772±3803 (10551–21663)4.7±5.7 (0.2–19.93)54.6±0.9 (3.63–6.15)1776±356.3 (1388–2385)

Abbreviations: HT = helical tomotherapy; DCAT = dynamic conformal arc radiotherapy; PTV = planning target volume; MF = modulation factor; BOT = beam-on time; MU = monitor unit.

The patient setup time for both systems and the time in-between arcs for DCAT were not included.

p < 0.01.

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Discussion 

Performances of DCAT vs. HT 

In this study, we found that the HT plans had significantly better conformity to the PTV than DCAT plans. Regarding the homogeneity of PTV, both plans gave satisfactory results and no significant difference was observed. By contrast, the dose gradient outside the PTV (GSI value) obtained with the DCAT plans was significantly better than that with the HT plans. We found that the dosimetric advantages of the 2 plans were inconsistent for individual OARs. The HT plans had lower maximum doses in some OARs, because of the unique characteristic of the wide spread of the fan beams delivered in HT. By contrast, using noncoplanar beam directions to avoid some specific organs, DCAT could create better dose sparing in other OARs. However, as revealed by the CQI of 0.92 ± 0.45 for the 9 OARs, overall, marginally better dose sparing was observed in HT.

Because the 2 comparable rival plans have inconsistent advantages, an overall plan analysis using the PQI was designed to incorporate 3 independent variables, namely the healthy tissue conformity index (H), a merit function (M), and a penalty function (P), to quantify the comparisons between the rival plans completely. In some situations, one plan may have a better M, while the other plan may have a better P. In this scenario, clinicians would inevitably be required to decide which plan would benefit the patient most. In our results, the HT plan had a significantly higher (better) H value (0.70 ± 0.11 vs. 0.45 ± 0.10; p < 0.01), M value (0.70 ± 0.11 vs. 0.45 ± 0.10; p = 0.05), and P value (0.84 ± 0.15 vs. 0.75 ± 0.19; p = 0.03), with a final better PQI score (0.37 ± 0.12 vs. 0.65 ± 0.08; p < 0.01) compared with the DCAT plan. This result indicates a significant dosimetric gain in HT. To further identify the discerning power of PQI, the PQDP of the PQI and 2 conventionally used indices of CN or COIN were compared. As demonstrated in Table 6, the result indicated that the PQI is a reliable indicator.

There are some limitations with regard to our results. Theoretically, to our best knowledge, the dose distribution and contours should be exported for both systems and both datasets imported using the same comparison software (e.g., CERR)31 to perform the analysis without bias. Although we used the same resolution, voxel size, and binning of the DVHs in both systems, an intrinsic difference in the calculation algorithms might produce different results.

Literature comparison 

Dosimetric comparisons between HT and the gamma knife or linac-based SRS have been reported for small intracranial tumors. Han et al. evaluated 16 intracranial tumors and observed a better dose conformity and dose gradient, although increased dose heterogeneity was achieved with HT compared with step-and-shoot intensity-modulated radiosurgery (IMRS). In their series, the results of the dosimetric index for coplanar IMRS vs. noncoplanar IMRS vs. HT were 1.53 ± 0.38 vs. 1.35 ± 0.15 vs. 1.6 ± 0.10 for the CI, 1.15 ± 0.05 vs. 1.13 ± 0.04 vs. 1.10 ± 0.09 for the HI, and 1.37 ± 19.08 vs. 22.32 ± 19.20 vs. 43.28 ± 13.78 for the GSI, respectively. There was no further evaluation of the overall plan analysis comparing IMRS and HT. In our series, better dose conformity, similar dose homogeneity, but a poorer dose gradient, was achieved with HT compared with DCAT. Comparisons of different techniques delivered using linac-based SRS have been reported. Cardinale et al. compared 3 SRS techniques: arcs, noncoplanar fixed fields, and IMRS.8 For an ellipsoidal target, they found that the dose conformity was similar for all 3 techniques and normal brain isodose distributions were more favorable with the arcs plan; for the hemisphere and irregular targets, dose conformity and high/low isodose brain volumes were better with IMRS. Baumert et al. further evaluated the possible advantage of intensity-modulated stereotactic radiotherapy (IMSRT) over stereotactic conformal radiotherapy (SCRT) for 10 cases of meningioma in the skull base area.32 They observed that the shape of the target influenced the result, and an advantage of IMSRT over SCRT was seen, especially in targets with irregular and concave shapes. In most of our cases, the VS were regular, round, or ellipsoidal, and no differential impact of target shape was examined in this study.

Machine status 

Although a dosimetric advantage was observed for HT in our study, the overall beam-on time and average MUs used in the HT plans were significantly greater than for DCAT plans delivered using the Novalis system. Beam-on time depended on the limitations of gantry rotation and dose prescription in the HT system, whereas in the Novalis system, beam-on time is determined primarily by the number of treatment fields and segments. A speed limitation on gantry rotation exists in the HT system, which confines the doses delivered to a time of 60 s per cycle and, as a result, a single treatment fraction has to be separated into 3 portions to deliver sufficient prescribed doses. To save beam-on time, we could increase the jaw width of HT, but the coverage of PTV would be sacrificed. Increasing the pitch value would not significantly reduce the beam-on time, because the gantry rotation speed would decrease as the pitch increases to deliver sufficient dose to the target. Although we found that HT took more time than linac-based SRS or IMRT, as reported elsewhere,12 the time taken for pretreatment patient setup is not usually considered in these comparisons. It may take several hours to place a head ring and position linac-based SRS; therefore, the pretreatment process should be considered, as suggested by Jensen et al.33 Compared with linac, the impact of more MUs delivered in HT on the patients and the machine itself remains an important issue to be explored. Mechanically, large MUs delivered by the machine will shorten the lifetime of some major internal components, such as the thyratron, magnetron, target, and beam center line, and in some situations problems with transmission and radiation leakage may also be of concern. We have checked these issues, but our preliminary results revealed no significant difference between the two treatment systems and were comparable to those reported from other studies.34, 35, 36 They found that the beam-on time delivered by HT was 5−15 times longer than those delivered by conventional linacs. However, the radiation leakage outside the treatment field was <0.05% and might become negligible at the expense of increased shielding of the collimators. In addition, a narrow radiation field usually does not increase the leakage, which is dominated by the in-field scattering.

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Conclusions 

Using several dosimetric indices, we compared the planning outcome of HT and DCAT in 10 patients with VS. We found that both planning systems satisfied the required PTV prescription, but better dose conformity, similar dose homogeneity, and a poorer dose gradient were achieved with HT compared with DCAT. An overall plan analysis using the PQI, which was verified by the PQDP, confirmed the dosimetric advantage of HT, although not all indices revealed a better outcome for HT. Whether this dosimetric advantage translates into a clinical benefit merits further investigation.

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Acknowledgments 

The authors thank the anonymous reviewers for their helpful comments on the original manuscript and Ms. H-F Lee, Ms. H-M Ting, and Mr. C. Lee for their assistance in data collection. Dr. An Liu helped with the technical assistance. This study was supported financially in part by grants from the CGMH (CMRPG870791) and NSC (98-2221-E-151-038).

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PII: S0958-3947(09)00131-9

doi:10.1016/j.meddos.2009.11.005

Medical Dosimetry
Volume 36, Issue 1 , Pages 62-70, Spring 2011

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