|Year : 2015 | Volume
| Issue : 2 | Page : 488-491
A technique to reduce low dose region for craniospinal irradiation (CSI) with RapidArc and its dosimetric comparison with 3D conformal technique (3DCRT)
Roopam Srivastava1, Gagan Saini2, Pramod Kumar Sharma1, Manish Chomal2, Anchal Aagarwal3, Sapna Nangia3, Madhur Garg4
1 Department of Medical Physics and Radiation Safety, International Oncology Center, Fortis Hospital, Noida, Uttar Pradesh, India
2 Department of Radiation Oncology, International Oncology Center, Fortis Hospital, Noida, Uttar Pradesh, India
3 Department of Radiation Oncology, Indraprastha Apollo Hospital, New Delhi, India
4 Department of Radiation Oncology, Montefiore Cancer Center, New York, USA
|Date of Web Publication||7-Jul-2015|
Department of Medical Physics and Radiation Safety, International Oncology Center, Fortis Hospital, Sector - 62, Noida - 201 301, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
We proposed a method to reduce the volume of normal tissues irradiated by low doses in patients receiving CSI with RapidArc (RA) using Avoidance-Sector technique (RA+AS) and to compare its dosimetric implications with RA using full-arc (RA+FA) and 3D conformal technique (3DCRT). Four patients of CSI were retrospectively planned with 3DCRT, RA+FA, and RA+AS. Conformity-Index (CI), Homogeneity-Index (HI), and Paddick Gradient-Index (GI) were calculated. Quantitative evaluation was done using DVH analysis for PTVs and OARs. When compared with 3DCRT, GI, CI, and HI were favorable to RA based techniques. In comparison with 3DCRT the doses to OARs were lower with RA+AS with the difference being statistically significant in most instances. RA+AS significantly decreases the dose to OARs and their volumes receiving low doses in comparison with RA+FA and 3DCRT.
Keywords: Avoidance sector technique in RapidArc, craniospinal irradiation, RapidArc, V5 in lung
|How to cite this article:|
Srivastava R, Saini G, Sharma PK, Chomal M, Aagarwal A, Nangia S, Garg M. A technique to reduce low dose region for craniospinal irradiation (CSI) with RapidArc and its dosimetric comparison with 3D conformal technique (3DCRT). J Can Res Ther 2015;11:488-91
|How to cite this URL:|
Srivastava R, Saini G, Sharma PK, Chomal M, Aagarwal A, Nangia S, Garg M. A technique to reduce low dose region for craniospinal irradiation (CSI) with RapidArc and its dosimetric comparison with 3D conformal technique (3DCRT). J Can Res Ther [serial online] 2015 [cited 2020 Jul 14];11:488-91. Available from: http://www.cancerjournal.net/text.asp?2015/11/2/488/144556
| > Introduction|| |
Craniospinal irradiation (CSI) is an integral part of the medulloblastoma management and may be required in treating germ cell tumors/NHL/anaplastic ependymoma with spinal metastases. CSI may be planned with the standard two-dimensional (2D) technique using a direct posterior field to spine and matched lateral-opposed brain fields. This technique is complex since it requires matching field edges and may lead to high exit doses organs at risk (OARs) such as thyroid, mediastinum, and heart. ,
RapidArc, a Volumetric Modulated Arc Therapy (VMAT), is easier to plan as the optimization takes care of the heterogeneity at the field edges; however, the drawback is a possible increase in volumes of regions receiving low doses of radiation. ,
Avoidance Sector (AS) is a technique of optimization for RapidArc (RA) plans in which the radiation beam is turned off for the certain sectors in an arc rotation. In this study, we retrospectively generated and compared radiation therapy plans for CSI using 3DCRT, RapidArc using Full-arc without any Avoidance Sector (RA + FA), and RapidArc with Avoidance Sector (RA + AS) techniques. We aimed to compare the conformity and homogeneity associated with each of the above techniques and the effect on dose delivered to various critical organs using the above three techniques, especially their effect on low doses delivered to lungs.
| > Materials and methods|| |
Four consecutive patients treated with CSI were evaluated retrospectively. They were diagnosed as intracranial teratoma, anaplastic astrocytoma grade-3 with seeding of cerebrospinal fluid, standard risk medulloblastoma and pineoblastoma. Computed Tomography scans were done in supine position for two patients and in prone position for the other two.All four patients had actually been treated with conventional 3DCRT technique on a Varian Trilogy Linear Accelerator equipped with High Definition MLC (HDMLC) using a 6-MV photon beam.
Clinical target volume (CTV) brain and CTV spine were contoured for each patient. CTV brain was expanded uniformly by 5 mm to create planning target volume (PTV) brain. PTV spine was generated by 7-mm volume expansion around CTV spine. Organs at Risk (OARs) including lungs, kidneys, heart, liver, oesophagus, thyroid, eyes, and lenses were delineated. The prescribed PTV doses were 36 Grays (Gy) in 20 fractions.
Due to a limitation of field size with HDMLC, PTV spine was split into Upper PTV Spine and Lower PTV Spine in order to cover entire PTV Spine using MLCs. 
3DCRT plans were made for PTV Brain, Upper PTV Spine, and Lower PTV Spine with three separate isocentres. PTV brain was planned with two opposing laterals, whereas Upper PTV spine and Lower PTV spine were planned with single posterior field. Suitable couch andcollimator rotations were used in lateral cranial fields to match their divergence with posterior spine field. Junction feathering in 3DCRT plans was done to avoid cold and hot spots at the junctions[Figure 1].
RA planning with FA and AS
plans were generated with FA and AS techniques using three isocentres located in three different regions of PTV. At each isocentre, inverse planning was done with a single arc from 179°-181° in anticlockwise direction.  For patients planned with AS, the technique was applied for Upper PTV Spine [Figure 1]. The angle spans that were avoided during optimization using AS were 100°-30° and 330°-260° for patients simulated in supine position [Figure 2], while it was 140°-80° and 280°-220° for patients simulated in prone position. These values were reached upon by considering the position of target and the lungs in body.
|Figure 1: Above shows field placement in a patient planned with 3DCRT, RA+FA and RA+AS|
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|Figure 2: Sector avoidance angles span in a supine position of patient for middle arc in RA|
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evaluation of 3DCRT and RA plans was done using Dose Volume Histograms (cDVH) for PTVs and OARs. For target coverage conformity, various parameters such as RTOG Conformity Index (CI), Paddick Conformity Index (CI), Paddick Dose Gradient Index (GI), Oliver Moderate Dose Homogeneity Index (HI), and RTOG Homogeneity Index (RTOG HI) were calculated for each individual plan to compare the three techniques in each of the four patients. 
Paddick CI = (TVPI) 2 /(PI Χ TV)
RTOG CI = PI/TV
Paddick GI = PI 50% /TV
RTOG HI = (D 5% − D 95% )/D mean .
Oliver Dose HI = D 95% /D 5%
PI is the volume of the prescription isodose line
PI 50% is the volume of 50% of the prescription isodose line
TV is the target volume.
TVPI is the target volume within the prescribed isodose volume PI.
D 5% is the dose received by 5% volume of the PTV
D 95% is the dose received by 95% volume of the PTV
The mean and standard deviations were computed for all parameters of OARs for all the three techniques. Intragroup comparisons were made by a two-tailed paired t-test to test the hypothesis that there is no significant difference among the means of any two techniques. All statistical tests were evaluated at α =5% level of significance; thus, a P value of <0.05 was considered of statistical significance.
| > Results|| |
RTOG CI was (2.56 ± 0.98) with 3DCRT, (1.1 ± 0.06) with RA + FA, and (1.11 ± 0.06) with RA + AS plans. Paddick CI was found to be (0.51 ± 0.22) for 3DCRT, (0.88 ± 0.03) for RA + FA, and (0.86 ± 0.03) for RA + AS. 3DCRT plans with a GI of (6.18 ± 0.97) were achieved, while RA + FA and RA + AS had a GI of (2.02 ± 0.29) and (1.74 ± 0.18), respectively. Oliver HI was calculated to be (0.89 ± 0.01) in 3DCRT and (0.92 ± 0.01) for RA + FA and RA + AS. V5 in lung was observed to be (43.8% ±0.9) with 3DCRT, (77.6% ±2.0) using RA + FA, and (48.1% ±0.7) using RA + AS. V20 for lung was found to be (0.5% ±0.3) on using RA + AS, which was reduced from (2.7% ±1.4) in RA + FA and (30% ±3.4) with 3DCRT. Thyroid max doses were found to be lower with RA + AS (26.5 ± 5.0) than that of RA + FA (31.4 ± 3.8) and 3DCRT (34.6 ± 2.5). The mean dose for heart was found to be less with RA + AS (9.1 ± 1.6) and RA + FA (9.3 ± 2.1) as compared with 3DCRT (19.2 ± 2.4). Liver mean doses were (10.0 ± 0.7) with 3DCRT, (8.1 ± 2.0) with RA + FA, and (6.3 ± 0.2) with RA + AS. Combined Kidney mean doses were (16.5 ± 2.6) with 3DCRT,(13 ± 5.5) with RA + FA, and (8.1 ± 0.5) with RA + AS.
| > Discussion|| |
RTOG and Paddick CI were favorable to both RA-based techniques in comparison to 3DCRT [Table 1]. Paddick GI was in general better for RA as compared to 3DCRT. We also found that the use of RA + AS further decreased the value in comparison to RA + FA[Figure 3]. Oliver HI values calculated for all three techniques were similar. The incidence of Grade 3 pneumonitis was 2%, 4%, and 24% for lungs V5 <35%, 35-50%, and >50%, respectively (P < 0.001).  V5 in lungs was least in 3DCRT plans as compared to both RA based techniques. Additionally, the difference in values obtained is statistically significantly better for RA + AS in comparison with 3DCRT technique for lungs (all parameters except V5), heart, kidneys, liver, and thyroid gland [Table 2]. This finding is particularly encouraging because V5 obtained for RA + AS is still acceptably low.
|Figure 3: PTV coverage with 95% of the prescribed dose and low dose spill with 20% of the prescribed dose in a patient planned with 3DCRT,RA+FA and RA+AS|
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|Table 1: Mean and standard deviation of dose indices for PTV for all four patients by means of all three techniques |
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|Table 2: Below tabulates the doses to OARs and their P values obtained using 3DCRT, RA+FA, and RA+AS |
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| > Conclusion|| |
We found a consistent and statistically significant decrease in doses to OARs and volume of normal structures receiving low doses in RA + AS not only in comparison with RA + FA but also with 3DCRT making it a valuable technique for planning CSI.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]