|Year : 2013 | Volume
| Issue : 3 | Page : 138-142
Dosimetric analysis of intensity modulated radiotherapy plans having one or more pairs of parallel opposed beams among the set of beams in some special cases
Om Prakash Gurjar, Surendra Prasad Mishra
Department of Physics, Mewar University, Chittorgarh, Rajasthan, India
|Date of Web Publication||28-Jul-2014|
Om Prakash Gurjar
(Ph.D. Scholar - Mewar University), Asstt. Professor (Med. Phys.) and RSO (III), ROS SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore - 453 111
Source of Support: None, Conflict of Interest: None
The purpose of this study was to analyze the dosimetric parameters of intensity modulated radiotherapy (IMRT) plans with a set of beams having one or more pairs of parallel opposed beams for the pelvis, head-and-neck (H and N) and brain cases having large secondary nodes. We selected three pelvis (carcinoma of prostate with pelvic bony mets), three H and N (two cases of carcinoma of the parotid with large secondary nodes and one case of carcinoma of the base of the tongue with large secondary nodes), and three brain (glioblastoma multiform) IMRT patients. IMRT plans were done with a set of 6-9 beams having one or more pairs of parallel opposed beams. Each plan was done using 6 MV photon energy. Each plan was analyzed on the basis of planning target volume (PTV) coverage with 93%, 95%, 100%, 107%, and 110% of the prescribed dose (PD), organs at risk (OAR) doses, homogeneity index (HI), conformity index (CI), and normal tissue integral dose (NTID). The doses to OARs were well within tolerance limits and the PTV coverage for 93%, 95%, and 100% of PD was obtained very well (followed the radiation therapy oncology group criteria "95% and 99% volume of PTV should receive 95% and 93% of the PD, respectively"), and values of HI, CI, and NTID were also satisfactory. In summary, very good OAR sparing and PTV coverage were observed in all plans. Hence, it can be concluded that use of one or more pairs of parallel opposed beams in IMRT plans in some special cases as selected for this study offers the benefit in terms of critical target volume irradiation, while maintaining the OAR within tolerable limits.
Keywords: Dosimetric parameters, organs at risk, parallel opposed beams
|How to cite this article:|
Gurjar OP, Mishra SP. Dosimetric analysis of intensity modulated radiotherapy plans having one or more pairs of parallel opposed beams among the set of beams in some special cases. Radiat Prot Environ 2013;36:138-42
|How to cite this URL:|
Gurjar OP, Mishra SP. Dosimetric analysis of intensity modulated radiotherapy plans having one or more pairs of parallel opposed beams among the set of beams in some special cases. Radiat Prot Environ [serial online] 2013 [cited 2019 Jun 25];36:138-42. Available from: http://www.rpe.org.in/text.asp?2013/36/3/138/137480
| Introduction|| |
Treatment planning system (TPS) plays an important role in working out the treatment plans with the objectives of maximum dose to tumor while minimizing the dose to normal organs. For this choosing the appropriate energy, couch angle, gantry angle, and additional necessary accessories like a wedge and bolus plays a very important role. In the case of intensity modulated radiotherapy (IMRT), the couch angle and gantry angle is important, and the quality of the plan depends on gantry angles.  In general, any pair of parallel opposed beams is not used in IMRT because parallel opposed beams do not show a significant beam shaping potential.  except for the case of metastasis cases such as carcinoma of the parotid with large secondary nodes, carcinoma of base of tongue with large secondary nodes, carcinoma of prostate with pelvic bony mets and glioblastoma multiforme (GBM), etc., The main objective of using one or more pairs of parallel opposed beams is to minimize the dose to normal organs, which generally receive very high doses in regular IMRT plans were nonparallel opposing beams are used in above mentioned cases. In IMRT, the use of parallel opposed beams increases dose spillage outside the target. However, if the target (primary plus secondary) is in almost whole volume in any direction of patient separation (except some tissue volume) and specially if there is no organ at risk (OAR) in the path and at the same time some OARs are near to target (perpendicular to that direction), then the use of pairs of parallel opposed beams may be of advantage. In this paper, we therefore analyzed those plans having one or more pairs of parallel opposed beams on the basis of dosimetric parameters.
| Materials and methods|| |
Three pelvic (carcinoma of prostate with pelvic bony mets), three head-and-neck (H and N) (two cases of carcinoma of the parotid with large secondary nodes and one case of carcinoma of the base of the tongue with the large secondary nodes) and three brain (GBM) cases were selected for this study.
Thermoplastic sheets (Orfit) were molded for each patient for immobilizing the patients during the course of treatment. Three fiducial lead markers were put on two bilateral points and one anterior point on the surface of the molded thermoplastic sheet in the same cross-sectional plan to make three reference points. Then, computed tomography (CT) images of 3 mm slice thickness were taken by using a Siemens SOMATOM Definition AS scanner (Siemens Medical Systems, Germany). After importing the CT images in to the TPS Oncentra Master Plan Version: 3.3SP1 (Nucleotron Pvt. Limited, The Netherland), target volumes viz., gross tumor volume (GTV), clinical target volume (CTV), planning target volume (PTV) and normal OARs were contoured by a radiation oncologist fo llowing the guidelines of International Commission on Radiation Units and Measurements (ICRU 83).  [Figure 1]a, [Figure 2]a, and [Figure 3]a show the relevant contoured slices for brain, H and N and pelvis cases, respectively.
|Figure 1: Brain case: (a) Computed tomography slice showing contoured structures. (b) Beam arrangement where parallel opposed beams are at gantry angles 90° and 270°|
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|Figure 2: Head-and-neck case: (a) Computed tomography slice showing contoured structures. (b) Beam arrangement where three pairs of all six fields are parallel opposed viz. 20-200°, 340-160°, and 90-270°|
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|Figure 3: Pelvis case: (a) Computed tomography slice showing contoured structures. (b) Beam arrangement where parallel opposed beams are at gantry angles 90° and 270°|
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For brain cases
Clinical target volume was marked at 3 mm margin around GTV and PTV was also at 3 mm margin around CTV.
For head-and-neck cases
Gross tumor volume and CTV64 (64 Gy/32#) were marked on the right side, while CTV46 (46 Gy/32#) was marked on the left side of the patient anatomy. CTV64 was marked around GTV and PTV64 around CTV64 with 5 mm margin. PTV46 was marked around CTV46 with 5 mm margin.
For pelvis cases
Prostate was marked as GTV (70 Gy/35#), CTV (70 Gy/35#) was marked around GTV and PTV (70 Gy/35#) was marked around CTV with 5 mm margin. CTV50 (50 Gy/35#) was marked separately, and PTV50 (50 Gy/35#) was marked around CTV50 with 5 mm margin. Bladder outside PTV was marked as "bladder-PTV" and rectum outside PTV was marked as "rectum-PTV".
Nine fields were placed for the brain cases with gantry angles 60°, 90°, 120°, 150°, 180°, 210°, 240°, 270°, and 300°, where two fields were parallel opposed at gantry angles 90° and 270°. Six fields were placed for the H and N cases with gantry angles 20°, 200°, 340°, 160°, 90°, and 270° where three pairs of all six fields were parallel opposed viz., 20-200°, 340-160°, and 90-270°. Finally, seven fields were placed for the pelvis cases with gantry angles 180°, 140°, 90°, 55°, 305°, 270°, and 220°. The isocenter was placed at the geometrical center of the PTV in each case. [Figure 1]b, [Figure 2]b and [Figure 3]b show the field arrangement in the brain, H and N and pelvis cases, respectively. For the brain cases, the prescribed doses (PDs) were 54 Gy (GTV and CTV) in 30 fractions, for H and N cases 64 Gy (GTV) and 46 Gy (CTV) in 32 fractions, and for pelvis cases 70 Gy (GTV) and 50 Gy (CTV) in 35 fractions. Dose constraints were given to all OARs following the dose constraints explained in ICRU 83, in an article of Emami et al. and in Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC). ,, All IMRT plans were done with 6 MV photon beam. Pencil beam enhanced algorithm was used to calculate doses with grid size of 0.3 cm. Forty one pairs of multileaf collimators installed in the medical linear accelerator "SIEMENS PRIMUS PLUS" (Siemens Medical Systems, Germany) were selected for segmentation. In each plan, the total number of segments generated was from 80 to 110. Leaf thickness projected at isocenter of all middle leaves was 1 cm and 0.5 cm for the outer most leaves on both sides in both banks.
| Results|| |
In all plans a very good OAR sparing and PTV coverage was observed; these plans were done with beams including one or more pairs (as per required) of parallel opposed beams for the IMRT of prostate cancer with pelvic bony mets (femoral heads also covered in target volume), parotid cancer and base of tongue cancer with large secondary nodes and GBM with CTV very near to optic chiasma and optic nerves.
Dose volume histograms (DVHs) of each patient plan were studied, and dose statistics were recorded. Using this dose statistics dose to the PTV and OAR was seen to be as given below:
Planning target volume
In all plans PTV coverage for 93% and 95% of PD was obtained very well, and the maximum dose had-for all cases-an average value <107%, which fulfills the radiation therapy oncology group criteria, which is "95% and 99% volume of PTV should receive 95% and 93% of the PD, respectively and maximum dose should not exceed 107% of PD".
Organs at risks
In all plans, the dose to the OARs was within the limits as quoted in ICRU 83, in the article of Emami et al. and in QUANTEC which is as follows:
Limit at 45 Gy (up to 48 Gy counted as deviation), brainstem: Limit at 54 Gy, eyes: Limit at 45 Gy, eye Lens I and II: Limit at 7 Gy, optic nerve I and II: Limit at 50 Gy, parotid I and II: ≤30 Gy to 50% of the volume, bladder: ≤80 Gy to 2/3 rd and ≤65 Gy to 3/3 rd volume, rectum: ≤60 Gy to 3/3 rd volume, femoral head I and II: ≤52 Gy to 3/3 rd volume.
[Figure 4] shows the DVH of the plan for one case of the brain where we can clearly see that PTV, CTV, and GTV has a good dose coverage and doses to OARs viz., both optic nerves, both eyes and both lenses are well within their tolerance limits. [Figure 5] shows the DVH of a tough plan for one case of H and N (parotid cancer with large secondary nodes), where the spinal cord and opposite parotid is very near to target volume as shown in [Figure 2]. Simultaneously, integrated boost (SIB) plan was done for this, in DVH we can see that target volume are covered satisfactorily and dose to the spinal cord (D max 7.8 Gy) is also not much high. Dose to opposite parotid (D 50 = 25.6 Gy) is also well-below to its tolerance limit. Similarly, [Figure 6] shows the DVH of SIB plan for one case of pelvis in which target volumes are excellently covered with PD and at the same time bladder-PTV (normal bladder) and rectum-PTV (normal rectum) are well-saved.
|Figure 4: Dose volume histogram of plan for the case of glioblastoma multiforme where planning target volume is very close to optic chiasma and optic nerves|
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|Figure 5: Dose volume histogram of plan for the case of parotid cancer with large secondary nodes (only opposite parotid and spinal cord was to save)|
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|Figure 6: Dose volume histogram for the case of prostate cancer with pelvic bony mets (femoral heads also covered in target volume)|
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Physical analysis of the intensity modulated radiotherapy plans
The plans were evaluated for approval and different scoring indices were calculated. [Table 1], [Table 2], [Table 3] give the different indices from the approved plans;
HI - Homogeneity Index = D 5% /D 95%
CI - Conformity Index (for 95% of PD) = Volume receiving 95% of PD/PTV
Homogeneity index and CI are the objective tools for assessment of the treatment plan, HI indicates the dose homogeneity in the target volume and CI indicates the dose confinement near to target volume and level of dose spillage in normal tissue volume around the target volume. Value of HI and CI = 1 is best; although it is not possible practically and the value of both indices always remain higher than 1. The value of these indices in any plan is as near to 1 the plan will be that much better. In above [Table 1], [Table 2], [Table 3], we can see that mean HI is 1.06, 1.08 and 1.09 for brain, H and N and pelvis cases, respectively, which shows the dose distribution with good homogeneity in the target volumes. The mean CI is 1.3, 1.27 and 1.35 in brain, H and N and pelvis cases, respectively, which is also acceptable however the value remains up to 1.1-1.2 in a good plan, but as target volumes in our cases were very large and we used some pairs of parallel opposed beams to achieve acceptable target coverage and dose tolerance of OARs, so CI is comparatively little higher than in the regular normal plans.
| Discussion|| |
We note that the delivery of radiation therapy with the use of one or more pairs of parallel opposed beams in IMRT plans in cases where large secondary nodes must be treated, makes good sparing of OARs while possibly increasing dose spillage in the normal tissues falling in the path of parallel opposed beams; however, still maintaining the latter within the range of tolerability. In most of such cases, which has been selected for this study, we were unable to achieve desired target coverage and OAR sparing by plans with nonopposing fields and so those plans were not suitable to treat the patients, so we need to make plans using one or more pairs of parallel opposed beams. The perception is based on the fact that with IMRT field arrangements, opposed beams are not recommended. Indeed Bortfeld, on behalf of the American Association of Physicists in Medicine (AAPM, Monograph 29), recommends that "parallel-opposed beams should be avoided in IMRT-the reason (being) that parallel opposed beams add much less beam-shaping potential".  A similar conclusion is drawn by Clifford Chao KS in his book. 
Combination of IMRT and anterior- posterior/posterior- anterior (AP-PA) techniques has been utilized by Soyfer et al. for centrally located lung tumors. In their study, the combined IMRT and AP-PA techniques offer better lung tissue sparing compared to plans predicated solely on IMRT for centrally-located lung tumors, which is also a good example of using one pair of parallel opposed beams along with other nonopposed beams in IMRT plans in some special selected cases.
The importance of selection of the number of beams and beam angles plays an important role, and unnecessary higher number of beams does not improve the plans. For example, the studies done on five beam plans and seven plus beam plans by Takamiya et al. show that less beam plans with better beam angle selection can result in a better overall plan. In their study, all fields were nonopposing. Similarly, in the study of Ashamalla et al.,  where intensity modulated arc therapy versus IMRT has been analyzed. There was the need of choosing the beam angles with one or more pairs of parallel opposed beams in our cases, which made the quality plan and good results were obtained as desired for quality treatment. The results of our study are in concurrence with the results of above-mentioned studies.
| Conclusion|| |
Our results show that the use of one or more pairs of parallel opposed beams in IMRT plans in some special cases as selected for this study, offers benefit in terms of critical target volume irradiation, while maintaining the OAR within tolerable limits.
| Acknowledgment|| |
We thank to Dr. Dinesh Mangal, Director, SEAROC Cancer Centre, Jaipur for allowing to do this research work and providing the necessary equipments.
| References|| |
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3]
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