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A Comparison of IMRT and VMAT treatment techniques on centrally-located lung

tumors and their effects on V5 lung dose

Amber Mehr, B.S.; Andrew Edel, B.S; Jenny Huang, B.S., R.T.(T); Ruha Siddiqui, B.S.;

Ashley Hunzeker, M.S., C.M.D.; Nishele Lenards, R.T.(R)(T), M.S., C.M.D., FAAMD; 

Alyssa Olson, M.S., R.T.(T), C.M.D

University of Wisconsin – La Crosse Medical Dosimetry Program

ABSTRACT   

The goal of this study was to determine if there was a difference in the percentage of lung

receiving 5 Gy or more (V5) in patients planned with intensity modulated radiation therapy

(IMRT) or volumetric modulated arc therapy (VMAT) treatment planning. Fourteen patients

with centrally-located lung tumors and planning target volumes (PTVs) between 100-1500cc

were selected for this research study. The heart, lungs, spinal cord and esophagus were contoured

by the medical dosimetrist for dose tracking purposes to organs at risk (OAR). Each patient was

planned using IMRT and VMAT techniques for comparison purposes. When creating the VMAT

and IMRT treatment plans, the medical dosimetrist used similar optimization techniques to

ensure that the planning objectives were met. Upon plan completion, a paired t-test was used to

determine if there was a significant difference between the planning techniques with regard to

the V5. The t-test score for the lung V5 dose was 3.02 and was considered statistically significant.

Therefore, when comparing IMRT and VMAT techniques for lung treatments, the results of this

study demonstrated that IMRT provided an advantage of sparing lung V5 dose.

Keywords: Intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy

(VMAT), V5, centrally-located lung tumors   

   

Introduction   

In the past, three-dimensional conventional radiation therapy (3DCRT) planning was

primarily used for lung treatments.1 The onset of 3DCRT first began in the early 1980s and was

an important advancement in the world of radiation therapy. Using 3DCRT allowed the

manipulation of spatial orientation, field number selection, beam energy selection, field

weighting and more.2

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Through innovative developments in technology, 3DCRT is slowly being replaced by

IMRT and VMAT. Intensity modulated radiation therapy allows radiation dose to conform

precisely to the 3D shape of the tumor by allowing the manipulation of radiation intensity

through modulation of various static beams in small volume.3 On the contrary, VMAT is a

different type of IMRT where the linear accelerator continuously rotates around the patient while

simultaneously reshaping and altering the intensity of the beam. Both are more advanced types

of planning techniques that utilize more sophisticated dose computation algorithms and

optimization methods.5 These optimization methods allow both techniques to target lung tumors

specifically while minimizing radiation exposure to OAR via inverse planning. Khalil et al4

proposed that 3DCRT posed difficulties when treating large lung volumes, whereas IMRT was

able to successfully treat large tumor volumes without increasing standard lung dose constraints

such as mean lung dose (MLD) and the percentage of lung receiving 20 Gy or more (V20).

Additional research by Liao et al3 further supported this claim.

Radiation therapy is an integral component of treating lung cancer; however, as with

many treatment modalities, the potential side-effects, such as radiation pneumonitis (RP) must be

considered. A comparative study between IMRT and 3DCRT techniques found that RP was

present in 3.5% and 7.9% of the studied population respectively.3 Dosimetric parameters such as

lung V5, V20, the percentage of lung receiving 30 Gy or more (V30), and MLD, have been

considered variables used to predict the occurrence of RP.4 Recent research on V5 indicated an

association with RP and should be kept as low as possible in addition to the aforementioned

dosimetric parameters.4 Yet, a study by Lievens et al6 stated that lung V5 dose is not predictive of

RP nor indicated if lung V5 dose levels are higher when dose is delivered with dynamic arcs.

While there still remains some uncertainty regarding the importance of V5, it should still be

considered when creating a treatment plan as it can be an indicator of RP.7,8 The purpose of this

study was to determine if there was a difference in the lung V5 dose between IMRT and VMAT

treatment planning.

Materials and Methods    

Patient Selection & Setup     

To prevent bias of the data set, plans chosen for comparison purposes had to meet certain

criteria to be considered eligible for data collection. These criteria included a specified range for

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the prescription dose, PTV volume (cm3), PTV vertical length (cm), and lung volume (cm3).

Fourteen patients with centrally-located lung tumors, treated at 3 different cancer centers, were

selected for this study. Patients were prescribed a dose range between 45 to 70 Gy. The tumor

pathology varied among patients, which included non-small cell lung carcinoma (NSCLC), small

cell lung carcinoma (SCLC), and squamous cell carcinoma (SCC). All patients had centrally-

located lung tumors with a PTV between 100-1500 cm3. The lung PTV volume of the studied

patients varied from 1900-3350 cm3 with the vertical lengths of 6-17 cm (Table 1). Patients

excluded from this study included those with a laterally-located lung tumor or prior radiation

treatment.

All 14 patients were positioned in a similar fashion. Patients were set up in the supine

head-first position using immobilization devices during their CT simulation (Figure 1). The

patient’s arms were placed over their heads using a T-Bar device to remove the upper extremities

from the treatment field. A vacuum bag was placed underneath the patient’s chest and arms to

provide stability and comfort. Once the CT scan was performed, the isocenter was established by

the radiation oncologist and medical dosimetrist within the clinic. The radiation therapist placed

marks on the patient’s skin surface to denote the isocenter position; these marks were used for

daily treatment for alignment purposes.    

Contouring    

 After the CT simulation was performed, the patient CT images were imported into the

treatment planning system (TPS) to be contoured by the radiation oncologist and medical

dosimetrist. The radiation oncologist contoured the gross tumor volume (GTV), clinical target

volume (CTV) and PTV. The GTV was created around the cancerous tissue as visualized from

the CT. The GTV was then expanded by 0.7 cm in all directions to create the CTV. To create the

PTV, the CTV was expanded 1.0 cm superiorly and inferiorly and 0.5 cm anteriorly, posteriorly,

laterally and medially coinciding with the Radiation Therapy Oncology Group (RTOG) 0617

Protocol.9 Once the target volumes were completed, the medical dosimetrist contoured the

thoracic OAR following RTOG 0617 protocol and included the spinal cord, lungs, esophagus,

and heart.9   

Treatment Planning     

Planning was performed using Pinnacle 9.8, Pinnacle 14.0 and Eclipse 13.7 TPS and was

used to plan 4, 10, and 4 patients respectively. Patient treatments were completed on Varian

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21iX, Trilogy and Elekta Infinity and Synergy linear accelerators. All 14 patients received

thoracic radiation treatment using a VMAT technique and were re-planned using the IMRT

technique with the same TPS and linear accelerator.

The original prescription for each patient ranged between 45 –70 Gy and was maintained

for the purpose of this study. Applying proper technique depended on the tumor size, tumor

location, OAR and dose tolerance criteria. Both techniques used the same planning objectives.

None of the 28 plans were optimized using a V5 objective to allow for an effective evaluation of

the metric. Other required planning objectives included the ipsilateral lung V20 < 30-35%,

ipsilateral lung V30 < 20-25%, a maximum dose to the spinal cord ≤ 50 Gy, spinal cord dose to

0.03 cm3 < 44 Gy, and to reduce the volume of both the esophagus receiving 45 Gy (V45) and the

volume of the heart receiving 60 Gy (V60) to 33% (Table 2).

The VMAT beams were arranged as either 2 partial arcs, 2 full arcs, 3 partial arcs or 3

full arcs with varying collimator angles to create a conformal plan around the PTV (Figure 2).

The beams were planned with different rotational directions, clockwise (CW) and counter

clockwise (CCW) and utilized a photon energy of 6 MV. The plans were optimized according to

the desired constraints for the lung(s), spinal cord, esophagus, and heart, while simultaneously

optimizing to obtain PTV coverage. All VMAT plans were normalized so that 95% of the PTV

received prescription dose.

For the IMRT plan, the treatment planning objectives, target volumes, isocenter and

prescription were kept the same as the initial VMAT plan. For each IMRT plan, the medical

dosimetrists used a median of 6 coplanar beams (range 4-9 beams) and 6 MV to deliver the

prescribed dose to 95% of the PTV. The beams were placed at optimal angles to avoid the OAR

and create a conformal plan (Figures 3 and 4).

Results

For comparison purposes, data was collected on the V5 lung dose metric in all IMRT and

VMAT plans. A paired t-test was conducted to evaluate the significance of V5 and a statistical

analysis was performed to compare the 2 techniques. A visual evaluation of the 5 Gy isodose

distribution in each plan as was also completed for trend mapping.

With regard to the lung V5 dose, VMAT plans averaged 6% higher V5 dose than IMRT.

Upon further evaluation, a statistical analysis concluded that the noticed variance was

statistically significant. The mean V5 value for the VMAT plans was 69.57% ± 17.16%, while the

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mean V5 value for the IMRT plan was 63.5% ± 14.73%. A 2-tailed t-test for paired samples

resulted in a t13-score of 3.02 and a p-value of .001.

For each plan, a visual inspection of the isodose distribution for 5 Gy was compared. A

common trend when comparing the isodose distributions for both techniques was an increased

amount of 5 Gy dose inferiorly within the lungs for the VMAT treatment technique. However, as

the dose progressed more superiorly, less lung tissue was spared from the 5 Gy dose for both

techniques. In addition, it was noticed that the 5 Gy isodose line for both techniques was found

close to the entry points for each treatment beam. Therefore, due to the delivery method of the

VMAT technique, the lung V5 dose for the VMAT plans looked more continuous due to the

constant supply of dose as the beam traveled around the patient (Figure 5).

Discussion  

Notable differences between IMRT and VMAT treatment planning techniques were

observed throughout this study. For VMAT, the mean V5 lung dose percentage across patient

cases was greater than 65% which has been suggested as a reasonable lung V5 dose

constraint. For both planning techniques the standard deviation was over 15% between patients.

This high degree of variance limits the conclusions that can be drawn. It is unclear if the results

were skewed by a few cases with large changes in V5 dose. Large standard deviations between

patients could be due to differences in anatomy, prescription dose, beam arrangement, PTV size,

and PTV location.5,9 Planning technique is one of many factors that also can affect lung V5

dose. Further research is needed to discover which of these factors are covariable.

Upon completion of this study, it was obvious that IMRT and VMAT have different

characteristic dosimetric outcomes that become more pronounced at the lower doses. With

VMAT, the dose entered the patient continuously while the gantry moved along the arc. This

resulted in dose being delivered to the patient’s body through more entry points when compared

to IMRT treatment planning. With this change in dose delivery, patients received a higher

integral dose due to the increased access to the lungs.5 Therefore, as concluded in this study, the

lung V5 dose for IMRT was lower when compared to the VMAT plans. 

When comparing the isodose distribution, the 5 Gy isodose line was greater superiorly

within the lung and greater for VMAT plans. This makes sense because the volume of lung

becomes greater when traveling more inferiorly through the patient’s thorax. The V5 dose was

also more prominently observed wherever the beams entered the patient. This meant that when

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the patients were treated using IMRT, there was less V5 dose because there were gaps between

beam entry points unlike when the patient was treated with VMAT.

In a study conducted by Boyle et al,10 the IMRT technique produced lower doses to the

lungs, heart, and esophagus in comparison to 3DCRT. However, few studies have analyzed the

significance of lung dose when comparing newer methods such as VMAT versus IMRT. This

study aimed to present the statistical difference and evaluate results in lung dose received in

IMRT and VMAT planning. Therefore, IMRT may provide better results for physicians

inquiring into the significance of lung dose, especially V5, with regards to treatment planning.

Conclusion   

Through advancements in technology, IMRT and VMAT planning are now being used to

treat lung cancer. With the introduction of these techniques, the difference in lung V5 dose

between planning techniques required more analysis due to the risk of RP. Patients with

centrally-located lung tumors were selected and IMRT and VMAT comparison plans were

created to determine the differences in lung V5 dose. When comparing IMRT and VMAT

treatment plans, there were differences found among both techniques. The VMAT planning

technique resulted in higher lung V5 dose in patients. Both techniques, however, were beneficial

to the medical dosimetrists because coverage was maintained while limiting dose to critical

structures due to blocking and beam placement.

The limitations of this study included a small sample size and the exclusion of 3DCRT as

one of the planning techniques for comparison. For future studies, a larger sample size of IMRT

and VMAT treatments plans should be created focusing solely on laterally-located lung tumors

to see if there is a difference in overall lung dose. The beams should also be limited to the side of

the patient’s body that contains the tumor to control dose to the contralateral lung.7 Medical

dosimetrists should also look at IMRT planning versus 3DCRT and VMAT versus 3DCRT to

identify changes in lung dose with planning technique advancements.

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References

1. Graham MV, Purdy JA, Emami B, et al. Clinical dose-volume histogram analysis for

pneumonitis after 3D treatment for non-small cell lung cancer (NSCLC). Int J Radiat Oncol

Biol Phys. 1999;45(2):323-329. https://dx.doi.org/10.1016/S0360-3016(99)00183-2  

2. Cai J, Malhotra HK, Orton CJ. A 3D- conformal technique is better than IMRT or VMAT for

lung SBRT. Med Phys. 2014;41(4):040601-040602. https://dx.doi.org/10.1118/1.4856175 

3. Chun SG, Hu C, Choy H, et al. Impact of intensity-modulated radiation therapy technique for

locally advanced non-small-cell lung cancer: a secondary analysis of the NRG Oncology

RTOG 0617 randomized clinical trial. J Clin Oncol. 2017;35(1):56-62.

https://dx.doi.org/10.1200/jco.2016.69.1378

4. Khalil AA, Hoffman L, Moeller DS, et al. New dose constraint reduces radiation-induces

fatal pneumonitis in locally advanced non-small cell lung cancer patients treated with

intensity-modulated radiotherapy. Acta Oncologica. 2015;54:1343-1349.

https://dx.doi.org/10.3109/0284186x.2015.1061216

5. Li Y, Wang J, Tan L, et al. Dosimetric comparison between IMRT and VMAT in irradiation

for peripheral and central lung cancer. Oncol Lett. 2018;15(3):3735-3745.

https://dx.doi.org/10.3892/ol.2018.7732 

6.  Aaron A, Czerminska M, Jänne P, et al. Fatal pneumonitis associated with intensity-

modulated radiation therapy for mesothelioma. Int J Radiat Oncol Biol Phys.

2006;65(3):640–645. https://dx.doi.org/10.1016/j.ijrobp.2006.03.012   

7. Helen H, Jauregui M, Zhang X, et al. Beam angle optimization and reduction for intensity-

modulated radiation therapy of non–small-cell lung cancers. Int J Radiat Oncol Biol Phys.

2006;65(2):561–572. https://dx.doi.org/10.1016/j.ijrobp.2006.01.033  

8. Lievens Y, Nulens A, Gaber MA, et al. Intensity-modulated radiotherapy for locally

advanced non-small-cell lung cancer: a dose-escalation planning study. Int J Radiat Oncol

Biol Phys. 2011;80(1):306-313. https://dx.doi.org/10.1016/j.ijrobp.2010.06.025 

9.  Bradley J, Choy H, Komaki R, et al. RTOG 0617: A randomized phase III comparison of

standard-dose (60 Gy) versus high dose (74 Gy) conformal radiotherapy with concurrent and

consolidation carboplatin/paclitaxel +/- cetuximab (IND #103444) in patients with stage

IIIA/IIIB non-small cell lung cancer. Lancet Oncol. 2015(2):187-199.

https://dx.doi.org/10.1016/S1470-2045(14)71207-0  

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10. Boyle J, Ackerson B, Gu L, et al. Dosimetric advantages of intensity modulated radiation

therapy in locally advanced lung cancer. Adv Radiat Oncol. 2017;2(1):6-11.

https://dx.doi.org/10.1016/j.adro.2016.12.006

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Figures

Figure 1.  Demonstration of patient positioning for CT simulation and treatment delivery. 

 

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Figure 2. The conformal dose distribution for a lung plan treated to 61.2 Gy (red) with 3-partial arcs using a VMAT treatment planning technique.

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Figure 3. An example of a lung plan with a 5-field beam arrangement using an IMRT treatment technique.

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Figure 4. The conformal dose distribution, for a 61.2 Gy prescription dose, of a 7-field beam arrangement using an IMRT treatment technique.

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Figure 5. A comparison of the lung V5 dose (red) distribution for VMAT (left) and IMRT (right) treatments within the lung.

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Tables

Table 1. Patient qualifiers which included the following: prescription (Gy), lung volume (cm3), PTV volume (cm3), PTV length (cm).PatientNo.

Prescription(Gy)

Lung Volume(cm3)

PTV Volume(cm3)

PTV length(cm)

1 60 2728.13 362.8 13.52 70 2690.28 629.2 9.33 60 3211.2 1087.9 12.34 60 3044.3 916.5 14.05 61.20 2086 724.1 11.76 50 2816.82 582.3 10.27 45 3347.97 1409.5 17.38 61.20 1868.36 535.6 17.79 60 2682.9 320.8 12.610 60 2851.84 120.2 6.011 50 2364.43 340.6 11.812 50.40 1916.4 273.2 8.013 50 2521.7 336.2 10.914 60 2325.9 240.8 8.7

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Table 2. The thoracic constraints used for patient treatment planningin IMRT and VMAT. Organ at risk ObjectivesSpinal Cord Maximum Dose (point dose) < 50 Gy

Maximum Dose (0.03 cm3) < 44 GyLung V20 < 30-35%

V30 < 20-25%Esophagus V45 < 33%Heart V60 < 33%