andrewedel.weebly.comandrewedel.weebly.com/uploads/1/1/1/5/111569887/dr… · Web viewComparison of IMRT and VMAT treatment techniques on centrally-located lung tumors and their effects

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

and their effects on V5 Lung doses   

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   

ABSTRACT   

The percentage of lung receiving 5 Gy or more (lung V5 dose) can be crucial in determining

future long-term effects such a pneumonitis. The goal of this study was to determine if there was

a difference or increase in the percentage of lung V5 dose during intensity modulated radiation

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

treatment techniques are relatively new to the industry and their impact on critical organ dose

requires more exploration. Fourteen patients with centrally located tumors and planning target

volumes (PTV) between 100-1500cc were selected for this research study. Each patient had an

IMRT treatment plan and a VMAT treatment plan created for comparison purposes. A paired t-

test was used to determine if there was a significant difference between the planning techniques.

The t-scores for the lung V5 dose, percentage of lung volume receiving 20 Gy or more (lung V20

dose), percentage of lung volume receiving 30 Gy or more (lung V30 dose) were 3.02, 2.42, and

2.01, respectively. The t-score for lung V30 dose was statistically insignificant meaning that the

percentage of lung volume receiving 30 Gy or more was similar. The t-score for the lung V5 dose

and lung V20 dose was statistically significant, meaning there was an increase in the volume of

lung receiving 5 Gy or higher and 20 Gy or higher when using VMAT instead of IMRT. The

results of this study determined that both IMRT and VMAT planning techniques are comparable

when creating lung treatment plans. The organs at risk (OAR) doses were kept below their

constraints (Table 1). The IMRT planning technique obtained similar PTV coverage when

compared to the VMAT planning technique but IMRT was able to maintain lower lung V5 and

lung V20 dose.   

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

(VMAT), Lung V5 dose, centrally located lung tumors   

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Introduction:   

In the past, 3D conventional radiation therapy (3DCRT) planning was primarily used for

lung treatments and dose constraints were based on dose volume histograms (DVHs) of these

plans.1 The onset of 3DCRT first began in the early 1980s and was an important feat in the world

of radiation therapy. Using 3DCRT allowed the manipulation of spatial orientation, selecting the

number of fields, beam energy, field weighting and more.2 It involved a high degree of technique

among treatment planners to achieve the best plan.   

Through advancements in technology, 3DCRT is slowly being overcome by VMAT and

IMRT. Plans using 3DCRT have minor deviations in comparison to IMRT and VMAT planning

and is the sole reason 3DCRT is slowly phasing out for certain circumstances and type of

diseases. Both IMRT and VMAT are types of planning techniques that allow for better dose

computation algorithms, better and precise optimization methods, and ability to target the tumor

more precisely, which in turn creates a better conformal plan in comparison to 3DCRT.2 When

comparing 3DCRT to IMRT and VMAT, the impact on high lung dose has become a topic of

discussion among the radiation oncology field. The advanced treatment planning techniques have

led to more entry points for radiation dose by using multiple beams and continuous arcs. In

retrospect, this has created a risk for increasing the lung V5 dose.3  

The purpose of this study was to determine if there was a difference in the lung V5 dose

during IMRT or VMAT treatment planning. The increased lung dose is a concern for patients

because when lung V5 dose increases, there becomes a higher risk for radiation pneumonitis and

other complications.4,5,6 Previous research studies, such as Lievens et al.2, have proposed that lung

V5 dose is not predictive of radiation pneumonitis nor has the study indicated if lung V5 dose

levels are higher when dose is delivered with dynamic arcs. Another factor to consider is whether

to evaluate lung V20 dose as a risk for pneumonitis, proposed by a study by Graham et al.1 While

looking at this criterion alone may not be as predictive as previously thought, data now suggests

that lung V5 dose must be considered in addition to lung V20 dose.5,6    

Materials and Methods:    

Patients     

Fourteen patients with lung tumors that were treated at 3 different cancer centers were

selected for this study. Patients were prescribed with a dose range between 4500 to 7000 cGy.

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The tumors ranged in the type of cancer and their progression which included non-small cell

lung carcinoma (NSCLC), malignant neoplasm of lobe, squamous cell carcinoma (SCC), small

cell lung carcinoma (SCLC) and left upper lobe (LUL). All patients had centrally located lung

tumors with a PTV between 100-1500 cc (Table 2). To prevent bias of the data set, plans chosen

for comparison purposes had to meet certain criteria to be considered eligible for data collection.

This criterion included remaining within a certain range for the prescription dose, PTV volume

(cc), PTV vertical length (cm), and lung volume (cc).   

There were various factors that would prevent patients from being part of the study

including a laterally-located lung tumor, and prior radiation treatments. Plans using lung V5 dose

as an optimization parameter were also excluded to prevent bias within the results. Controlling

this within the study was essential because if lung V5 dose was used as an optimization

parameter, the plan could not be used to determine if IMRT or VMAT planning technique

resulted in greater V5 dose.    

For treatment, all 14 patients were positioned in a similar fashion.  Patients were set up in

the supine head-first position using several immobilization devices during their computed

tomography (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-lock bag was

placed underneath the patient’s chest and arms to provide stability and comfort. Lastly, an elastic

band was placed around their feet to limit patient mobility. 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 to ensure reproducibility and patient

alignment to the isocenter.    

Contouring    

 After the CT simulation was performed, the patient CT images were uploaded into either

Pinnacle or Eclipse treatment planning systems (TPS) to be contoured by the radiation

oncologists and medical dosimetrists. The radiation oncologists contoured the gross tumor

volume (GTV), clinical target volume (CTV) and PTV. The GTV was created around the

cancerous tissue determined by imaging. 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 laterally and medially coinciding with the Radiation Therapy Oncology Group

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(RTOG) 0617 Protocol.7 Once the target volumes were completed, the medical dosimetrists

contoured the thoracic OAR following RTOG 0617 protocol to include spinal cord, lungs,

esophagus, and heart (Table 1).7    

Treatment Planning     

Ten IMRT and VMAT plans were made on Pinnacle 9.3 and 14.0 workstation and treated

on Varian 21iX, Trilogy and Elekta Infinity and Synergy accelerators.  Four IMRT and VMAT

plans were made on Eclipse (Version) workstation and treated on Varian Trilogy accelerators.

Applying proper technique depended on the tumor size, tumor location, OAR and dose-tolerance

criteria. The medical dosimetrists used 6 MV photon beams for both IMRT and VMAT. The

prescriptions ranged between 45 Gy-70 Gy for treatments but was kept consistent when re-

planning using the different treatment technique for the current study (Table 3).  

A common treatment planning technique for the treatment of lung cancer is IMRT. The

medical dosimetrists used 5-9 beams for each IMRT plan using a static step-and-shoot approach

(Figure 2) (Table 3). The beams were placed at angles to avoid the OAR.   

In this study, the actual IMRT plans were retrieved and re-planned for the VMAT study

using the same treatment objectives. Volumetric modulated arc therapy produces highly

conformal dose distribution, improves the delivery efficiency by reducing treatment time and

produces accurate dosimetric calculations.3 The VMAT beams were arranged as 2 partial arcs, 2

full arcs, 3 partial arcs or 3 full arcs with varying collimator angles (Figure 3). The beams were

planned with different rotational directions, clockwise (CW) and counter clockwise (CCW)

(Table 3). The arcs and collimator angles were chosen to avoid OAR while maintaining the best

coverage of the PTV.     

To determine the constraints for the OAR, used for planning and optimization, the

radiation oncologist referred to RTOG 0617 for creating the IMRT and VMAT treatment plans.

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 (Table 1).   

Plan Comparisons    

Once an IMRT and a VMAT plan was created for each patient, the plans were compared

to identify the differences. A paired t-test was selected because it compared the two plans with

the same data set. Differences in anatomy have a large effect on plan results and this statistical

test accounted for this variability. Both IMRT and VMAT were normalized to meet PTV

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coverage. Specific metrics were recorded for each plan including lung V30 dose, lung V20 dose,

lung V5 dose, and objectives of OAR (Table 1).  The IMRT and VMAT plans were acceptable if

at least 95% of PTV volume received 100% of the prescribed dose.  

A statistical analysis was performed for all lung metrics to allow for a clear plan

comparison of the 2 techniques. The lung V5 dose, lung V20 dose, lung V30 dose and mean dose

were recorded. The percentage volume of heart receiving 60 Gy or more (heart V60 dose) was

also recorded.  

Results:  

 Treatment plans of 14 patients met the previously established requirements. Patients that

were selected were diagnosed with one of the following; NSCLC, SCLC, SSC, or undefined

malignant neoplasms of the lung (Table 4). The physician prescription ranged between 4500 and

7000 cGy (table 5). Only 2 cases were initially planned with IMRT.   

The lung volume showed differences in lung V20 and lung V5 dose for VMAT and IMRT

planning.  Lung V30 dose in VMAT plans (mean= 19.17, SD= 5.93) was not significantly greater

than IMRT plans (mean= 16.77, SD= 5.03) based on a 2-tailed test for paired samples (t13 = 2.01,

P = .066). However, lung V20 dose in VMAT plans (mean= 27, SD=6.29) was significantly

greater than IMRT plans (mean= 25.72, SD= 6.95) based on a 2-tailed test for paired samples (t13

= 2.42, P = .031). The lung V20 dose is significantly different, however the difference between

plans is under 2%. Lung V5 dose in VMAT plans (mean= 69.57, SD=17.16) was significantly

greater than IMRT plans (mean= 63.5, SD= 14.73) based on a 2-tailed test for paired samples (t13

= 3.02, P = .001). Note that the VMAT mean is greater than 65% which has been suggested as a

reasonable lung V5 dose constraint.   

Discussion   

Similarities and differences between IMRT and VMAT treatment planning techniques

were observed throughout this study. The lung V30 dose was comparable for each type of

planning technique. Treatment planning with IMRT instead of VMAT, resulted in less lung V5

and lung V20 dose.  

The difference between the percentage of lung V30 dose for IMRT and VMAT was not

statistically significant. When looking at lung V30 dose delivered to the lung, the differences

between treatment techniques became less noticeable. This was due to medical dosimetrists

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limiting the amount of dose received by the lung during optimization at lung V30. Through this

optimization, both techniques could lower the lung V30 dose.   

When comparing the two treatment techniques, the difference between the lung V5 dose

and lung V20 dose increased significantly. 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 increase in access to the lungs during

movement of the arcs and resulting in more dose entry points. Therefore, the lung V5 dose and

lung V20 for IMRT was lower than the lung V5 dose and lung V20 dose for VMAT plans.   

Conclusion:   

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

perform lung treatments. With the introduction of these techniques, the difference of lung V5

dose required more analysis due to the risk of pneumonitis. Patients with centrally located lung

tumors were selected and IMRT and VMAT plans were created to determine the differences in

lung V5, lung V20, and lung V30 dose. When comparing IMRT and VMAT treatment plans, there

were differences found amongst both techniques. Planning with IMRT and VMAT, resulted in

similar lung V30 dose.  Using the VMAT planning technique however, resulted in higher lung V5

dose and lung V20 dose in patients with centrally-located lung tumors. Both techniques were

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

critical structures due to blocking and beam placement. The limitations for this study included

the small sample size and the exclusion of 3DCRT as one of the planning techniques for

comparison.    

For future studies, IMRT and VMAT treatments plans should be created focusing only on

laterally-located lung tumors to see if there is a difference in lung V5 dose. The beams should

also be limited to the side of the patient’s body that contains the tumor to lower dose to the

contralateral lung.5 Medical dosimetrists should also look at IMRT planning versus 3DCRT and

VMAT versus 3DCRT to see how much the lung V5 dose has increased with planning technique

advancements. Knowing the amount of lung V5 dose a patient is receiving is important because

increased low lung doses could lead to future radiation damage.   

References

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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://doi.org/10.1118/1.4856175. 

3. 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  

4. 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   

5. 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  

6. 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  

7. 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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Figures

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

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Figure 2. The arrangement of the beams, for the 7-beam IMRT treatment technique, were placed to avoid the spinal cord and produced dose conformity.  

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Figure 3. An example of a 3-arc VMAT utilizing clockwise and counterclockwise motion to produce a highly conformal dose distribution.  

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Tables  Table 1. The thoracic constraints used for patient treatment planning in IMRT and VMAT. Organ at risk   Objectives  Spinal Cord   Dmax (point dose) < 50 Gy  

Dmax (0.03 cc) < 44-48 Gy  Lung   V20 < 30-35%  

V30 < 20-25%  Esophagus   V45 < 33%  Heart   V60 < 33%  Abbreviations: Dmax = maximum dose to organ in Gy;  V20 = percentage of organ volume receiving 20 Gy or more; V30 = percentage of organ volume receiving 30 Gy or more; V45 = percentage of organ volume receiving 45 Gy or more; V60 = percentage of organ volume receiving 60 Gy or more 

Table 2. Patient characteristics Patient No.  Age  TNM  PTV Volume (cc) 1      362.8 2      629.15 3      1087.87 4      916.54 5      724.13 6      582.27 7      1409.5 8      535.58 9      320.78 10      120.21 11      340.59 12      273.2 13      336.23 14      240.81 Abbreviations: TNM = T describes the size of the tumor, N describes spread of cancer to nearby lymph nodes, and M describes metastasis.  

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Table 3. Summary of treatment planning for VMAT and IMRT  Patient No. 

Prescription  TPS  VMAT, 6 MV  IMRT, 6 MV 

1  50.4Gy  Pinnacle     2  70 Gy  Pinnacle     3  60 Gy  Pinnacle     4  60 Gy  Pinnacle     5  61.2 Gy   Pinnacle  Arc A: 184-176 CW, coll. 15 

Arc B: 176-184 CCW, coll. 15 

Arc C: 186-22 CW, coll. 30 

Gantry: 180, 140, 20, 340, 300, 260, 220 

Coll. 0 

6  50 Gy  Pinnacle  Arc A: 184-176 CW, coll. 15 

Arc B: 176-184 CCW, coll. 15 

Gantry: 0, 40, 80, 120, 160, 200 

Coll. 0 7  45 Gy  Pinnacle  Arc A: 358-2 CW, coll. 190 

Arc B: 2-358 CCW, coll. 190 

Gantry: 0, 40, 80, 120, 160, 200 

Coll. 0 8  61.2 Gy  Pinnacle  Arc A: 182-178 CW, coll 10 

Arc B: 182-178 CW, coll 95 

Arc C: 178-182 CCW, coll 10 

Arc D: 178-182 CCW, coll 95 

Gantry: 0, 40, 80, 120, 160, 200, 240, 280, 320 

Coll. 0 

9  60 Gy  Pinnacle  Arc A: 242-2 CW, coll 190 

Arc B: 2-242 CCW, coll 190 

Gantry: 0, 40, 80, 120, 160, 200 

Coll. 0 10  60 Gy  Pinnacle  Arc A: 330-178 CW, coll 15 

Arc B: 178-330 CCW, coll 15 

Gantry: 0, 51, 103, 154, 206, 257, 309 

Coll. 0 11  50 Gy  Eclipse     12  50.4 Gy  Eclipse     13  50 Gy  Eclipse     14  60 Gy  Eclipse     Abbreviations: TPS = treatment planning system; VMAT = volumetric modulated arc therapy; IMRT = intensity-modulated radiation therapy; CW = clockwise; CCW = counter clockwise; coll = collimator 

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Table 4. Patient diagnosis frequency  Patient diagnosis  Number of patients Squamous cell carcinoma   4 Non-small cell lung cancer  4 Small cell lung cancer  1 Malignant neoplasm of the upper lung  3 Malignant neoplasm of the lower lung  2 

Table 5. Prescription dose frequency Prescription dose (cGy)  Number of patients 4500  1 5000  3 5040  2 6000  5 6120  2 7000  1