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ORIGINAL ARTICLE
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Year : 2020  |  Volume : 57  |  Issue : 4  |  Page : 388--392

Is 9-field IMRT superior to 7-field IMRT in the treatment of nasopharyngeal carcinoma?

Mohamed S Ibrahim1, Ehab M Attalla2, Mostafa El Naggar3, Wael M Elshemey4,  
1 Department of Radiation Physics, Oncology and Hematology Hospital, Maadi Armed Forces Medical Compound, Cairo, Egypt
2 Department of Radiotherapy, National Cancer Institute, Cairo University, Giza, Egypt
3 Department of Oncology, Medical Research Institute, Alexandria University, Giza, Egypt
4 Department of Biophysics, Faculty of Science, Cairo University, Giza, Egypt

Correspondence Address:
Mohamed S Ibrahim
Department of Radiation Physics, Oncology and Hematology Hospital, Maadi Armed Forces Medical Compound, Cairo
Egypt

Abstract

Background: To evaluate the pros and cons of 9-field intensity modulated radiotherapy (IMRT) compared to 7-field IMRT in the treatment of nasopharyngeal carcinoma (NPC). Methods: Ten NPC patients were treated with 7F-IMRT and 9F-IMRT. A dose prescription of 70 Gy was delivered in 35 fractions to gross planning target volume (PTV1). Plan verification was performed via 2D-array and film dosimetry. Dose-Volume Histogram (DVH) parameters were used to evaluate the quality of IMRT plans. Results: Dose data for the investigated planning techniques obeyed the Radiation Therapy Oncology Group (RTOG) protocol no. 0615. The dose delivered to PTV1 and organs-at-risk (OARs) for 9F-IMRT was significantly better than 7F-IMRT, except for OARs which were at a distance from PTV1, such as eyes, optical nerves, and chiasma. Ninety five percent of PTV1 was covered by more than 95% of the prescribed dose (67.75 ± 1.1 Gy and 68.57 ± 1.2 Gy for 7F-IMRT and 9F-IMRT, respectively). The maximum dose to 1% of brainstem was 50.06 ± 2.7 Gy and 47.75 ± 2.6 Gy for 7F-IMRT and 9F-IMRT, respectively. Dose verification showed good agreement with treatment planning system with a maximum deviation for 2D-array of 2.16% ± 0.86 and 1.73% ± 0.33 for 7F-IMRT and 9F-IMRT, respectively. Similarly, radiochromic film reported maximum dose deviations of 3.38% ± 1.68 and 2.77% ± 1.3, respectively. Conclusion: 9F-IMRT provides better homogenous dose to PTV1 and more sparing of OARs over 7F-IMRT for NPC patients, except for OARs which are are a distance from PTV1.



How to cite this article:
Ibrahim MS, Attalla EM, El Naggar M, Elshemey WM. Is 9-field IMRT superior to 7-field IMRT in the treatment of nasopharyngeal carcinoma?.Indian J Cancer 2020;57:388-392


How to cite this URL:
Ibrahim MS, Attalla EM, El Naggar M, Elshemey WM. Is 9-field IMRT superior to 7-field IMRT in the treatment of nasopharyngeal carcinoma?. Indian J Cancer [serial online] 2020 [cited 2020 Dec 2 ];57:388-392
Available from: https://www.indianjcancer.com/text.asp?2020/57/4/388/297025


Full Text



 Introduction



Nasopharyngeal carcinoma (NPC) is a not uncommon malignant radiosensitive tumor of the nasopharynx that has been documented all over the world. The main treatment of NPC is radiotherapy which offers a means to control the disease through delivery of adequate radiation dose confined mainly to target volume.[1]

Intensity modulated radiotherapy (IMRT) is one of the current radiation therapy modalities that is used in the treatment of NPC. It has the advantage of delivering a highly precise and conformed dose to planning target volume (PTV) using nonuniform radiation beamlet intensities offered by multileaf collimator (MLC).[2] The MLC is either static or dynamic and it helps to spare organs-at-risk (OARs) while delivering the prescribed dose to target volume.[3],[4],[5] The therapeutic index is thus increased, while acute and late morbidity reduced.[6],[7] Moreover, IMRT has been reported to significantly improve the quality of life parameters including sticky saliva, dry mouth, and swallowing.[8]

Increasing the number of fields in an IMRT plan is expected to enhance the dose homogeneity in PTV. Therefore, 7-field and 9-field IMRT are in routine use in current radiotherapy workflow. Dose–Volume–Histogram (DVH) parameters (such as mean dose, Dmean, maximum dose, Dmax, Dose to 95% of target volume, D95%, Dose to 50% of target volume, D50%, and doses to 5% and 1% of target volume, D5% and D1%, respectively) are reliable parameters for the evaluation of plan quality of 7-field IMRT compared to 9-field IMRT.[9]

Quality assurance (QA) tests are essential to ensure accurate patient treatment. QA includes measuring beam flatness and stability in addition to evaluating the accuracy of MLC leaf positions and ensuring accurate modelling of the linear accelerator at the commissioning stage of the treatment planning system (TPS). Verification procedures for 7F-IMRT and 9F-IMRT are carried out for each patient using radiation dosimeters such as 2D-array and radiochromic films.[10]

In this work, the performance of 7F-IMRT and 9F-IMRT in the treatment of NPC are compared with special emphasis on elucidating the best in dose homogeneity to PTV and minimum dose to OARs. The pros and cons of both techniques are extensively discussed.

 Materials and Methods



Planning systems and radiotherapy machine

Computerized Medical Systems (CMS) Inc.'s (St. Louis, MO) Xio three-dimensional TPS (software release 4.64) was used in this work. It utilizes pencil beam calculation algorithm in the calculation of dose. A Primus K, (Siemens, Munich, Bayern, Germany) linear accelerator was also used. It has dual energies of 6 and 15 MV and operates up to 300 MU/minute. It comes with a 58-leaf, 3D MLC with two parallel sets of 29 independent leaves each at the isocenter. The outer most leaf pairs width measure 0.5 cm each, while the inner 27 leaf pairs width measure 1.0 cm each. The maximum over travel is 10 cm over the beam central axis for each leaf. The minimum leaf gap is 0.0 cm. A leaf position accurate to within 2 mm is used in treatment.

Patient and staging evaluation

This was a retrospective study (Ayadi El-Mostabal hospital, Alexandria, Egypt) where ten NPC patients' computed tomography (CT) scans were imported to the TPS. New treatment plans were made as per the CT reports of each patient, using 7F-IMRT and 9F-IMRT techniques and exported to a phantom to measure the dose distribution.

Patients were diagnosed with cancer of tumor sizes 1 and 2 (T1,2), negative lymph nodes (No) and no metastasis Mo, i.e., T1,2 No Mo stage.

Computed tomography and target volumes

Patients were immobilized in the supine position using head and neck thermoplastic mask. CT was carried out using a slice spacing of 1.25 mm for all patients. Scan limits were defined from the region of vertex to 5 cm inferior to the clavicular heads. CT images were exported electronically to TPS.

The clinical target volume (CTV) was defined as the gross target volume (GTV) plus a margin of 5 mm in all directions.[11] Delineation of GTV, CTV, and organs-at-risk (OARs) was performed on a number of treatment planning CT images. The planning target volume (PTV) is defined as the CTV plus a margin of 5 mm in all directions. PTV consists of primary tumor only (PTV1) and bilateral neck lymph node (PTV2) treated with anterior–posterior conformal fields. All fields are half-beam blocked to the central axis to avoid overlapping dose. Radiation Therapy Oncology Group (RTOG) protocol 0615 was used as the reference for all volume definitions and planning evaluations.[12],[13]

Dose prescription

For all treatment planning techniques, the dose prescriptions were 70 Gy in 35 fractions, to PTV1. The treatment was delivered once daily, 5 fractions per week, over 7 weeks. All targets were treated in the same time.[12],[13],[14]

Inverse-planned IMRT

IMRT plan was performed using 6 MV photons with 7 to 9 equally spaced coplanar beams using commercial inverse planning software. The beams were spread around the target at equal spaces. To avoid opposing fields, odd numbers were used for the treatment fields.

Inverse-planned 7F-IMRT and 9F-IMRT

Seven fields were shaped at the beam's eye view to conform to the shape of PTVs using MLCs at gantry angles of 30°, 80°, 130°, 180°, 230°, 280°, and 320° while nine fields are shaped at the beam's eye view to conform to the shape of PTVs using MLCs at gantry angles of 20°, 60°, 100°, 140°, 180°, 220°, 260°, 300°, and 340°.

Treatment planning evaluation tools

Qualitative evaluation of treatment plans were carried out using visual slice-by-slice review with the help of isodose line distribution. This was essential to detect the sites of hot and cold spots in treatment plans. It includes Dmean, Dmax, D95%, D50%, D5%, and D1% in addition to DVHs.

DVHs of PTV1 and OARs were used for plan evaluations and comparisons. All plans were normalized based on the DVHs to ensure that 95% of PTVs received 100% of prescribed dose. The dose inhomogeneity (DI) in PTVs was given by (D5%− D95%)/Dmean.[15] For OARs, the mean and maximum doses for brain stem, optic nerves, and eyes were used for the evaluation of treatment plan.

Treatment plans were considered acceptable if they fulfilled the dose recommendations by the RTOG protocol 0615 for NPC patients.[12],[13]

Plan verification

IMRT treatment plans are known by their large number of treatment fields divided into many segments. This prevents any possibility to carry out manual calculation of monitor units (MUs) as a quality assurance for the treatment plan calculation. The phantom substitution method is commonly used because verification of dose distributions in a real patient is impossible.[16]

For MU/segment evaluation, quality assurance for calculated IMRT treatment plans was done by exporting the plans to the phantom. A defined point dose and the dose distribution in phantom were measured and compared to the corresponding treatment planning calculations using gamma index with individual acceptance criteria of 3% dose difference (DD) and 3 mm distance-to-agreement (DTA).[17]

Water equivalent phantom, PTW RW3, and 0.6-cm3 Farmer-type ionization chamber were utilized for point dose measurements. A 2D-array (model Seven29TM, PTW, Freiburg, Germany) was employed to measure dose distribution.[18] Point dose and dose distribution measurements were carried out at the machine isocenter, at a depth of 15 cm. Patients' plans were transferred to the CT images of the 2D-array to verify the doses calculated by the TPS for IMRT plans.

On the other hand, patients' plans were transferred to CT images of an Alderson Rando anthropomorphic phantom (PTW, Freiburg, Germany) and another group of plans were compared to film measurements in the Alderson phantom. Extended dose range (EDR2, Gammex rmi, Middleton, WI) film was inserted axially into the Alderson phantom at the primary tumor region for 2D dose measurements. Measured and calculated dose planes were imported and compared using Varisoft software (version 3.1, PTW, Freiburg, Germany). The gamma index criterion was used to evaluate the accuracy of TPS calculations with individual acceptance criteria of 3% DD and 3 mm DTA.[19]

Prior to the delivery of investigated plans, a radiation oncologist reviewed the individual plans, slice-by-slice, and approved it after checking all biological parameters. Also, routine machine output measurements were performed for 6 MV photon beam in order to set the dosimetric base line for ion chamber and film measurements. Film calibration was carried out using five Kodak EDR2 films and 30 × 30 × 30 cm3 solid water phantom (PTW-29672, Freiburg, Germany).[20],[21] Quantitative analysis of dose distribution was done to compare the measured and calculated dose distributions based on gamma values to determine the percentage pixels in the scanned areas that go beyond the acceptance criteria (percentage failed pixels).

Integral dose (ID) definition

Integral Dose is the total energy absorbed by the body and is computed based on the average organ density, dose, and volume as defined in the following equation:[22]

Integral Dose = D ρ V (Gy Kg)[22]

Where D is the average organ dose, ρ is the average organ density, and V is the organ volume.

It is often stated that the large number of beamlets and monitor units used in IMRT leads to an increase in ID.[23]

This work was been carried out at Ayadi Almostakbal Hospital, Alexandria, Egypt, and approved by its Institutional Review Board (IRB).

 Results



Dose analysis of PTV1

In terms of dose coverage to PTV1, all techniques achieved the constraint that 95% of the volume was covered by more than 95% of the prescribed dose. [Table 1] shows that, for PTV1, despite respecting the RTOG constrains, all investigated 7F-IMRT dosimetric parameters were significantly (P < 0.05) inferior compared to 9F-IMRT. The DI in PTV1 for 7F-IMRT and 9F-IMRT were 0.09 ± 0.005 and 0.06 ± 0.007, respectively. These values all indicate a satisfactory degree of similarity between the doses that covers 95% and 5% of the volume and respect the RTOG constrains as well.{Table 1}

Dose analysis of OARs

Brain stem and spinal cord

[Table 2] lists Dmax and D1% values for brain stem and spinal cord. Despite the fact that all results agreed with the constrains of RTOG 0615 protocol, brain stem and spinal cord showed statistically significant (P < 0.05) sparing in case of 7F-IMRT compared to 9F-IMRT. Moreover, 9F-IMRT provided better sparing than 7F-IMRT.{Table 2}

Left and Right parotids

The Dmean and D50% values for left and right parotids are given in [Table 2]. One can notice that for left and right parotids, there were statistically significant (P < 0.05) differences between 7F-IMRT and 9F-IMRT for Dmean and D50%. All results agree with the constraints of RTOG 0615 protocol and 9F-IMRT provided the best sparing.

Optical chiasma

For optical chiasma, despite respecting the RTOG constrains, the Dmax values for 9F-IMRT dosimetric parameters were significantly (P < 0.05) inferior compared to 7F-IMRT. The 7F-IMRT shows the best sparing for optical chiasma.

Left and right optical nerve and left and right eye

Dose data for these OARs all respected the RTOG 0615 protocol. The 7F-IMRT showed the best sparing, where its dosimetric parameters were significantly (P < 0.05) superior compared to 9F-IMRT.

Delivery time and number of monitor units (MUs) per fraction

As shown in [Table 3], 7F-IMRT technique had the advantage of having the shortest delivery time compared to 9F-IMRT technique. This was also reflected in having the lowest number of MUs.{Table 3}

Integral dose (ID) evaluation

To calculate the ID to the normal tissues outside the target volumes, all of the target volumes were subtracted from the body volume (body minus target volumes) and referred to as the Normal Healthy Tissue (NHT).[24] By comparing the ID for 7F-IMRT to that of 9F-IMRT techniques it was found statistically significant (P < 0.05), since 9F-IMRT was higher than 7F-IMRT by 1.26%. This agreed with the large number of beamlets and monitor units used in IMRT which led to an increase in ID.[23]

 Discussion



The effect of number of treatment fields on plan quality is remarkable. As the number of beams increases, isodoses conform more tightly to PTV1 with improvement in target dose homogeneity.[25],[26] For PTV1, 9F-IMRT improved Dmean compared to 7F-IMRT, to be 99% instead of 97.9%. Also, Dmax was improved to be 107.5% instead of 109.5%; therefore, no hot spots were reported [Table 1].

Using gamma index for MU/segment evaluation, the average percent failed pixels for PTV1 level were 3.06% and 2.98% for 7F-IMRT and 9F-IMRT plans, respectively.

The DI in PTV1 was better with increase in the number of treatment fields,[25],[26] where DI were 0.06 and 0.09 for 9F-IMRT and 7F-IMRT, respectively [Table 1].

For OARs nearby PTV1 (like parotids, brain stem, and spinal cord), as the number of beams increases, better sparing is achieved. This is due to the reduction in the dose delivered to OARs by each beam. On the other hand, for OARs which are distant from PTV1 and nearby skin (like eyes, optic nerves, and chiasma), better sparing is achieved with decrease in the number of beams. This is due to the increase in surface dose with increase in treatment fields.[25],[26]

Plan verification methods all ensured that for both techniques (7F-IMRT and 9F-IMRT) measured and calculated dose plans were compatible and respected the gamma index criteria and agreed with RTOG 0615 protocol.

The advantages of using 9F-IMRT lied in achieving maximum homogeneous dose to PTV1 and minimum required dose to nearby OARs. On the other hand, the disadvantage was in delivering higher dose (compared to 7F-IMRT) to OARs which were at a distance from PTV1. Nevertheless, it still respected the RTOG 0615 constraints.

 Conclusion



The number of treatment fields had a remarkable effect on IMRT plan quality. The 9F-IMRT technique provided a sufficiently homogenous dose to PTV1 and pronounced sparing of nearby OARs compared to 7F-IMRT for NPC patients. On the other hand, for OARs which were at a distance from PTV1 and nearby skin, 7F-IMRT was superior to 9F-IMRT. All dose data obtained by both techniques for PTV1 and OARs agreed with RTOG 0615 protocol and therefore are practically applicable.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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