A Case Report on the Effect of Fan Beam Thickness in Helical Tomotherapy of Nasopharyngeal Carcinoma
Article Outline
Abstract
The fan beam thickness (FBT) in helical tomotherapy is defined by a pair of collimators parallel to the rotational orbit of the radiation beam and is fixed for a specific patient treatment. The aim of this case study is to evaluate the dosimetric influence of changing the FBT in the treatment of a nasopharyngeal carcinoma (NPC) patient. The subject was a T2N1M0 stage NPC patient. The planning target volumes (PTVs) of the primary nasopharyngeal tumor and the left and right cervical lymphatics were delineated along with the organs at risk (OARs) in the corresponding computed tomography slices. Three treatment plans with FBT of 1.0 cm, 2.5 cm, and 5.0 cm (FBT-10, FBT-25, and FBT-50) were generated separately based on similar dose constraints and planning parameters. The dosimetric results of the PTV and OARs were collected and compared among the 3 treatment plans. The differences in the dose parameters of the PTVs were small among the 3 plans. The FBT-10 plan demonstrated the most homogeneous PTV doses with the smallest homogeneity indices (HIs). The FBT-50 plan delivered the highest dose to the OARs and the FBT-10 plan delivered the lowest. The differences between the 2 plans were more significant in the spinal cord, optic chiasm, optic nerves, and lens. This case study demonstrated that the variation of FBT in tomotherapy affected the quality of the treatment plan mainly in the OAR doses, but not so much in the PTV. Increasing the FBT reduced the effectiveness in the sparing of OARs.
Key Words: Fan beam thickness, Helical tomotherapy, Nasopharyngeal carcinoma
Introduction
Helical tomotherapy is a specialized technique of delivering intensity-modulated radiotherapy (IMRT). The machine generates a modulatable fan beam from a 6-MV radiation source that is rotated around the patient in a helical manner.1, 2 The beam intensity is modulated by sliding the leaves into and out of the path of the fan beam across its width, at the same time as the radiation beam rotates round the patient and the treatment table advances toward the gantry along the patient's cranio-caudal direction. As well, helical tomotherapy is equipped with the megavoltage-computed tompgraphy (MVCT), which provides daily image guidance in treatment. This specialized technique has allowed precise IMRT and offered new treatment options in a great variety of challenging oncology cases in clinical routines.3, 4, 5
The commercially available TomoTherapy Hi-Art system (TomoTherapy, Inc., Madison, WI) produces a fan beam radiation using a 64-leaf collimator. Each leaf projects a shadow 0.625 mm wide at the isocenter. The width of the beam in the y-direction, the fan beam thickness (FBT), is defined by a pair of collimators parallel to the rotational orbit of the radiation beam and is fixed for a specific patient treatment. The dimension of FBT for a specific treatment can be selected between 1.0 cm, 2.5 cm, and 5.0 cm. It has been demonstrated that increasing the FBT reduces the overall irradiation time. For instance, a treatment time reduction of up to 61% was obtained by changing the FBT and/or lower modulation factor using standard pitch in the cranio-spinal axis irradiation.6 However, the potential trade-off is the degradation of the dose distribution to the target volume and organs at risk (OARs). Therefore, to achieve an optimum treatment in helical tomotherapy, a planner has to consider between treatment time and target volume dose distribution, which in turn will be dependent on the choice of FBT.
Nasopharyngeal carcinoma (NPC) is a prevalent disease in Southern China. The average annual incidences in Hong Kong from 1996–2007 were 20–25 (male) and 5–10 (female) per 100,000 persons.7 Because of its deep-seated anatomical location and close proximity to many critical normal structures, IMRT (including tomotherapy), which can provide highly conformal target dose with steep gradient at the target periphery, has been introduced in the treatment of this disease. Preliminary results showed that helical tomotherapy was able to improve the PTV dose coverage and better OAR sparing compared with the linac-based IMRT in the simultaneous integrated boost treatment of this disease.8
In the IMRT delivered by the helical tomotherapy, the planning target volume (PTV) of NPC includes the primary lesion at the nasopharynx and the bilateral cervical lymphatics down to around the level of supraclavicular nodes. The PTV usually follows an irregular inverted U shape, with lengths of >20 cm along the cranio-caudal direction. This relatively long and irregularly shaped target volume often creates conflict between the treatment time and target dose conformity, and a carefully selected FBT would be important to provide the optimal treatment.
The aim of this case study is to evaluate the dosimetric influence of changing the FBT in the IMRT of a NPC patient using helical tomotherapy. The results will provide references in selecting the optimal FBT in the treatment planning of NPC cases treated by tomotherapy.
Methods
A NPC patient staged T2N1M0 (AJCC 1997) treated by helical tomotherapy according to a routine protocol was retrospectively recruited for this study. The computed tomography (CT) images of the patient, which covered from the vertex down to the upper mediastinum at 3-mm interval, were taken and transferred to the TPS. Three PTVs were routinely defined, which included the primary nasopharyngeal tumor and the left and right cervical lymphatics. The OARs including the parotid gland, pituitary gland, spinal cord, brainstem, optic chiasm, optic nerve, and lens were contoured in the corresponding CT slices. Fifty-one point five (51.5) Gy was prescribed to 95% of the PTVs of the nasopharynx and cervical lymphatics (Phase I treatment). The standard plan, which was delivered to the patient, was planned at the tomotherapy planning workstation using FBT of 2.5 cm, with the routine pitch and modulation factors set at 0.3 and 2.0, respectively. After the calculation of the beamlets by the system, the plan optimization was performed based on the reference dose constraints and planning objectives of the target volumes and OARs, as shown in Table 1. The same patient was replanned by changing FBT to 1.0 cm and 5.0 cm, but keeping the same prescription, pitch, modulation factors and dose requirements of the OARs as the standard plan. The dose calculation grid for the 3 treatment plans were set at 0.25 cm. The plan reports including the DVHs of the 3 plans were printed and recorded. Apart from the DVH, the homogeneity index (HI) was used for the dose comparison. The HI, which indicated the dose uniformity within the PTV, was calculated by dividing the difference of the maximum and minimum doses by the median dose of the PTV. DVHs were also used to evaluate the doses to the OARs. For the serial organs such as the neurological structures, their maximum dose and D5, which was the dose received by the highest 5% volume of the OAR, were taken, whereas the maximum and median doses were used for the other OARs. To study the dosimetric influence of the variation of FBT, the dose results of the 3 treatment plans were compared and evaluated.
Table 1. The dose constraints and importance factors of the planning target volumes (PTVs) and organs at risk (OARs) of the tomotherapy plan delivered to the NPC patient (FBT = 2.5 cm)
| Maximum Dose (Gy) | Minimum Dose (Gy) | Importance Factor | Dose-Volume Requirement | ||
|---|---|---|---|---|---|
| % Volume | Dose (Gy) | ||||
| NP-PTV | 50.5 | 50.5 | 270 | 95 | 51.5 |
| LN-PTV | 50.0 | 50.0 | 90 | 95 | 50.0 |
| Brainstem | 32.0 | 90 | 5 | 25.0 | |
| Spinal cord | 28.0 | 90 | 8 | 20.0 | |
| Pituitary | 30.0 | 7 | 15 | 25.0 | |
| Optic chiasm | 30.0 | 90 | 7 | 27.0 | |
| L parotid | 50.0 | 25 | 15 | 15.0 | |
| L optic nerve | 20.0 | 16 | 10 | 15.0 | |
| R lens | 3.0 | 5 | 10 | 2.5 | |
Results
PTV
All of the 3 treatment plans—FBT-10, FBT-25, FBT-50 (generated with FBT set at 1.0 cm, 2.5 cm, and 5.0 cm, respectively)—fulfilled the prescribed dose requirement (95% volume of PTV received 51.5 Gy). The PTV dose in the 3 treatment plans did not show great difference. For instance, the differences in the dose parameters (maximum, minimum, and median) of the NP and cervical lymphatics PTVs were within 3.0 Gy (6% of the prescribed dose) (Figs. 1 and 2). Similar results were also shown in the PTV DVHs of the 3 treatment plans. The FBT-10 plan demonstrated the most homogeneous PTV doses with the smallest HI, followed by the FBT-50 plan (Fig. 3).
Fig. 1.
Comparison of DVH of NP PTV among the 3 treatment plans.
Fig. 2.
Comparison of DVH of neck lymphatics PTV among the 3 treatment plans.
Fig. 3.
Comparison of homogeneity index (HI) to the PTVs among the 3 treatment plans.
OAR
The FBT-10 plan was able to meet the dose requirements in most OARs, except for pituitary and parotid glands. Both the FBT-25 and FBT-50 plan had most of the OAR doses exceeding the dose requirements, with the latter showing greater magnitudes. For the dose parameters, the FBT-10 plan demonstrated the lowest dose values whereas those of the FBT-50 plan were the highest (Table 2). A similar pattern was observed from the DVHs of the OARs (Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10). The differences between the FBT-10 plan and FBT-50 plan were more significant (>40% in maximum dose) in the spinal cord, optic chiasm, optic nerves, and lens.
Table 2. Comparison of the dosimetric results of the organs at risk (OARs) among the 3 helical tomotherapy plans with different fan beam thickness
| Maximum Dose (Gy) | D5 (Gy) | Median Dose (Gy) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| FBT-10 | FBT-25 | FBT-50 | FBT-10 | FBT-25 | FBT-50 | FBT-10 | FBT-25 | FBT-50 | |
| Brainstem | 34.4 | 38.9 | 46.1 | 25.0 | 30.0 | 41.0 | |||
| Spinal cord | 26.4 | 30.1 | 37.7 | 20.1 | 25.0 | 34.5 | |||
| Pituitary gland | 38.2 | 40.7 | 45.2 | 32.0 | 36.3 | 41.4 | |||
| Optic chiasm | 20.7 | 32.6 | 42.2 | 17.5 | 30.0 | 38.5 | |||
| Parotid gland | 52.2 | 53.4 | 53.0 | 21.3 | 20.4 | 33.5 | |||
| Optic nerve | 13.1 | 24.8 | 38.3 | 12.8 | 24.5 | 37.8 | |||
| Lens | 2.4 | 6.0 | 12.6 | 2.2 | 5.0 | 11.8 | |||
Fig. 4.
Comparison of DVH of brainstem among the 3 treatment plans.
Fig. 5.
Comparison of DVH of spinal cord among the 3 treatment plans.
Fig. 6.
Comparison of DVH of pituitary gland among the 3 treatment plans.
Fig. 7.
Comparison of DVH of optic chiasm among the 3 treatment plans.
Fig. 8.
Comparison of DVH of parotid gland among the 3 treatment plans.
Fig. 9.
Comparison of DVH of optic nerve among the 3 treatment plans.
Fig. 10.
Comparison of DVH of lens among the 3 treatment plans.
Discussion
The highly irregular shape of the PTV and the close proximity of many OARs in NPC patients always pose a challenge to radiotherapy, even in IMRT treatment. Helical tomotherapy is capable of shaping the high-dose region around the target volume and producing dosimetric advantages over conventional radiotherapy,9 linac-based IMRT,10, 11 and stereotactic radiosurgery (SRS),12 and is therefore expected to be useful in treating NPC. By following the routine protocol, the current study revealed that the influence of FBT variation did not produce obvious effect to the dose distribution of PTV. The 3 treatment plans could largely fulfill the PTV dose requirements with small differences among them. This uncovered the fact that the 5.0-cm FBT was capable of achieving a reasonable dose to the PTV. The main reason for the small difference was because much higher importance weights were applied on the PTVs dose requirements relative to the OARs during the plan optimization process. As a result, the PTV received doses close to the dose requirements in all treatment plans. However, PTV dose homogeneity did not demonstrate a definite relationship with the FBT because the FBT-25 plan gave the least homogeneous PTV dose among the 3 plans.
A consistent pattern was observed in the OAR dose differences among the 3 treatment plans. The smallest FBT (FBT-10 plan) provided the best sparing of the OARs, whereas the poorest sparing was found in the largest FBT. The results illustrated that the thickest FBT was not able to fulfill all dose requirements. It was expected that it would be more difficult for larger FBT to provide fine dose distribution compared with the smaller FBT. As a result, to achieve the dose objectives of the PTV, the doses to the OARs were compromised. Many of the doses to the OARs in the FBT-50 plan were not acceptable. For instance, the D5 to the spinal cord and brainstem approached 70% of the prescribed dose. Also, the doses to the optical organs, including the lens, optic nerve, and optic chiasm, also significantly exceeded their dose constraints, and were not acceptable. Comparing between the FBT-10 and FBT-25 plans, the former plan was superior in sparing the OARs and their differences were obvious except for the parotid glands.
Purely from the dosimetry point, smaller FBT was preferred because it would improve the PTV dose homogeneity and better spare the OARs, which is often a challenge to NPC cases because there are many OARs in close proximity to the PTV. The trade-off would be the increase of treatment time, which could be significant for long PTV as in the NPC cases. As for this NPC patient with stage T2N1, the PTV was moderate in size and was at reasonable distances from the OARs, so the dosimetric outcome of the FBT-25 plan was acceptable. It is recommended that the 1.0-cm FBT would be required for the more extensive tumor with close proximity with many OARs, so as to provide effective sparing of the OARs. On the other hand, the 5.0-cm FBT was not recommended for NPC because of its poor sparing of OARs.
Conclusion
This case study demonstrated that the variation of FBT in tomotherapy affected the quality of the treatment plan mainly in the OAR doses, but not so much in that of the PTV. Increasing the FBT reduced the effectiveness in OAR sparing. The treatment plan generated from the 2.5-cm beamwidth was acceptable for the selected case. It was recommended that more advanced tumor involvements with OARs in close proximity to the PTV would require FBT of 1.0 cm.
Acknowledgment
This work was supported by a research grant from Hong Kong Polytechnic University.
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PII: S0958-3947(09)00130-7
doi:10.1016/j.meddos.2009.11.004
© 2011 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights reserved.
