|Year : 2010 | Volume
| Issue : 4 | Page : 585-587
Integral dose to the carotid artery in intensity modulated radiotherapy of carcinoma nasopharynx: Extended field IMRT versus split-field IMRT
A Bahl1, K.S.J. Basu1, DN Sharma1, GK Rath1, PK Julka1, S Thulkar2
1 Department of Radiation Oncology, Delhi State Cancer Institute, New Delhi, India
2 Department of Radiotherapy and Radiology, All India Institute of Medical Sciences, New Delhi, India
|Date of Web Publication||24-Feb-2011|
Assistant Professor, Clinical Oncology, Delhi State Cancer Institute, New Delhi
Source of Support: None, Conflict of Interest: None
Aim : To evaluate the integral dose to carotid vessels in extended field intensity modulated radiotherapy (IMRT) (including the lower neck nodes in IMRT field) and split field IMRT (using separate single anterior field to treat lower neck nodes) in cancer nasopharynx.
Materials and Methods : Dosimetric data from 10 patients of carcinoma nasopharynx, undergoing IMRT, were evaluated in this prospective study. The carotid vessels were contoured from sternoclavicular joints upto the base of skull. IMRT plans were generated for all patients with extended field and split field IMRT techniques using nine coplanar beams with 6 MV photons. A dose of 70 Gy to planning target volume (PTV) 70Gy , 59.4 Gy to PTV 59.4Gy and 54 Gy to PTV 54Gy was delivered in 33 fractions. The dose constraints were similar for both the techniques. The integral dose to the carotid arteries was calculated as the mean dose times the volume (mean dose Χ volume) in units of liter-gray.
Results : The mean dose to the carotid vessels in the extended field IMRT was 63.88 ± 0.97 Gy (mean dose ± SD) and it was 64.43 ± 0.73 Gy for the split field technique. The integral dose in the extended field versus split field technique was 0.29 ± 0.0207 and 0.32 ± 0.0213 liter-gray, respectively. The difference was statistically significant (P < 0.013).
Conclusions : Extended field IMRT delivers a slightly lower integral dose to carotid arteries in treatment of cancer nasopharynx while maintaining good dose homogeneity to the PTV 54Gy and can be preferred over split field radiotherapy.
Keywords: Carotid artery, intensity modulated radiotherapy, nasopharynx
|How to cite this article:|
Bahl A, Basu K, Sharma D N, Rath G K, Julka P K, Thulkar S. Integral dose to the carotid artery in intensity modulated radiotherapy of carcinoma nasopharynx: Extended field IMRT versus split-field IMRT. J Can Res Ther 2010;6:585-7
|How to cite this URL:|
Bahl A, Basu K, Sharma D N, Rath G K, Julka P K, Thulkar S. Integral dose to the carotid artery in intensity modulated radiotherapy of carcinoma nasopharynx: Extended field IMRT versus split-field IMRT. J Can Res Ther [serial online] 2010 [cited 2020 Jul 9];6:585-7. Available from: http://www.cancerjournal.net/text.asp?2010/6/4/585/77089
| > Introduction|| |
Intensity modulated radiotherapy (IMRT) is an established treatment modality in the management of head and neck cancers. , IMRT is based on the principle of treating the tumor area while sparing the surrounding normal structures. A lot of attention is focused on contouring and sparing normal structures like paortid glands, spinal cord, eyes, brainstem, temperomandiular joints, optic nerves, etc. , Vessels, by virtue of their proximity to the lymph nodes, are included in the high-dose gross tumor volume (GTV) or clinical target volume (CTV) when treating the neck, and receive a high dose of radiation. Recent studies are now stressing on identifying and sparing the carotid arteries while doing head and neck IMRT. , Dose received by the carotid artery can be restricted using IMRT compared to that using conventional techniques, with an aim to reduce long-term morbidity of carotid artery stenosis. IMRT in head and neck can be delivered by extended field IMRT (including the lower neck nodes in IMRT field) versus split field IMRT (using separate single anterior field to treat lower neck nodes). Our study was designed to compare the carotid artery doses while using these two different treatment strategies of extended field IMRT versus split field IMRT in carcinoma naspharynx patients treated with radical intent.
| > Materials and Methods|| |
Dosimetric data from 10 adult patients of carcinoma nasopharynx (T1-2, N0-2, M0) were evaluated in this prospective study. The inclusion criteria were: (1) patients with biopsy proven nasopharangeal cancers; (2) patients planned for definitive radiotherapy; (3) patients willing to give informed consent and participate in follow-up. Exclusion criteria included (1) previously irradiated cases and (2) patients younger than 14 years.
Contrast-enhanced computed tomography (CT) scans of patients were obtained with a slice thickness of 2.5 mm with thermoplastic cast immobilization in place. Contouring was done as per the Radiation Therapy Oncology Group consensus guidelines. , The carotid vessels were contoured from sternoclavicular joints upto base of the skull and included the common carotid, right and left carotid vessels on both sides of neck. IMRT plans were generated for all patients with extended field and split field IMRT techniques using nine co-planar beams with 6 MV photons. A margin of 2 mm was added to the GTV [planning target volume (PTV) 70Gy ] and 5 mm for CTV 59.4Gy (PTV 59.4Gy ) and CTV 54Gy (PTV 54Gy ). A dose of 70 Gy to PTV 70Gy , 59.4 Gy to PTV 59.4Gy and 54 Gy to PTV 54Gy was delivered in 33 fractions. For the split field techniques, isocenter was placed at the junction of PTV 59.4Gy and PTV 54Gy and the jaws were adjusted in the beam's eye view to cover PTV 70Gy and PTV 59.4Gy while blocking the lower portion of the field at isocenter in order to match the lower anterior field with the upper IMRT fields. In the optimization process, fixed-jaws option available in Eclipse treatment planning system version 6.5 (Varian, Palo Alto, CA, USA) was used to avoid automatic jaws adjustment usually done in IMRT planning. The lower anterior field used in the split field technique to treat PTV 54Gy was normalized in the center of the PTV and 90% isodose line was selected for prescription. The dose constraints were similar for both the techniques [Table 1]. The integral dose of the carotid arteries was calculated as the mean dose times of volume (mean dose × volume) in units of liter-gray. Data were analyzed using SPSS 16.0. Statistical analyses were performed using a paired two-tailed Student t test to determine whether there was any statistically significant difference in any of the parameters examined. Differences were considered statistically significant with P ≤ 0.05.
| > Results|| |
The mean dose to the carotid vessels in the extended field IMRT was 63.88 ± 0.97 Gy (mean dose ± SD) and it was 64.43 ± 0.73 Gy for split field technique. The integral dose in the extended field versus split field technique was 0.29 ± 0.0207 and 0.32 ± 0.0213 liter-gray, respectively [Figure 1]. A percent difference of +9% was observed in the integral dose of carotid arteries in split field technique. The difference was statistically significant with a "P" value of 0.01337. No significant differences were observed in the dose to critical structures and target volumes except for PTV 59.4Gy where the coverage and dose uniformity was slightly lower than in extended field IMRT.
|Figure 1: Integral dose profile of carotid arteries in patients under study|
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| > Discussion|| |
IMRT in head and neck can be done using extended field, with the entire neck being included in the treatment as is our instituional policy, or using split field technique where a separate anterior field is used to treat the supraclavicular area. Carotid artery atherosclerosis and stenosis accompanies head and neck irradiation , and can lead to an increased risk of stroke. Using conventional radiotherapy techniques, a 12% risk of stroke in 15 years has been reported.  A recent study showed that the median mean dose to the carotid arteries in carcinoma naspharynx patients was 65.7 Gy with IMRT versus 58.4 Gy with 3-field conformal radiotherapy (P < 0.001). However, with the application of dose constraints to the carotid arteries, it could be reduced to 54 Gy.  A 11-29% incidence of second malignacies has been reported in head and neck cancer patients. , In early glottic cancers, these second primaries are reported to be a leading cause of death.  Re-irradiating such patients needs a thorough evaluation of doses already received by critical structures and further stresses on the need for a dose reduction to all critical structures. Vitolo et al. reported a higher dose to carotids with IMRT compared to conformal radiotherapy. However, with re-planning using carotid constraints, they were able to demonstrate a significant reduction in carotid doses (65 Gy versus 54 Gy). Rosenthal et al.  reported median carotid V35, V50, and V63 doses of 100, 100, and 69.0%, respectively, in T1-2 glottic cancers treated by conventional planning. These reduced to 2, 0, and 0%, respectively (P < 0.01) with IMRT using carotid sparing. The results of our study show a reduction in both the maximum dose and integral dose to carotid artery using extended field radiotherapy. To the best of our knowledge, no such comparison has been reported so far. Our preliminary results show extended field IMRT to be marginally better than split field IMRT. However, there are no data to suggest that such a reduction will positively impact the rate of carotid artery injury and its subsequent effect on patients. A dose-injury effect on carotids has also not been demonstrated. There are studies showing no difference in the rate of carotid injury with doses used in head and neck treatments compared to lower doses.  Extended field IMRT can be delivered faster than split field technique and with lesser effort. Extended IMRT can deliver a more uniform dose to the lower neck region, which is not possible with the split field technique due to highly irregular surface area. However, long-term implication on treatment outcome needs to be studied. The impact of such an intervention on rate of carotid stenosis will also require a longer follow-up and a larger study. A judicious selection of cases, particularly those with minimal involvement of lower neck, may benefit from such an endeavor.
| > Conclusions|| |
Extended field IMRT delivers a lower integral dose to carotid arteries in treatment of cancer nasopharynx while maintaining good dose homogeneity to the PTV 54Gy and can be preferred over split field IMRT. Its long-term implications on the rate of carotid stenosis need to be studied.
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