Physics Contribution
Do We Need Daily Image-Guided Radiotherapy by Megavoltage Computed Tomography in Head and Neck Helical Tomotherapy? The Actual Delivered Dose to the Spinal Cord

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Purpose

To quantify the actual delivered dose to the cervical spinal cord with different image-guided radiotherapy (IGRT) approaches during head and neck (HN) cancer helical tomotherapy.

Methods and Materials

Twenty HN patients (HNpts) treated with bilateral nodal irradiation were analyzed. Daily megavoltage computed tomography MVCT) scans were performed for setup purposes. The maximum dose on the planning CT scan (plan-Dmax) and the magnitude and localization of the actual delivered Dmax (a-Dmax) were analyzed for four scenarios: daily image-guided radiotherapy (dIGRT), twice weekly IGRT (2×WkIGRT), once weekly IGRT (1×WkIGRT), and no IGRT at all (non-IGRT). The spinal cord was recontoured on 236 MVCTs for each scenario (total, 944 fractions), and the delivered dose was recalculated for each fraction (fx) separately.

Results

Fifty-one percent of the analyzed fx for dIGRT, 56% of the analyzed fx for the 2×WkIGRT, 62% of the analyzed fx for the 1×WkIGRT, and 63% of the analyzed fx for the non-IGRT scenarios received a higher a-Dmax than the plan-Dmax. The median increase of dose in these fx was 3.3% more for dIGRT, 5.8% more for 2×WkIGRT, 10.0% more for 1×WkIGRT, and 9.5% more for non-IGRT than the plan-Dmax. The median spinal cord volumes receiving a higher dose than the plan-Dmax were 0.02 cm3 for dIGRT, 0.11 cm3 for 2×WkIGRT, 0.31 cm3 for 1×WkIGRT, and 0.22 cm3 for non-IGRT. Differences between the dIGRT and all other scenarios were statistically significant (p < 0.05).

Conclusions

Compared to the Dmax of the initial plan, daily IGRT had the smallest increase in dose. Furthermore, daily IGRT had the lowest proportion of fractions and the smallest volumes affected by a dose that was higher than the planned dose. For patients treated with doses close to the tolerance dose of the spinal cord, we recommend daily IGRT. For all other cases, twice weekly IGRT is sufficient.

Introduction

Many head and neck cancer patients (HNpts) experience anatomical changes throughout the course of radiation therapy due to tumor and soft tissue shrinkage and body weight loss. These have an impact on patient setup and delivered dose (1). As intensity-modulated radiotherapy (IMRT) provides a steep dose falloff to normal tissue and organs at risk, the accuracy of the daily setup of patients is essential. This is especially true with regard to the spinal cord, where exceeding the tolerance dose could be extremely harmful. Several studies of the changes in delivered doses to the spinal cord during fractionated radiotherapy due to soft tissue changes are available. However their findings are not consistent: either no changes or increases up to 15 Gy were reported 2, 3, 4, 5. Furthermore, these studies were carried out with new planning kV computed tomography (kVCT), performed at predefined times. These kVCTs were rigidly registered by bony landmarks to the initial planning kVCT and did not take the actual position of the patient on the treatment table into account. Thus, they do not answer the question of how much actual dose was delivered to the spinal cord, as delivered in the treatment position. Knowledge of the actual delivered dose and recommendations for image-guided RT (IGRT)-based treatments are extremely important, as two trends can be observed currently in IMRT and IGRT for HNpts: dose escalation (e.g., by simultaneous integrated boost) and reirradiation.

This work focused on radiotherapy to the spinal cord only. We investigated the impact of setup uncertainties and morphological changes on the delivered dose to the spinal cord throughout the course of head and neck tomotherapy, with daily, twice weekly, or once weekly image-guidance by megavoltage CT (MVCTs) scans and without any image guidance at all. Furthermore, this study addressed additional open questions regarding the actual delivered dose to the spinal cord during fractionated head and neck IGRT: for example, is every fraction affected by an increase in dose compared to the initial plan; is the volume affected by a dose increase different from approach to approach; and is there a trend regarding the affected regions?

Section snippets

Patient characteristics

We analyzed 20 HNpts treated with helical tomotherapy (HT) (TomoTherapy Inc., Madison, WI) at our institution. The cohort consisted of 10 nasopharynx cancer patients who received definitive radiotherapy and 10 tonsil cancer patients who underwent postoperative radiotherapy. Sixteen patients underwent concomitant platinum-based chemoradiotherapy. Table 1 shows the patients’ characteristics.

Treatment planning and delivery

We previously described treatment planning and delivery of HT for HNpts treated in our clinic (1). Briefly,

Results

The absolute overall setup errors of the analyzed directions and the 3D vector absolute setup error are presented in Table 2. Assessment of setup errors when setup direction error was taken into account revealed no statistically significant differences to the zero position for longitudinal (mean difference, 0.13 mm; 95% CI, −0.57 to 0.83; p = 0.714), vertical (mean difference, 0.36 mm; 95% CI, −0.41 to 1.14; p = 0.359), and lateral (mean difference, −0.75mm; 95% CI, −0.65 to 0.50; p = 0.800)

Discussion

In our study, the overall magnitude of the 3D vector and the magnitude of translational and rotational setup errors were within previously reported tomotherapy setup errors 9, 10, 11. No systematic translational setup errors were observed. However, we detected a small (<1°) systematic clockwise rotation error. This can be explained by the standardized procedure of setup of patients in our clinic: the setup is performed by a single technician, by fastening the mask of the patient first on the

Conclusions

The Dmax delivered to the myelon during fractionated radiotherapy of HNpts is different than the planned dose. The proportion of fractions affected by a higher-than-planned dose, the magnitude of the dose increase, and the volumes of the myelon affected by a higher-than-planned dose were less pronounced for dIGRT. We strongly recommend daily IGRT if patients are treated with doses close to the known tolerance dose of the spinal cord. For all other cases, twice weekly IGRT is sufficient.

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Conflict of interest: none.

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