Risk assessment of radiation-induced malignancies based on whole-body equivalent dose estimates for IMRT treatment in the head and neck region

doi:10.1016/S0167-8140(99)00079-1

Radiotherapy and Oncology

Volume 53, Issue 3, 1 December 1999, Pages 199-203

https://doi.org/10.1016/S0167-8140(99)00079-1 Get rights and content

Abstract

Background and purpose: Intensity modulated radiation therapy (IMRT) has been introduced in our department for treatment of the head and neck region with the intention of reducing complications without compromising treatment outcome. However, these new treatment modalities inevitably require a substantial increase in monitor units per target dose yielding an increased risk of secondary malignancies induced by the treatment. This study aims at assessing the increased risk by means of in vivo measurements of the whole-body equivalent dose of both the conventional and the IMRT treatment techniques for head and neck lesions.

Material and methods: A conventional technique using parallel opposed, wedged treatment fields has been compared with a slice-by-slice arc rotation technique for IMRT. Both techniques were used to treat head and neck lesions with a 6-MV photon beam. Thermoluminescent badges and neutron bubble detectors designed for personnel monitoring have been applied to obtain the estimated whole-body equivalent dose on three patients for each treatment technique. The nominal probability coefficient for a lifetime risk of excess fatal cancer, recommended by the ICRP 60 has been used for risk estimates based on the estimated dose values.

Results: An estimated whole-body equivalent dose per monitor unit equal to 1.2×10⁻² mSv/MU and 1.6×10⁻² mSv/MU have been obtained with the conventional and IMRT technique, respectively. Applying the average amount of MU necessary to realize a 70 Gy target dose the estimated whole-body equivalent dose for both treatment techniques becomes 242 mSv (conventional) and 1969 mSv (IMRT), yielding an increase in the risk for secondary malignancies with a factor 8.

Conclusions: Historically the risk of secondary malignancies has been accepted to take advantage of the possible benefits of improved local control and treatment outcome. However, the introduction of new and sophisticated treatment techniques will also increase the risk of radiation induced malignancies. Therefore, these risk estimates become important to assess whether the benefits of the treatment technique outweigh the possible risks.

Introduction

When radiation is used as a therapeutic modality in the treatment of malignant diseases, the patient is exposed to the risk of harmful effects that might occur many years after a successful treatment. The physician must be familiar with the risk related to a particular treatment and the potential benefit to the patient. The latter being the hope of a complete cure or at least, a temporary remission of the effects of the disease. With advances in therapeutic technology and the use of combined treatment modalities, the survival time has been extended to many years and the need for constant re-evaluation of the risks associated with therapeutic techniques persists. Because of the inefficient use of the photon beam that has been generated by the linear accelerator (linac), intensity modulated radiation therapy (IMRT) techniques require a substantial increase in beam-on time with respect to conventional treatment techniques to deliver equivalent target doses

Inevitably, the ratios of tumor integral dose to patient integral dose that are currently known with conventional high energy irradiation techniques will decrease. Together with improved treatment outcome the potential risks of radiation-induced second malignancies will increase and the cost-benefit may need to be re-evaluated. Therefore, the absorbed dose that the patient receives outside the treatment volume needs to be investigated for these particular treatment techniques.

Intensity modulated radiation therapy treatment has been initiated in our department in 1995 for treatment of the head and neck (H and N) region [15], [16]. The commercially available system for IMRT (The Peacock System^®: NOMOS corporation, Sewickley, PA) is based on a slice-by-slice arc rotation approach, also referred to as ‘tomotherapy’. Both the process of intensity modulation of the treatment beam and the fact that patients are being treated in consecutive slices of 1 or 2 cm result in a significant increase in the total amount of monitor units that is required per Gy of target dose. To investigate the possible impact of this relatively new technique on the risk of inducing a secondary cancer, the contribution of photons and neutrons to the dose delivered outside the treatment field has been measured in vivo on patients treated at the head and neck region by means of conventional radiotherapy as well as tomotherapy.

Followill et al. [4], [5] estimated the risk of secondary cancers as a sequela of IMRT, based on extrapolations of scatter dose usually encountered in conventional treatment techniques and theoretical considerations. Yet the values of whole-body equivalent dose for patients undergoing radiotherapy are highly variable, depending strongly on beam energy, linac design, treatment set-up and treatment technique. Neutron doses in particular could differ significantly as shown in different studies [3], [4], [6], [10], [12], [13]. Recently, Mutic et al. [11] measured whole-body dose in tomotherapy of the head and neck region based on phantom studies. Apart from evaluating the uniformity of the whole-body dose distribution, these investigators observed significantly lower risk estimates than the theoretical predictions previously published. In this study, the whole-body equivalent dose produced in tomotherapy has been obtained in vivo in a clinical set-up for treatment of head and neck lesions and compared with similar data from conventional techniques. It is important to note, however, that the here obtained whole-body equivalent dose can be used as a first approximation only because of the non-uniform dose distributions encountered in radiotherapy and the specific organ response in terms of tumor induction. The presented dose measurements will be compared against both the theoretical approach [4], [5] as well as the phantom measurements [11] from literature.

Section snippets

Conventional treatment technique of the head and neck region

The patient treatments analyzed in this study consisted of a 70 Gy target dose administered in 35 fractions of 2 Gy. From 0 to 50 Gy, two lateral opposed, wedged, 6 MV photon beams from a dual energy linac (KDS-2 MEVATRON: Siemens, OCS-SMS, Concord, CA) were used. After 40 Gy the fields were reduced to exclude the spinal cord to deliver a total dose of 70 Gy. The excluded posterior strips received an additional 10 Gy electron boost to bring the total dose in this region to 50 Gy (if the nodes

Results

The three conventional treatments showed a mean MU setting of 458 MU, whereas a mean of 4492 MU were needed with the IMRT experiments. The estimates of whole-body equivalent dose per MU, H_p(10)_conv and H_p(10)_IMRT for both treatment techniques are listed in Table 1.

Applying the absorbed dose at the sternum from Table 1 and multiplying this value with the number of fractions needed for a 70 Gy target dose (i.e. 35 fractions) and MU_conv or MU_IMRT, respectively, yields H_p,_conv(70Gy)=1.18×10⁻²

Discussion

Any absorbed dose the patient receives outside the treatment volume must be considered undesirable. In addition to the primary beam absorption in overlying and underlying healthy tissues, the major sources of absorbed dose outside the treatment volume are: (a) photons leaking through or scattered by the head shielding and different collimators (referred to as X-ray leakage), (b) photons scattered out of the treatment volume, and (c) neutrons originating in the treatment head and leaking through

Conclusions

Historically the risk of secondary malignancies has been accepted to take advantage of the possible benefits of improved local control and treatment outcome. However, with the introduction of new and sophisticated treatment techniques the currently accepted risk factors for radiation induced malignancies will change. The present study showed that the risk may be increased by a factor 8 when replacing the conventional treatment technique for head and neck lesions with tomotherapy. Therefore,

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Risk assessment of radiation-induced malignancies based on whole-body equivalent dose estimates for IMRT treatment in the head and neck region

Abstract

Introduction

Section snippets

Conventional treatment technique of the head and neck region

Results

Discussion

Conclusions

Int. J. Radiat. Oncol. Biol. Phys.

Radiother. Oncol.

Int. J. Radiat Oncol. Biol. Phys.

Int. J. Radiat. Oncol. Biol. Phys.

Int. J. Radiat. Oncol. Biol. Phys.

Int. J. Radiat. Oncol. Biol. Phys.

Int. J. Radiat. Oncol. Biol. Phys.

Radiation Measurements

Int. J. Oncol. Radiat. Biol. Phys.