Background and purpose: The purpose of this study was to perform in-vivo measurements of extracranial doses received by patients undergoing serial tomotherapy of the head and neck.
Material and methods: Intensity modulated radiotherapy treatment (IMRT) plans were designed for nine patients using the CORVUS treatment planning system (NOMOS Corp.). These plans were delivered using a tertiary collimator dedicated for serial tomotherapy attached to a 10-MV linear accelerator. For each patient, one optically stimulated luminescence dosimeter (OSLD) was placed on the sternum and one on the lower abdomen. The OSLDs were then processed, thereby estimating the in vivo absorbed doses to the sternum and gonads as a function of distance from the treatment site.
Results: The OSLDs were shown to measure known doses to within 5%, thereby validating their accuracy for this dose and energy range. In the patient studies, the dose received by the OSLDs varied in direct proportion to the number of monitor units delivered and inversely with the distance from the target volume; the patient dose at a distance of 15 cm from the target is approximately 0.4% of the total monitor units delivered, and drops to below 0.1% of the total MUs at approximately 40 cm from the center of the target. The average sternal dose was 1353 mSv and the average abdominal dose was 327 mSv for an average prescribed dose of 60.1 Gy. This can be attributed, at least partially, to the inefficient treatment delivery that on average required 9.9 MU/0.01 Gy.
Conclusions: While IMRT reduces the normal tissue volume in the high-dose region, it also increases the overall monitor units delivered, and hence the whole-body dose, when compared with conventional treatment delivery. As has been noted in existing literature, these increases in whole-body dose from radiotherapy delivery may increase the likelihood of a radiation-induced secondary malignancy. Therefore, it is important to assess the risk of secondary malignancies from IMRT delivery, and compare this relative risk against the potential benefits of decreased normal tissue complication probabilities.