Interfractional and intrafractional errors assessed by daily cone-beam computed tomography in nasopharyngeal carcinoma treated with intensity-modulated radiation therapy: a prospective study

Abstract

This prospective study was to assess interfractional and intrafractional errors and to estimate appropriate margins for planning target volume (PTV) by using daily cone-beam computed tomography (CBCT) guidance in nasopharyngeal carcinoma (NPC). Daily pretreatment and post-treatment CBCT scans were acquired separately after initial patient setup and after the completion of each treatment fraction in 10 patients treated with IMRT. Online corrections were made before treatment if any translational setup error was found. Interfractional and intrafractional errors were recorded in the right–left (RL), superior–inferior (SI) and anterior–posterior (AP) directions. For the translational shifts, interfractional errors >2 mm occurred in 21.7% of measurements in the RL direction, 12.7% in the SI direction and 34.1% in the AP direction, respectively. Online correction resulted in 100% of residual errors ≤2 mm in the RL and SI directions, and 95.5% of residual errors ≤2 mm in the AP direction. No residual errors >3 mm occurred in the three directions. For the rotational shifts, a significant reduction was found in the magnitudes of residual errors compared with those of interfractional errors. A margin of 4.9 mm, 4.0 mm and 6.3 mm was required in the RL, SI and AP directions, respectively, when daily CBCT scans were not performed. With daily CBCT, the margins were reduced to 1.2 mm in all directions. In conclusion, daily CBCT guidance is an effective modality to improve the accuracy of IMRT for NPC. The online correction could result in a 70–81% reduction in margin size.

The ratio of weight loss to planning target volume significantly impacts setup errors in nasopharyngeal cancer patients undergoing helical tomotherapy with daily megavoltage computed tomography

“…Nonetheless, a prior study that employs post-treatment cone-beam CTs has shown a small intrafraction motion of 1.2 mm during RT delivery to NPC with standard thermoplastic mask immobilization. 33 Our study also lacks treatment toxicity data that would further determine the causes of weight loss. The retrospective nature of this analysis poses the risks of patient selection bias and uncontrolled confounding factors.…”

Quantification and Assessment of Interfraction Setup Errors Based on Cone Beam CT and Determination of Safety Margins for Radiotherapy

“…One limitation of this study is that it does not account for residual errors because a second (verification) CBCT scan after the repositioning was not carried out. In other studies, for prostate bed [ 11 ] and head and neck [ 23 ] the residual errors were determined by performing a second CBCT after the treatment and matching it with the planning CT. Of course this procedure will also lead to a higher dose exposure to the patient. The assessment of the residual errors will allow to calculate CTV-to-PTV expansion margins when image guidance is used which should in general be smaller than the values obtained in this study.…”

Faculty of Radiation Oncology Position Paper on the use of Image–Guided Radiation Therapy

“…A follow-up from this study also showed that the incidence of post-treatment gastrostomy dependence and oesophageal stricture was lower with reduced margins. 44 These results are reflected in a number of other dosimetric studies which have found that without daily IGRT, PTV margins need to be larger to account for set-up errors [45][46][47][48][49][50] and that the improved setup accuracy with IGRT can potentially lead to improved sparing of normal tissue structures. 51 Frequent volumetric imaging also allows for the detection of soft tissue changes during treatment.…”