Interfractional anatomic variation in patients treated with respiration-gated radiotherapy

Abstract

As quality assurance for respiration-gated treatments using the Varian RPM system, we monitor interfractional diaphragm variation throughout treatment using extra anterior-posterior (AP) portal images. We measure the superior-inferior (SI distance between one or more bony landmarks and the ipsilateral diaphragm dome in each such radiograph and calculate its difference, D, from the corresponding distance in a planning CT scan digitally reconstructed radiograph (DRR). For each patient, the mean of D represents the systematic diaphragm displacement, and the standard deviation of D represents random diaphragm variations and is a measure of interfractional gating reproducibility. We present results for 31 sequential patients (21 lung, 10 liver tumors), each with at least 8 such portal images. For all patients, the gate included end-exhale. The patient-specific duty cycle ranged from 30% to 60%. All patients received customized audio prompting for simulation and treatment, and 14 patients also received visual prompting. Respiration-synchronized fluoroscopic movies taken at a conventional simulator revealed patient-specific diaphragm excursions from 1.0 cm to 5.0 cm and diaphragm excursion within the gate from 0.5 cm to 1.0 cm, demonstrating a significant reduction of intra-fractional diaphragm (and by inference tumor) motion by respiratory gating. One standard deviation of the systematic displacement (the mean of D) was 0.63 cm and 0.48 cm for the lung and liver patient groups, respectively. The average +/-1 SD of the random displacements (i.e., the average of the standard deviations of D) was 0.42 +/- 0.11 cm and 0.50 +/- 0.19 for the two groups, respectively. The similar magnitude of the systematic and random displacements suggests that both derive from a common distribution of interfractional variations. Combining visual with audio prompting did not significantly improve performance, as judged by D. Guided by these portal images, field changes were made during the course of treatment for 6 patients (1 lung, 5 liver).

Figure 1

The RPM screen and visual feedback hardware. (a) The RPM v1.5 PC screen for amplitude‐gated treatment or simulation at end‐inhale. The treatment beam would be on when the marker motion trace is between the two horizontal lines labeled “Gate Region.” (b) The small LCD monitor mounted on the couch, on which the patient can view the visual prompt

Figure 2

The determination of D. D is found by comparing the distance of isocenter to diaphragm on an AP DRR from the planning scan with the distance of isocenter to diaphragm on an AP or PA portal image. Correction for setup error on the portal image is made by finding the distance from isocenter to a vertebral landmark visible on both images.

Figure 3

The mean and standard deviation of D ($D_{\text{pt}}$ and $S_{\text{pt}}$) for the lung cancer patients

Figure 4

Frequency distribution of D for (a) patient 3_LU and (b) patient 6_LU

Figure 5

The mean and standard deviation of D ($D_{\text{pt}}$ and $S_{\text{pt}}$) for the liver cancer patients. Note that there are two sets of bars for patient 3, for whom both right and left diaphragms were monitored.

Figure 6

Chronological variation of D over the course of treatment for four patients. These show four qualitatively different types of variation in diaphragm position.