Doses delivered by portal imaging quality assurance in routine practice of adjuvant breast radiotherapy worth to by monitored and compensated in some cases

Abstract

Background: Imaging, in radiotherapy, has become a routine tool for repositioning of the target volume at each session. The repositioning precision, currently infracentimetric, evolves along with the irradiation techniques. This retrospective study aimed to identify practices and doses resulting from the use of high energy planar imaging (portal imaging) in daily practice.

Methods: A retrospective survey of portal images (PIs) was carried out over 10 years for 2,403 patients and for three linacs (1 Elekta SLi, 2 Varian Clinac) for postoperative mammary irradiations. Images were taken using a standardized number of monitor units (MU) for all patients. Due to the variable sensitivities of the detectors and the possibility of adjustment of the detector-patient distance, the number of MU were 3; 2 and 1 respectively, for Elekta SLi^®, Clinac 600^® and Clinac 2100^®. Then, a representative cumulated dose was calculated in simplified reference conditions (5 cm depth, beam of 10 cm × 10 cm, 6 MV), considering the total number of images taken during the whole treatment course. The consistency between the representative doses and the actual absorbed doses received by the patients was verified by simulating a series of typical cases with the treatment plan dose calculation system.

Results: The delivered doses differ significantly between the three linacs. The mean representative dose values by complete treatment were 0.695; 0.241 and 0.216 Gy, respectively, for SLi, Clinac 600 and Clinac 2100. However, 15 patients were exposed to a dose >2 Gy with a maximum dose of 5.05 Gy. The simulated doses were very similar to the representative doses.

Conclusions: A significant dose delivery was highlighted by this study. These representative doses are presently communicated weekly to the radiation oncologist for the radiation protection of their patients. Moreover, they should be taken into account in a possible study of long-term stochastic risks.

Keywords: Portal imaging; breast cancer; dose; radiotherapy.

Figure 1

Screen capture of the repositioning imaging performed on a Clinac 2100^®, by double exposure for a patient irradiated on the left breast. The left part of the screen shows the matching obtained between the daily imaging and the reference one (digitally reconstructed radiograms, DRR), the right window shows the DRR (top) and the daily image (bottom). In the lower strip of the window are displayed the corrections to apply and the status of medical approval.

Figure 2

Example of table as issued from the Record and verify system a case of left breast cancer. After the identification lines, the first line gives the cumulative of all imaging doses, the second part of the table details by beam the images made and the resulting doses, the last part of the table gives the cumulative dose per treatment course (main volume vs. boost). For this case, an additional cumulative dose of 0.4 Gy was delivered by the imaging procedure. The doses were calculated according to Eq. [1]. Two irradiated fields external and internal and two simulated fields for positioning were used. EXT, external; INT, internal; SIM, simulated fields; ANT, anterior; LAT, lateral; R, right; num images, number images; MU, monitor units.

Figure 3

Weekly table summarizing cumulative doses delivered per patient. The first and second columns correspond respectively to the name and identification number of the patient. The following columns correspond respectively to the radiation oncologist responsible for the patient, the name of the treatment course, the number of cumulative images made for the patients and the cumulative dose of the imaging. The color code facilitates the reading and warns about the doses received by imaging when they start to become important. R, right; L, left.

Figure 4

The classes of standardized dose from the portal imaging for the three machines performed in the context of the breast treatments according to the treatment apparatus, for 10 years of activity. The curves represent the doses distribution histograms by bins of 0.1 Gy. There is an almost equal peak for Clinac 2100^® and Clinac 600^® at the respective doses of 0.1 and 0.15 Gy. The highest doses are achieved by the Elekta SLi^® peaking at 0.55 Gy. Note the spreading of the data ranges, with the largest area under the curve for Elekta SLi^® then the Clinac 600^® and finally Clinac 2100^®.

Figure 5

The case-specific data of the total imaging dose values for real and representative treatments on each machine, calculated using three methods: (I) the total maximum dose as calculated by the TPS which takes account of the maximum dose value in the imaged volume which is generally the nipple region (D_TPS); (II) the total dose in the center of the breast (D_center) represented by the dose at mid-depth and mid-thickness of the breast (ICRU reference point) which is the most representative of the dose delivered; (III) the total imaging dose (D_imaging) in standardized condition (beam of 10×10 cm² at 5 cm depth). The average Dcenter, presented in green is shown to be slightly higher than the calculated Dimaging (“physician-reported”) for the Clinac 2100^® and slightly lower for the Clinac 600^® and the Elekta SLi^®. The box plot represent from top to bottom either the extreme or maximum values, the 75^th percentile, the median, the 25^th percentile and the minimum values.