The root cause analysis on failed patient-specific measurements of pencil beam scanning protons using a 2D detection array with finite size ionization chambers

Abstract

The aim of this report is to present the root cause analysis on failed patient-specific quality assurance (QA) measurements of pencil beam scanning (PBS) protons; referred to as PBS-QA measurement. A criterion to fail a PBS-QA measurement is having a <95% passing rate in a 3.0%-3.0 mm gamma index analysis. Clinically, we use a two-dimensional (2D) gamma index analysis to obtain the passing rate. The IBA MatriXX PT 2D detection array with finite size ionization chamber was utilized. A total of 2488 measurements performed in our PBS beamline were cataloged. The percentage of measurements for the sites of head/neck, breast, prostate, and other are 53.3%, 22.7%, 10.5%, and 13.5%, respectively. The measurements with a passing rate of 100 to >94%, 94 to >88%, and <88% were 93.6%, 5.6%, and 0.8%, respectively. The percentage of failed measurements with a <95% passing rate was 10.9%. After removed the user errors of either re-measurement or re-analysis, 8.1% became acceptable. We observed a feature of >3% per mm dose gradient with respect to depth on the failed measurements. We utilized a 2D/three-dimensional (3D) gamma index analysis toolkit to investigate the effect of depth dose gradient. By utilizing this 3D toolkit, 43.1% of the failed measurements were improved. A feature among measurements that remained sub-optimal after re-analysis was a sharp >3% per mm lateral dose gradient that may not be well handled using the detector size of 5.0 mm in-diameter. An analysis of the sampling of finite size detectors using one-dimensional (1D) error function showed a large dose deviation at locations of low-dose areas between two high-dose plateaus. User error, large depth dose gradient, and the effect of detector size are identified as root causes. With the mitigation of the root causes, the goals of patient-specific QA, specifically detecting actual deviation of beam delivery or identifying limitations of the dose calculation algorithm of the treatment planning system, can be directly related to failure of the PBS-QA measurements.

Keywords: 3D gamma index analysis; detector effects; patient-specific measurements; proton therapy; quality assurance.

FIGURE 1

Depth dose distributions for an ideal (top) and real (bottom) proton beam. The standard criteria of measurement depth selection for each field is shown for both cases. The depths selected are one‐half and one‐quarter of maximum penetration range. In the real case, this depth selection can lead to measurement in areas of high‐dose gradients with respect to depth as is indicated by the purple dashed line

FIGURE 2

Top: Solid lines present the spot size in air of the G2 (IBA Proteus+) and P#x02010;one (IBA Proteus One) as a function of beam range. The linear increasing of scattering for proton passing water over a length of beam range. Shown dashed lines are the spot size in water; it includes both effects of the scattering and the spot size in air. Bottom: An error function with specified sigma for each plateau was used to simulate a 1D single‐ or dual‐plateaus lateral profile. A sigma of 7.5 mm was used to generate the single plateau and the first peak of dual‐plateaus profile. To consider a high‐dose gradient shaped by a sharp distal penumbra along depth of beam path, a sigma of 1.8 mm was used for second peak of dual‐plateaus profile. The average dose within detectors with diameters of 2.0 and 5.0 cm and a spacing of 7.0 mm was calculated to present the effects of finite size detector and its spacing in used 2D array in this manuscript

FIGURE 3

Clinical plan and quality assurance (QA) calculation of prostate case with artificial hip. Top: A posterior–anterior (PA) field with a 74 mm range shifter was used to treat the prostate cancer with pelvic nodes of a patient with a metal hip replacement. Bottom Left: Shows calculated two‐dimensional (2D) dose distributions in water along the beam path for QA measurements. Bottom Right: The calculated 2D dose distributions in water at a depth perpendicular to the beam path. The noticeable difference between the two sides of the PA field stems from the presence of the artificial hip. This difference would be made up for with a lateral beam on the patient's right side but this is not feasible on the left side

FIGURE 4

Lateral dose profiles and measurements using different air gap distances. The extracted lateral profiles are at the 10 cm depth from the calculated 3D dose distributions following the solid arrow indicated in Figure 3 with an air gap (AG) of 6 and 12 cm as the dashed and solid lines, respectively. The location of extraction was selected to match one row of detectors in measurements. Measured lateral profiles using the DigiPhant with a 12 cm AG and using solid‐water with a 6 cm AG at same location are plotted by circle and rectangular points, respectively

FIGURE 5

Geant4 calculated fraction of the IDD_Mea fluence measured by an 8.2‐cm diameter detector as function of R80 and relative depth (i.e., measurement depth divided by R80)

FIGURE 6

Percentage deviations between the PPD_Cal of PB/MC and PDD_Mea at the depth of entrance and the averaging over depths at the plateau as a function of beam energy for an air gap of 180 mm. Solid/opened circles are for PPB_Cal of pencil beam algorithm, and solid/opened squares are for PPB_Cal of Monte Carlo algorithm.

FIGURE 7

Isodose and spot pattern distribution in three planes at requested depth of measurement. This Figure shows the three cut‐planes of two‐dimensional (2D) dose distribution of a typical field in breast cases. The location of three cut‐plan is indicated at top right corner. Measurement depth is at 3 cm. Measured 2D dose distribution will be compared with calculated one at bottom left. One depth dose and one lateral profile were extracted at the locations of dashed and solid arrows, respectively

FIGURE 8

Gamma map output of a quality assurance (QA) beam using two‐dimensional (2D) 3%/3 mm analysis and 2D/3D 3%/3 mm analysis Top: A measurement of dose distribution of an oblique field for a breast case. This is the measured distribution from the beam in the bottom left panel in Figure 4. Middle: Shows the obtained γ‐index map of this measurement using 2D γ‐index toolkit in myQA platform. Bottom: Shows the obtained γ‐index map by house‐built using 2D/3D γ‐index toolkit using criteria of 3%/3 mm

FIGURE 9

Extracted depth doses in areas of high gamma values for two clinical cases. Extracted depth doses along the dashed arrows indicated in Figures 5 and 7 for prostate and breast cases are shown. The dose gradient along depth is >6.0% per mm at the depth of measurement for the breast case and is <2.0% per mm at the depth of measurement for the prostate case

FIGURE 10

Comparison of lateral dose profiles extracted from TPS and measurements to an analytical simulation to examine possible detector effects. Square points are for measured lateral profiles for prostate and breast cases at top and bottom panels, respectively. Extracted TPSCalc lateral profiles are presented by dashed curves while the analytical simulated profiles are presented by dotted curves. Diamond and circle points, respectively, present effects for detector sizes of 2.0 and 5.0 mm with a spacing of 7.0 mm

FIGURE 11

Comparison of convolved measurements with TPS calculated dose. Left: TPSCalc and three‐dimensional (3D) convolved dose distribution in the top and bottom panels, respectively. The corners at each side of plateaus are smoothed with lower dose, while the lower doses of valley are filled in with higher doses. Right: The one‐dimensional lateral profiles of TPSCalc, two‐dimensional (2D), and 3D convolution are shown. The sharp corners at each plateau are reduced while the doses at valley are increased between the TPSCalc and 2D/3D convolution of detector sizes