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. 2022 Feb;23(2):e13491.
doi: 10.1002/acm2.13491. Epub 2021 Dec 10.

Scripted spot removal in PBS proton therapy planning

Samantha G Hedrick  1 , Bryant Walker  1 , Bart Morris  1 , Scott Petro  1 , Marc Blakey  1
Affiliations

Scripted spot removal in PBS proton therapy planning

Samantha G Hedrick et al. J Appl Clin Med Phys. 2022 Feb.

Abstract

Background: It is well known in proton therapy that the relative biological effectiveness (RBE) is not constant across the entire Bragg peak, with higher RBE at the distal end of the Bragg peak due to higher linear energy transfer (LET). Treatment planning systems are moving toward LET optimization to mitigate this potentially higher biological impact at a track end. However, using a simple script, proton users can begin to simulate this process by deleting spots from critical structures during optimization. In most cases, nominal target coverage and plan robustness remain satisfactory.

Methods: In our clinic, we developed a script that allows the user to delete spots in all organs at risk (OARs) of interest for one or more treatment beams. The purpose of this script is to potentially reduce side effects by eliminating Bragg peaks within OARs. The script was first used for prostate patients where spots in the rectum and sigmoid, outside of the overlap with the target, were deleted. We then began to use the script for head and neck (H&N) and breast/chestwall patients to reduce acute side effects of the skin by removing spots in a 0.5-cm skin rind.

Conclusions: By utilizing a simple script for deleting spots in critical structures, we have seen excellent clinical results thus far. We have noted reduced skin reactions for nearly all H&N and breast patients.

Keywords: PBS; outcomes; scripting.

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Conflict of interest statement

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Figures

FIGURE 1
FIGURE 1
Typical head and neck (H&N) spots distribution from a left posterior oblique beam, indicated by yellow arrow. Circles identify the areas of spots in the skin due to ending anteriorly (red circle) and skimming (purple circle)
FIGURE 2
FIGURE 2
“SkinSpots” contour (red) for (a) head and neck (H&N) and (b) breast plans
FIGURE 3
FIGURE 3
Example of spot removal script process for a two‐field prostate case. (a and b) right and left lateral beams with spots removed. (c) Isodose for plan with spots removed. (d and e) Right and left lateral beams with spots. (f) Isodose for plan with spots in rectum. Yellow arrows indicate beam direction. (g) Dose‐volume histogram (DVH) comparison. Rectum – brown, without spots – solid, with spots – dashed
FIGURE 4
FIGURE 4
Example of typical skin reaction locations for breast (left) and H&N (right) patients before spot removal technique. For the breast patient, the area of typical skin reaction is shaded in pink. For the H&N patient, the area of typical skin reaction is shaded in red
FIGURE 5
FIGURE 5
Example of spot removal script process for a two‐field breast case. (a and b) Medial and lateral beams with spots removed. (c) Isodose for plan with spots removed. (d and e) Medial and lateral beams with spots. (f) Isodose for plan with spots in skin rind. Yellow arrows indicate beam direction. (g) Dose‐volume histogram (DVH) comparison. SkinRind_0.5 cm – yellow, without spots – solid, with spots – dashed
FIGURE 6
FIGURE 6
Example of spot removal script process for a four‐field head and neck (H&N) case. (a and b) right anterior oblique (RAO) and left posterior oblique (LPO) beams with spots removed. (c) Isodose for plan with spots removed. (d and e) RAO and LPO with spots. (f) Isodose for plan with spots in skin rind. Yellow arrows indicate beam direction. (g) Dose‐volume histogram (DVH) comparison. NeckSkin_0.5 cm – red, without spots – solid, with spots – dashed
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