gms | German Medical Science

Background: Proton therapy with passive beam scattering typically requires creation of customized brass apertures for each field, which contribute to the overall cost of treatment. In our practice, prostate cancer patients are treated with standard GTV to PTV expansions based on prostate and seminal vesicle volumes. Four custom apertures are required: one each for the right and left lateral fields of both the initial and boost treatments. We assessed the variability of apertures across patients and tested the feasibility of creating a standardized aperture for the initial treatment field.

Material and methods: The apertures of 41 previously treated patients were reviewed and the area within each aperture was calculated. Initial fields included the prostate and seminal vesicles, treated to 50.4 Gy, followed by a boost to the prostate alone to 79.2 Gy. A standard initial field aperture was designed using the greatest vectors from the center of gravity across all patients, and was then tested against a separate cohort of 15 previously treated patients. From that cohort, DVHs were created for the four patients with the smallest initial PTVs (greatest difference between standard and custom apertures) using either an open, standard, or patient-specific initial field aperture.

Results: The mean area of the initial aperture was 66.7 cm2 (SD=8.6) and areas varied about the mean by -28% to +18%. The boost apertures varied more: from -35% to +32% about the mean of 37.9 cm² (SD=6.7). The mean aperture area did not differ between patients treated with or without neoadjuvant hormonal therapy. The standard initial aperture encompassed the patient-specific aperture area of the test cohort virtually completely in every case. DVH analysis of the four patients with the smallest initial PTVs revealed that compared to the customized apertures, both open and standard initial field apertures achieved identical coverage of the GTV and PTV. Compared to patient-specific apertures, the standard aperture increased mean dose to the bladder by 72% (27 vs. 46.5 Gy, p=0.005) and the anterior rectal wall by 13% (48.9 vs. 55.4 Gy, p=0.002). Compared to an open aperture, the standard aperture provided modest reduction in mean bladder dose (46.5 vs. 50.0 Gy, p=0.009), but no better sparing of the anterior rectal wall (mean dose 55.4 vs. 55.8 Gy, p=0.257).

Conclusion: Creation of a standardized initial field aperture is feasible and maintains similar dosimetric coverage of the GTV and PTV compared to patient-specific apertures, but at the cost of increased dose to the bladder and anterior rectal wall. Our data suggest that customized field apertures provide improved dose conformality and are warranted in proton radiotherapy for prostate cancer. Further studies to evaluate potential cost reductions and process efficiencies are important in management of finite medical resources and may ultimately improve patient access to proton therapy.