Preclinical assessment of comfort and secure fit of thermobrachytherapy surface applicator (TBSA) on volunteer subjects

Abstract

A thermobrachytherapy surface applicator (TBSA) was developed for simultaneous heat and brachytherapy treatment of chest wall (CW) recurrence of breast cancer. The ability to comfortably secure the applicator over the upper torso relative to the CW target throughout treatment is assessed on volunteers. Male and postmastectomy female volunteers were enrolled to evaluate applicator secure fit to CW. Female subjects with intact breast were also enrolled to assess the ability to treat challenging cases. Magnetic resonance (MR) images of volunteers wearing a TBSA over the upper torso were acquired once every 15 minutes for 90 minutes. Applicator displacement over this time period required for treatment preplanning and delivery was assessed using MR visible markers. Applicator comfort and tolerability were assessed using a questionnaire. Probability estimates of applicator displacements were used to investigate dosimetric impact for the worst-case variation in radiation source-to-skin distance for 5 and 10 mm deep targets spread 17 × 13 cm on a torso phantom. Average and median displacements along lateral and radial directions were less than 1.2 mm over 90 minutes for all volunteers. Maximum lateral and radial displacements were measured to be less than 1 and 1.5 mm, respectively, for all CW volunteers and less than 2 mm for intact breast volunteers, excluding outliers. No complaint of pain or discomfort was reported. Phantom treatment planning for the maximum displacement of 2 mm indicated < 10% increase in skin dose with < 5% loss of homogeneity index (HI) for -2 mm uniform HDR source displacement. For +2 mm uniform displacement, skin dose decreased and HI increased by 20%. The volunteer study demonstrated that such large and uniform displacements should be rare for CW subjects, and the measured variation is expected to be low for multifraction conformal brachytherapy treatment.

Figure 1. Rectangular shaped TBSA used in the volunteer study: (a) water bolus with thermal mapping catheters on skin contacting side; (b) HDR brachytherapy catheters on the outer side of TBSA; (c) TBSA with MR markers and contrast filled brachytherapy catheters wrapped around a mastectomy volunteer; (d) volunteer with Neoprene over‐garment secured over the TBSA.

Figure 2. MR images of volunteer CW: (a)–(c) axial images of mastectomy and healthy volunteers with excellent conformity of TBSA to CW; MR images indicate MR visible marker pairs on the skin and TBSA that were used to assess applicator lateral displacement. The bright dots above the water bolus from the liquid MR contrast filled inside the parallel array of brachytherapy catheter channels were used to assess radial displacement of HDR catheters from the CW surface.

Figure 3. Lateral displacement of applicator along the CW: (a) applicator displacement 30 minutes after the baseline scan (i.e., displacement at start of thermobrachytherapy treatment); (b) displacement at the end of study measured with reference to the data obtained 30 minutes after the baseline (i.e., displacement during treatment); (c) overall displacement measured between the first (planning) and last (end of treatment) scans.

Figure 4. Radial displacements of HDR brachytherapy catheters from the CW surface: (a) applicator displacement 30 minutes after baseline scan (i.e., displacement at start of thermobrachytherapy treatment); (b) displacement measured between the first and last scans.

Figure 5. Density distribution curves (a) of the 3M HDR catheter‐CW displacement measurements |ρ(0)−ρ(30)| which correspond to the relative shift in position of TBSA catheters at the start of thermobrachytherapy treatment, 30 minutes after the baseline data was obtained for HDR pretreatment planning. 95% confidence interval (b) (solid line) for the mean (cross marking) displacement of |ρ(0)−ρ(30)|. Density distributions and the estimated confidence intervals indicate very small variation in radial applicator displacement.

Figure 6. Transverse isodose distributions for a prescription of 100 cGy to 10 mm deep target in the case of undisturbed bolus (8 mm) and bolus thickness variation of ±2 mm.

Figure 7. Dose volume histograms for 5 mm (top) and 10 mm (bottom) deep targets calculated for the nominal 8 mm thick water bolus (solid lines) and for thicker (10 mm, dashed lines) and thinner (6 mm, dotted‐dashed lines) water bolus.

Figure 8. Numerical scores recorded by the volunteers on the comfort level of TBSA during the preclinical study. Note that the applicator and procedure were well‐tolerated (score ≥ 5 in and comfort remained almost the same (score ≤ 2 in during the study. No pain, breathing difficulty or discomfort was reported by any of the volunteers.