Objective method to report planner-independent skin/rib maximal dose in balloon-based high dose rate (HDR) brachytherapy for breast cancer

Is it important to define skin sub-volumes in breast brachytherapy?

Aim:

To evaluate clinically pertinent skin dose and volume parameters for the development of toxicities following accelerated partial breast irradiation (APBI).

Methods:

Three skin structures of various thicknesses inside the body (2 mm, 3 mm, 4 mm) were contoured over the treated breast retrospectively in a cohort of 62 women who underwent APBI using multi-catheter interstitial brachytherapy. The correlation statistics between the various skin structures and acute and late skin sequelae were evaluated using the Mann–Whitney U test and receiver-operating characteristic analysis. A p-value of <0·05 was considered significant.

Results:

At a median follow-up period of 54 months (range: 28–86), a significant correlation was seen between dose received by 0·2 (D0·2) cc of skin 4 mm inside the body as well as volume receiving 100% dose (V100) of skin 3 mm inside the body with cosmesis at 2 years and at last follow-up. The threshold for the two parameters for prediction of excellent or good cosmesis was 90% of the prescribed dose and 0·05 cc, respectively. No significant dosimetric or volumetric correlation was seen with other sequelae like wound dehiscence, fat necrosis, telangiectasia and atrophy.

Conclusion:

The results of this study support the use of dose and volumetric indices of the sub-volumes of the skin for correlation with clinical endpoints. However, the same should be validated prospectively in a larger cohort of women undergoing breast brachytherapy.

Dosimetric impact of source-positioning uncertainty in high-dose-rate balloon brachytherapy of breast cancer

Purpose

To evaluate the dosimetric impact of source-positioning uncertainty in high-dose-rate (HDR) balloon brachytherapy of breast cancer.

Material and methods

For 49 HDR balloon patients, each dwell position of catheter(s) was manually shifted distally (+) and proximally (–) with a magnitude from 1 to 4 mm. Total 392 plans were retrospectively generated and compared to corresponding clinical plans using 7 dosimetric parameters: dose (D₉₅) to 95% of planning target volume for evaluation (PTV_EVAL), and volume covered by 100% and 90% of the prescribed dose (PD) (V₁₀₀ and V₉₀); skin and rib maximum point dose (D_max); normal breast tissue volume receiving 150% and 200% of PD (V₁₅₀ and V₂₀₀).

Results

PTV_EVAL dosimetry deteriorated with larger average/maximum reduction (from ± 1 mm to ± 4 mm) for larger source position uncertainty (p value < 0.0001): from 1.0%/2.5%, 3.3%/5.9%, 6.3%/10.0% to 9.8%/14.5% for D₉₅; from 1.0%/2.6%, 3.1%/5.7%, 5.8%/8.9% to 8.7%/12.3% for V₁₀₀; from 0.2%/1.5%, 1.0%/4.0%, 2.7%/6.8% to 5.1%/10.3% for V₉₀. ≥ ± 3 mm shift reduced average D₉₅ to < 95% and average V₁₀₀ to < 90%. While skin and rib D_max change was case-specific, its absolute change (∣Δ(Value)∣) showed that larger shift and high dose group had larger variation compared to smaller and lower dose group (p value < 0.0001), respectively. Normal breast tissue V₁₅₀ variation was case-specific and small. Average ∣Δ(V₁₅₀)∣ was 0.2 cc for the largest shift (± 4 mm) with maximum < 1.7 cc. V₂₀₀ was increased with higher elevation for larger shift: from 6.4 cc/9.8 cc, 7.0 cc/10.1 cc, 8.0 cc/11.3 cc to 9.2 cc/ 13.0 cc.

Conclusions

The tolerance of ± 2 mm recommended by AAPM TG 56 is clinically acceptable in most clinical cases. However, special attention should be paid to a case where both skin and rib are located proximally to balloon, and the orientation of balloon catheter(s) is vertical to these critical structures. In this case, sufficient dosimetric planning margins are required.

Dosimetric evaluation of multilumen intracavitary balloon applicator rotation in high-dose-rate brachytherapy for breast cancer

The objective of this work is to evaluate dosimetric impact of multilumen balloon applicator rotation in high-dose-rate (HDR) brachytherapy for breast cancer. Highly asymmetrical dose distribution was generated for patients A and B, depending upon applicator proximity to skin and rib. Both skin and rib spacing was ≤ 0.7 cm for A; only rib spacing was ≤ 0.7 cm for B. Thirty-five rotation scenarios were simulated for each patient by rotating outer lumens every 10° over ± 180° range with respect to central lumen using mathematically calculated rotational matrix. Thirty-five rotated plans were compared with three plans: 1) original multidwell multilumen (MDML) plan, 2) multidwell single-lumen (MDSL) plan, and 3) single-dwell single-lumen (SDSL) plan. For plan comparison, planning target volume for evaluation (PTV_EVAL) coverage (dose to 95% and 90% volume of PTV_EVAL) (D95 and D90), skin and rib maximal dose (Dmax), and normal breast tissue volume receiving 150% (V150) and 200% (V200) of prescribed dose (PD) were evaluated. Dose variation due to device rotation ranged from -5.6% to 0.8% (A) and -6.5% to 0.2% (B) for PTV_EVAL D95; -5.2% to 0.4% (A) and -4.1% to 0.7% (B) for PTV_EVAL D90; -2.0 to 18.4% (A) and -7.8 to 17.5% (B) for skin Dmax; -11.1 to 22.8% (A) and -4.7 to 55.1% (B) of PD for rib Dmax, respectively. Normal breast tissue V150 and V200 variation was &lt; 1.0 cc, except for -0.1 to 2.5cc (B) of V200. Furthermore, 30° device rotation increased rib Dmax over 145% of PD: 152.9% (A) by clockwise 30° rotation and 152.5% (B) by counterclockwise 30° rotation. For a highly asymmetric dose distribution, device rotation can outweigh the potential benefit of improved dose shaping capability afforded by multilumen and make dosimetric data worse than single-lumen plans unless it is properly corrected.