Accelerated partial-breast irradiation using intensity-modulated proton radiotherapy: do uncertainties outweigh potential benefits?

Abstract

Objective: Passive scattering proton beam (PSPB) radiotherapy for accelerated partial-breast irradiation (APBI) provides superior dosimetry for APBI three-dimensional conformal photon radiotherapy (3DCRT). Here we examine the potential incremental benefit of intensity-modulated proton radiotherapy (IMPT) for APBI and compare its dosimetry with PSPB and 3DCRT.

Methods: Two theoretical IMPT plans, TANGENT_PAIR and TANGENT_ENFACE, were created for 11 patients previously treated with 3DCRT APBI and were compared with PSPB and 3DCRT plans for the same CT data sets. The impact of range, motion and set-up uncertainties as well as scanned spot mismatching between fields of IMPT plans was evaluated.

Results: IMPT plans for APBI were significantly better regarding breast skin sparing (p<0.005) and other normal tissue sparing than 3DCRT plans (p<0.01) with comparable target coverage (p=ns). IMPT plans were statistically better than PSPB plans regarding breast skin (p<0.002) and non-target breast (p<0.007) in higher dose regions but worse or comparable in lower dose regions. IMPT plans using TANGENT_ENFACE were superior to that using TANGENT_PAIR in terms of target coverage (p<0.003) and normal tissue sparing (p<0.05) in low-dose regions. IMPT uncertainties were demonstrated for multiple causes. Qualitative comparison of dose-volume histogram confidence intervals for IMPT suggests that numeric gains may be offset by IMPT uncertainties.

Conclusion: Using current clinical dosimetry, PSPB provides excellent dosimetry compared with 3DCRT with fewer uncertainties compared with IMPT.

Advances in knowledge: As currently delivered in the clinic, PSPB planning for APBI provides as good or better dosimetry than IMPT with less uncertainty.

Figure 1.

Comparison of dose distributions among (a) three-dimensional conformal photon radiotherapy (3DCRT), (b) passive scattering proton beam (PSPB), (c) intensity-modulated proton radiotherapy (IMPT) with TANGENT_PAIR and (d) IMPT with THANGENT_ENFACE treatment plans for one of the study patients.

Figure 2.

Comparison of dose–volume histogram (DVH) variations for various normal structures and clinical target volume (CTV) between passive scattering proton beam (PSPB; blue band) and intensity-modulated proton radiotherapy (IMPT; red band) using (a) TANGENT_ENFACE and (b) TANGENT_PAIR owing to patient set-up uncertainties for one example patient. The shadowed band represents the DVH variation range, and the solid line represents the DVH from the original plan. brst skin, breast skin; ip. lung, ipsilateral lung; ip. norm. brst, ipsilateral normal breast.

Figure 3.

Comparison of dose–volume histogram (DVH) variations between passive scattering proton beam (PSPB; blue band) and intensity-modulated proton radiotherapy (IMPT; red band) using (a) TANGENT_ENFACE and (b) TANGENT_PAIR owing to range uncertainties for the patient shown in Figure 2. The shadowed band represents the DVH variation range, and the solid line represents the DVH from the original plan. brst skin, breast skin; CTV, clinical target volume; ip. lung, ipsilateral lung; ip. norm. brst, ipsilateral normal breast.

Figure 4.

Dose–volume histogram (DVH) variations owing to spot mismatching with intensity-modulated proton radiotherapy (IMPT; red band) using (a) TANGENT_ENFACE and (b) TANGENT_PAIR for the patient shown in Figure 2. The shadowed band represents the DVH variation range, and the solid line represents the DVH from the original plan. brst skin, breast skin; ip. lung, ipsilateral lung; ip. norm. brst, ipsilateral normal breast PSPB, passive scattering proton beam.

Figure 5.

Dose–volume histogram (DVH) variation range (wide band) owing to (a) patient set-up uncertainties and (b) spot mismatching using the worst-case robust optimisation method and DVH comparison among intensity-modulated proton radiotherapy (IMPT) with robust optimisation (solid line), IMPT with non-robust optimisation (dashed line), and 3DCRT (dotted line) for TANGENT_PAIR and TANGENT_ENFACE, for the patient shown in Figure 2. 3DCRT, three-dimensional conformal photon radiotherapy; brst skin, breast skin; CTV, clinical target volume; ip lung, ipsilateral lung; ip norm brst, ipsilateral normal breast.

Figure 6.

Impact of range overshoot on the chest wall and lung for intensity-modulated proton radiotherapy with (a) en face beams and (b) tangential beams. The red colour wash represents the area with dose difference >15 Gy.

Figure 7.

Dose–volume histograms (DVHs) on free-breathing (FB) CT (solid line) and breath-hold (BH) CT (dashed line) for (a) en face beams and (b) tangential beams. brst skin, breast skin; CTV, clinical target volume; ip lung, ipsilateral lung; ip norm. brst, ipsilateral normal breast.