A comparison of clinical and dosimetric outcomes in patients receiving partial breast irradiation with photon-only versus mixed photon/electron treatment plans.

Several series evaluating external-beam partial breast irradiation (PBI) have linked negative cosmetic outcomes to large normal tissue treatment volumes. We compared patients treated with PBI whose treatment plans included only photons to those whose plans incorporated electrons. Twenty-seven patients were identified: median age 67 years, pT1 82%, pN0 56%, margin negative 100%. All received 38.5 Gy using 3-5 noncoplanar photon beams (6-15X). Electrons (9-20 MeV) were included in 59%. Median follow-up was 22 months. Ninety percent experienced good/excellent cosmetic outcomes. Two patients had fair cosmesis, and both were treated with a mixed photon/electron approach. Median conformity index for photon-only treatment plans was 1.7 (range, 0.9-2.0) and for photon/electron plans, 1.0 (0.3-1.4). Median percent ipsilateral breast volume receiving 100% and 50% of prescription dose was 19 and 50 for photon-only plans vs. 10 and 38 for photon/electron plans (p < 0.05). Median percent target volume receiving 100% and 95% of prescription dose was 93 and 98 for photon-only plans vs. 75 and 94 for photon/electron plans (p < 0.05). A mixed photon/electron, noncoplanar technique decreases the volume of treated normal breast tissue at the cost of slightly decreased tumor bed coverage. Further study is needed to determine whether this results in a more favorable therapeutic ratio than photon-only approaches.

Dosimetric and radiobiologic comparison of 3D conformal versus intensity modulated planning techniques for prostate bed radiotherapy.

Adjuvant radiotherapy for locally advanced prostate cancer improves biochemical and clinical disease-free survival. While comparisons in intact prostate cancer show a benefit for intensity modulated radiation therapy (IMRT) over 3D conformal planning, this has not been studied for post-prostatectomy radiotherapy (RT). This study compares normal tissue and target dosimetry and radiobiological modeling of IMRT vs. 3D conformal planning in the postoperative setting. 3D conformal plans were designed for 15 patients who had been treated with IMRT planning for salvage post-prostatectomy RT. The same computed tomography (CT) and target/normal structure contours, as well as prescription dose, was used for both IMRT and 3D plans. Normal tissue complication probabilities (NTCPs) were calculated based on the dose given to the bladder and rectum by both plans. Dose-volume histogram and NTCP data were compared by paired t-test. Bladder and rectal sparing were improved with IMRT planning compared to 3D conformal planning. The volume of the bladder receiving at least 75% (V75) and 50% (V50) of the dose was significantly reduced by 28% and 17%, respectively (p = 0.002 and 0.037). Rectal dose was similarly reduced, V75 by 33% and V50 by 17% (p = 0.001 and 0.004). While there was no difference in the volume of rectum receiving at least 65 Gy (V65), IMRT planning significant reduced the volume receiving 40 Gy or more (V40, p = 0.009). Bladder V40 and V65 were not significantly different between planning modalities. Despite these dosimetric differences, there was no significant difference in the NTCP for either bladder or rectal injury. IMRT planning reduces the volume of bladder and rectum receiving high doses during post-prostatectomy RT. Because of relatively low doses given to the bladder and rectum, there was no statistically significant improvement in NTCP between the 3D conformal and IMRT plans.