Biological impact of dosimetric perturbations of a fiducial marker and the daily number of fields in proton therapy for prostate cancer

The purpose of this study was to estimate the biological impact of dosimetric perturbations of a fiducial marker and the daily number of fields in proton therapy for prostate cancer. Using a linear-quadratic model, normalized total doses (NTDs) of points where deposited dose was reduced from the prescribed dose by dosimetric perturbation of a fiducial marker were calculated in two hypothetical prostate cancer treatment schedules: a) irradiation of both parallel-opposed lateral fields and b) irradiation of alternate field in each daily treatment. The impact of hypofractionation and sublethal damage repair between irradiation on NTD was also estimated. The NTD of two fields/day schedule becomes lower than that of one field/day schedule. The difference becomes larger as dose reduction from one of two fields becomes more enhanced. The NTD reduction from the total dose in the two fields/day schedule is largest (30% of total dose) where the dose from one beam is completely lost by a fiducial marker. In contrast, the NTD reduction from the total dose in the one field/day schedule is largest (9% of total dose) where the half dose from one beam is decreased by a fiducial marker. In addition, the NTD reduction becomes larger as the fractional dose increases in a hypofractionated regimen, and when the effect of sublethal damage repair was incorporated. These influences become significant in prostate cancer since the radiobiological sensitivity α/β of prostate cancer is lower than other cancer types and normal tissues late complication. Treating with one alternate field in a daily treatment can improve a deteriorating treatment effect by dosimetric distortion of a fiducial marker in prostate cancer treatment. However, the choice of the number of beams in a fraction must also be determined by considering the sparing of normal tissues and patient-specific status.

Comparative toxicity outcomes of proton-beam therapy versus intensity-modulated radiotherapy for prostate cancer in the postoperative setting

BACKGROUND

Despite increasing utilization of proton-beam therapy (PBT) in the postprostatectomy setting, no data exist regarding toxicity outcomes relative to intensity-modulated radiotherapy (IMRT). The authors compared acute and late genitourinary (GU) and gastrointestinal (GI) toxicity outcomes in patients with prostate cancer (PC) who received treatment with postprostatectomy IMRT versus PBT.

METHODS

With institutional review board approval, patients with PC who received adjuvant or salvage IMRT or PBT (70.2 gray with an endorectal balloon) after prostatectomy from 2009 through 2017 were reviewed. Factors including combined IMRT and PBT and/or concurrent malignancies prompted exclusion. A case-matched cohort analysis was performed using nearest-neighbor 3-to-1 matching by age and GU/GI disorder history. Logistic and Cox regressions were used to identify univariate and multivariate associations between toxicities and cohort/dosimetric characteristics. Toxicity-free survival (TFS) was assessed using the Kaplan-Meier method.

RESULTS

Three hundred seven men (mean ± SD age, 59.7 ± 6.3 years; IMRT, n = 237; PBT, n = 70) were identified, generating 70 matched pairs. The median follow-up was 48.6 and 46.1 months for the IMRT and PBT groups, respectively. Although PBT was superior at reducing low-range (volumes receiving 10% to 40% of the dose, respectively) bladder and rectal doses (all P ≤ .01), treatment modality was not associated with differences in clinician-reported acute or late GU/GI toxicities (all P ≥ .05). Five-year grade ≥2 GU and grade ≥1 GI TFS was 61.1% and 73.7% for IMRT, respectively, and 70.7% and 75.3% for PBT, respectively; and 5-year grade ≥3 GU and GI TFS was >95% for both groups (all P ≥ .05).

CONCLUSIONS

Postprostatectomy PBT minimized low-range bladder and rectal doses relative to IMRT; however, treatment modality was not associated with clinician-reported GU/GI toxicities. Future prospective investigation and ongoing follow-up will determine whether dosimetric differences between IMRT and PBT confer clinically meaningful differences in long-term outcomes.