Purpose/Objective(s):
Continued advancements in prostate radiotherapy techniques have allowed for planning target volume (PTV) margin reductions, leading to improvements in radiation-related gastrointestinal (GI) side effects. Standard PTV expansions assume that the setup error is a normal distribution around a mean. This study evaluated the oncologic and toxicity outcomes of patients treated with prostate radiotherapy with the assumption that the PTV error is one-sided and removing the PTV overlap with an empty rectum. By reducing the PTV expansion overlapping with the rectum, we aimed to reduce GI toxicity, while maintaining oncologic efficacy.
Materials/Methods:
Data was collected from 365 consecutive patients treated with intensity-modulated radiotherapy (IMRT) for intermediate or high-risk prostate cancer. Patients were treated with 70 Gy in 28 fractions or 78 Gy in 39 fractions. All patients were simulated after at least one enema and re-simulated if bowel emptying was not achieved. The PTV was a 7 mm circumferential expansion from the prostate, except for 4 mm posteriorly, and all overlap with the rectum was removed from the PTV, effectively resulting in no posterior margin where the prostate and rectum touched. Biochemical recurrence-free survival (BCRFS), distant metastasis-free survival (DMFS), prostate-cancer specific survival (PCSS), and overall survival (OS), were calculated using Kaplan-Meier analysis. Acute and late toxicities were graded using CTCAE v5.0.
Results:
Median follow-up was 121.5 months. Most patients had unfavorable intermediate-risk or high/very-high risk disease (84.4%), and were treated with moderate hypofractionated RT (98.6%). The 10-year BCRFS, DMFS, PCSS, and OS rates were 82.8%, 92.3%, 97.5%, and 70.7%, respectively. Local recurrence as defined by biopsy or PSMA PET positivity (in the setting of rising PSA) at 10 years was 2.7%. 6 of the 11 patients with local failures occurred in the posterior of the prostate. Acute Grade 2 GI toxicity was observed in 2.2% of patients, and late Grade 2 GI toxicity was 0.5%, with no Grade 3+ GI toxicity observed. Acute Grade 2 genitourinary (GU) toxicity occurred in 32.6% of patients, while late Grade 2 GU toxicity was 8.2%, and late Grade 3 GU toxicity was 1.1%. Comparisons with modern trials revealed that our cohort had significantly lower rates of acute and late GI toxicity, suggesting that reducing the overlap of the PTV with an empty rectum minimizes toxicity without compromising oncologic control.
Conclusion: Reducing the PTV expansion posteriorly by ensuring a patient has an empty rectum at simulation resulted in expected oncologic outcomes with a near absence of GI toxicities. This technique offers a viable, non-invasive alternative to rectal spacers, simplifying treatment and reducing cost. Prospective studies are warranted to validate these findings and further explore their long-term impact on patient outcomes and quality of life.
