Comparison of estimated late toxicities between IMPT and IMRT based on multivariable NTCP models for high-risk prostate cancers treated with pelvic nodal radiation

Abstract PurposeTo compare the late gastrointestinal (GI) and genitourinary toxicities (GU) estimated using multivariable NTCP models, between pencil beam scanning proton bream therapy (PBT) and helical tomotherapy (HT) in patients of high-risk prostate cancers requiring pelvic nodal irradiation (PNI) using moderately hypo-fractionated regimen.Materials and MethodsTwelve consecutive patients treated with PBT at our centre were replanned with HT using the same planning goals. Six late GI and GU toxicity domains (stool frequency, rectal bleeding, fecal incontinence, dysuria, urinary incontinence and hematuria) were estimated based on the published multivariable NTCP models. ΔNTCP (difference in absolute NTCP between HT and PBT plans) for each of the toxicity domains was calculated. One-Sample Kolmogorov-Smirnov test was used to analyze distribution of data and either a Paired T-test or a Wilcoxon matched-pair signed rank test was used to test statistical significance. ResultsPBT and HT plans achieved adequate target coverage. PBT plans led to significantly better sparing of bladder, rectum and bowel bag especially in the intermediate range of 15-40Gy; whereas doses to penile bulb and femoral heads were higher with PBT plans. The average ΔNTCP for grade(G)2-rectal bleeding, fecal incontinence, stool frequency, dysuria, urinary incontinence and G1-hematuria were 12.17%, 1.67%, 2%, 5.83%, 2.42% and 3.91% respectively favoring PBT plans. The average cumulative ΔNTCP for GI and GU toxicities (ΣΔNTCP) was 16.58% and 11.41% respectively favoring PBT. Using a model-based selection threshold of any G2 ΔNTCP >10%, 67% (8 patients) would be eligible for PBT.ConclusionPBT plans led to superior OAR sparing compared to HT which translated to lower NTCP for late moderate GI and GU toxicities in patients of prostate cancer treated with PNI. For two-thirds of our patients, the difference in estimated absolute NTCP values between PBT and HT, crossed the accepted threshold for minimal clinically important difference.