Impact of Pelvic Intensity-modulated Radiotherapy (IMRT) on Lymph Node Coverage and Dose to Critical Organs, Compare to Three-Dimensional Conformal Radiation Therapy (3D-CRT), in Localized High-Risk Prostate Cancer

Abstract

Men with high risk prostate cancer (HR-PrCa) are typically treated with combined androgen deprivation therapy and curative radiotherapy (RT) consisting of pelvic nodal and prostate/seminal vesicle (PrSV) irradiation followed by PrSV boost RT. 3D-CRT or IMRT are used for the treatment of pelvic nodes. A phase II randomized study examining the effect of RT technique on patient Quality of Life (QoL) was completed in our institution. Here, we report an early analysis of the dosimetric impact of 3D-CRT versus IMRT in patients participating in this study. A total of 105 men with HR-PrCa were randomized at 1:1 ratio to receive either pelvic 3D-CRT (standard 4-field conformal technique) or multi-beam IMRT, consisting of 4500cGy in 25 fractions to planning target volume (PTV) 1, followed by boost RT to PTV2 of 2496cGy in 13 fractions. The latter was delivered using a 7-field IMRT in all patients. Organs at risk (OAR) and clinical target volumes (CTV) were delineated on CT simulation images. The lymph node target included the common, internal, external iliac, obturator and pre-sacral lymph nodes in accordance with RTOG guidelines. CTV1 consisted of nodal CTV plus PrSV. CTV2 was minimized to PrSV. The PTV consisted of an expansion of 7 mm beyond CTV. For consistency, in comparison between the two techniques, IMRT fields were modified to results to similar superior border coverage as 3D-CRT, at the level of L5-S1. Bladder and rectum were contoured as solid organs. To take inter-fractional bowel motion into account, bowel was delineated as whole pelvic and abdominal cavity excluding bones, muscle, and other OAR. Pelvic RT with 3DCRT missed significant percentage of pelvic nodal volume. PTV1 D95% showed mean difference of -4.96, 95% CI (-6.41, -1.74); p = 0.0013 and D99% showed mean difference of -11.56, 95% CI (-14.96, -8.17); p< 0.00001 in favor of IMRT. There was significant difference in PTV1 heterogeneity index (D2%-D95%/D50%) (Mean diff = 0.22, 95%CI (0.14, 0.29); P<0.00001). IMRT decreased dose to OAR. Rectum V30 was 97% in 3DCRT compare to 93.41% in IMRT arm with absolute difference of 3.81% (95% CI (1.52, 6.11); P = 0.0014). Bladder V40 and V50 were significantly reduced (V40: 84 % vs 62% for 3DCRT vs IMRT, absolute difference of 22% (95%, CI (15.55, 28.2); P<0.0001); V50 absolute reduction of 18% with IMRT (95%, CI (11.53, 24.48); P<0.0001). Similarly, IMRT reduced total body/bone irradiation significantly. This randomized study of 3D-CRT versus pelvic IMRT is an appropriate setting to investigate the dosimetric impact of pelvic IMRT and help define its potential benefits on patient QoL. This early analysis suggests that utilization of IMRT for pelvic RT could offer dosimetric advantages by improve lymph node coverage and reducing RT dose to OAR. The results are in agreement with an improved bowel-related QoL observed in this study. On-going analysis examines correlations between dosimetric and patient QoL data.