ESTRO 2024 - Abstract Book

S4699

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

1 Centro Nazionale Adroterapia Oncologica (CNAO), Clinical Department, Pavia, Italy. 2 Università di Pavia, Department of Internal Medicine and Medical Therapy, Pavia, Italy

Purpose/Objective:

To evaluate clinical outcomes and treatment plans for intermediate/high-risk prostate cancer (pCa) patients treated with Carbon Ions Radiotherapy (CIRT) at a single institution between July 2013 and June 2020.

Material/Methods:

We retrospectively analyzed 42 pCa patients treated with CIRT and neoadjuvant and/or concomitant adjuvant/hormonotherapy. The prescription dose was 66.4 Gy(RBE) in 16 fractions delivered with two horizontal opposing fields. Dose fractionation was converted from the Japanese protocol of 57.6 Gy(RBE) in 16 fractions, based on the modified microdosimetric kinetic model (mMKM), to be applied in our Local Effect Model (LEM)-based treatment planning system (TPS). The clinical target volume (CTV) included the prostate and the proximal 1/3 of the seminal vesicles. We defined two planning target volumes (PTV):

i) low-dose PTV (LD-PTV) treated for the first 9 fractions that was CTV + anterior and lateral margins of 10 mm, posterior, cranial and caudal margins of 5 mm;

ii) high-dose PTV (HD-PTV) in which the posterior margin was positioned in front of the rectal anterior wall while the other margins remained the same for the remaining 7 fractions.

PTV coverage goal was D 95% >95% of the prescribed dose, while the main dose constraints were D 1cc <66 Gy(RBE), D 5cc <61 Gy(RBE), D 10cc <54 Gy(RBE) for the rectum, D 1cc <66.4 Gy(RBE) for the bladder and D 0.1cc <46 Gy(RBE) for the bowel. Biochemical Relapse Free Survival (BRFS), Local Recurrence Free Survival (LRFS), Distant Metastasis Free Survival (DMFS) and Overall Survival (OS) were estimated by the Kaplan–Meier method. Early and late gastrointestinal (GI) and genitourinary (GU) toxicities were assessed according to the CTCAE version 4.0. Clinical factors (age, cancer risk category, Gleason score, initial pre-treatment PSA, T stage, CTV volume, comorbidities) and dosimetric parameters (PTV, rectum and bladder dose-volume points) were tested as predictive factors for clinical failures and observed toxicities with univariate analysis (Chi-square test; a p-value of <0.05 was considered statistically significant). Rectum and bladder volumes receiving doses from 10 to 66 Gy(RBE) (V D ), with an increment of 10 Gy were tested for correlation with the incidence of toxicities higher then G1 with univariate analysis. For direct comparison with the Japanese published data, all treatment plans were recalculated with the mMKM model. The average percentage variations (ΔD 95% ) between planned LEM/mMKM D 95% and clinical goal on target coverage (D 95% >95%) were calculated.

Results:

The median patient age was 68 years (range: 51-89). With a median follow-up (FU) time of 40 months (range 9-101), biochemical relapse and local recurrence occurred in 3 (7.1%) and 2 (4.8%) patients at 5-years FU, respectively. The overall 5-year BRFS, LRFS, DMFS and OS were 82.1%, 88.9%, 92.3% and 94%, respectively, which were unrelated to any clinical variable tested (p<0.05). Furthermore, D 95% did not correlate with biochemical or local recurrence. Concerning toxicities, no G≥3 were recorded both in acute and late FU phases. Overall, no acute GI toxicity was observed, while 6 (14%) patients experienced early G2 GU. Late G1 and G2 GU toxicities were reported in 8 (19%) and 3 (7.1%) patients, respectively, while late G1 and G2 GI toxicities were observed in 3 (7.1%) and 1 (2.4%) patients. None