Physics aspects of prostate tomotherapy : Planning optimization and image-guidance issues

Purpose. To review planning and image-guidance aspects of more than 3 years experience in the treatment of prostate cancer with Helical Tomotherapy (HT). Methods and materials. Planning issues concerning two Phase I-II clinical studies were addressed: in the first one, 58 Gy in 20 fractions were delivered to the prostatic bed for post-prostatectomy patients: in the second one, a simultaneous integrated boost (SIB) approach was applied for radical treatment, delivering 71.4-74.2 Gy to the prostate in 28 fractions. On-line daily MVCT image guidance was applied: bone match was used for post-operative patients while prostate match was applied for radically treated patients. MVCT data of a large sample of both categories of patients were reviewed. Results. At now, more than 250 patients were treated. Planning data show the ability of HT in creating highly homogeneous dose distributions within PTVs. Organs at risk (OAR) sparing also showed to be excellent. HT was also found to favorably compare to inversely-optimized IMAT in terms of PTVs coverage and dose distribution homogeneity. In the case of pelvic nodes irradiation, a large sparing of bowel was evident compared to 3DCRT and conventional 5-fields IMRT. The analysis of MVCT data showed a limited motion of the prostate (about 5% of the fractions show a deviation ≥3 mm in posterior-anterior direction), due to the careful application of rectal emptying procedures. Based on phantom measurements and on the comparison with intra-prostatic calcification-based match, direct visualization prostate match seems to be sufficiently reliable in assessing shifts ≥3 mm. Conclusions. HT offers excellent planning solutions for prostate cancer, showing to be highly efficient in a SIB scenario. Daily MVCT information showed evidence of a limited motion of the prostate in the context of rectal filling control obtained by instructing patients in self-administrating a rectal enema.