Prostate Cancer Radiotherapy Protocols: Comparison and Insights

The treatment planning and setup methods in both protocols have been described previously.¹⁴ (link), ¹⁵ (link), ¹⁶ (link), ¹⁷ (link) The clinical target volume (CTV) was defined as the whole prostate and proximal one‐third of the seminal vesicles. In T3b cases, all seminal vesicles were included in the CTV. The planning target volume (PTV) 1 was defined as the CTV plus 5‐mm margins in the cranial, caudal, and posterior directions and 10‐mm margins in the right, left, and anterior directions. The PTV2 was created by adding 2‐3‐mm margins to the CTV in the dorsal direction but was identical to the CTV in the cranial and caudal directions and to the PTV1 in the right, left, and anterior directions; PTV2 was used for the last four times of the treatment course. The prophylactic area of the pelvic lymph nodes was not included in the PTV. Purgatives or enemas were used for rectal reproducibility in the CT simulation and as necessary during treatment. There was no use of metallic markers in the prostate to improve reproducibility or SpaceOAR to decrease the rectum dose in either CPaI or SScI. All patients were treated using resinous shells and an image‐guided irradiation system, and the images were compared with reference images and confirmed for bone matching with the digitally reconstructed radiographs, under shallow natural breathing. The treatment couch was moved to the matching position until the largest deviation of all measured points was less than 1 mm. The two‐fields technique (opposing lateral fields) was routinely used for CIRT planning in CPaI and SScI (Figure 1).
In both CPaI and SScI, the dose prescription and dose constraints were the same. The irradiation dose is expressed as Gy (RBE; physical carbon ion dose [Gy] × RBE). The RBE value for CIRT was estimated to be 3.0 at the distal part of the spread‐out Bragg peak based on previous experience at our institution.²² CIRT was given once a day, 4 days a week (generally, Tuesday to Friday). The prescribed dose for all patients in this study was 51.6 Gy (RBE) administered in 12 fractions, and >95% of the dose was prescribed to the PTV2. The recommended dose constraints for the rectum are as follows: the rectal volume prescribed 53 Gy (RBE), 50 Gy (RBE), and 40 Gy (RBE) = 0%, ≤7%, and ≤16%, respectively. The dose constraints to other organs at risk were not defined.
The beam technique used for CIRT differed between CPaI and SScI. Compensators and multileaf collimators were used for each port individually in all patients enrolled in CPaI,¹⁸ (link) whereas these devices were not needed in any of the patients in SScI.¹⁹ (link)