Session Item

Up to 10% of patients with rectal cancer have a pelvic recurrence after surgery. As pre-operative radiotherapy is an integrated part of the primary treatment for advanced cases many have received radiotherapy as a part of the initial strategy.

Surgery is the main treatment for pelvic recurrence, and outcome is highly dependent on radical (R0) resection. To increase R0 resection rate, re-irradiation has been introduced, mainly with doses around 40 Gy, with manageable toxicity.  Tumor control probability studies indicate, that doses around or above 50 Gy are associated to better pathological response. Additionally, not all recurrences are eligible for surgery, in these instances low dose radiotherapy or palliative systemic treatment is often applied.

 A comparative planning study was performed to determine if dose escalation with pencil beam proton therapy was feasible for pelvic recurrences of rectal cancer

Previously irradiated patients, with pelvic recurrences from rectal cancer, treated according to a re-irradiation protocol (40.8 Gy, 1.2 Gy per fraction, twice daily and concomitant capecitabine) were selected for comparative proton planning. The clinical photon plans were done with 1 or 2 arcs (VMAT), Eclipse. Proton plans were done with 3 posterior field robust IMPT planning (Multi field optimization, Eclipse v13.7) to 40.8 Gy RBE, 55 Gy RBE or 65 Gy RBE.

OARs were delineated according to RTOG guidelines. Dose to OARs (bowel loops, bladder, femoral heads, sacral bone, penile bulb, vagina) where compared for photon vs. doseescalated proton plans.

For comparison of selected DVHs metrics Wilcoxon's signed ranks test was used. A p-value<0.05 was considered statistically significant.

Eight patients were included (50% male), time to re-irradiation was 4.1 years (range: 2.2-6.9). Re-irradiation volumes (CTVs) ranged from 84 cm³ to 1080 cm³.

Mean dose to all OARs was lower with 40.8 Gy proton vs. 40.8 Gy photon therapy. Comparing dose escalations to the 40.8 Gy photon plan, mean dose to bowel loops was still significantly lower with dose escalation to 55 and 65 Gy, both p<0.01. Mean bladder dose was lower for both dose escalations, but only significantly for 55 Gy p=0.04 and p=0.055. For ipsi- and contra lateral femoral head, mean doses were significantly lower despite dose escalations, all p<0.01. Mean sacral dose was similar for the 55 Gy escalation plan but higher for the 65 Gy plan p=0.17 and p=0.05, fig 1 and 2. Due to small numbers statistical comparison was not done for penile bulb and vagina, but organs were generally spared with proton plans. As expected max doses increased for OAR close to or in the CTVs.

Dose escalation to 55 or 65 Gy RBE for re-irradiation of pelvic recurrences is possible while keeping mean doses to OARs at lower or similar levels as 40.8 Gy photon plans. Dose escalation could be relevant for both increasing radical resection in resectable recurrences or as a definitive treatment for unresectable recurrences.